From e75660dad3827fab87d96e2c65a7836fd5511b69 Mon Sep 17 00:00:00 2001
From: Adrian Taylor <adetaylor@chromium.org>
Date: Fri, 25 Oct 2024 11:08:58 +0000
Subject: [PATCH 1/6] Arbitrary self types v2: use Receiver trait

In this new version of Arbitrary Self Types, we no longer use the Deref trait
exclusively when working out which self types are valid. Instead, we follow a
chain of Receiver traits. This enables methods to be called on smart pointer
types which fundamentally cannot support Deref (for instance because they are
wrappers for pointers that don't follow Rust's aliasing rules).

This includes:
* Changes to tests appropriately
* New tests for:
  * The basics of the feature
  * Ensuring lifetime elision works properly
  * Generic Receivers
  * A copy of the method subst test enhanced with Receiver

This is really the heart of the 'arbitrary self types v2' feature, and
is the most critical commit in the current PR.

Subsequent commits are focused on:
* Detecting "shadowing" problems, where a smart pointer type can hide
  methods in the pointee.
* Diagnostics and cleanup.

Naming: in this commit, the "Autoderef" type is modified so that it no
longer solely focuses on the "Deref" trait, but can now consider the
"Receiver" trait instead. Should it be renamed, to something like
"TraitFollower"? This was considered, but rejected, because
* even in the Receiver case, it still considers built-in derefs
* the name Autoderef is short and snappy.
---
 .../src/error_codes/E0307.md                  |  6 +-
 compiler/rustc_hir_analysis/messages.ftl      |  4 +-
 compiler/rustc_hir_analysis/src/autoderef.rs  | 37 +++++--
 .../rustc_hir_analysis/src/check/wfcheck.rs   | 27 ++++--
 compiler/rustc_hir_typeck/src/method/probe.rs | 97 ++++++++++++++-----
 compiler/rustc_middle/src/traits/query.rs     | 12 ++-
 .../inference_var_self_argument.stderr        |  4 +-
 tests/ui/async-await/issue-66312.stderr       |  4 +-
 ...-gate-arbitrary-self-types-pointers.stderr |  6 +-
 .../feature-gate-arbitrary-self-types.stderr  |  6 +-
 ...te-arbitrary_self_types-raw-pointer.stderr |  6 +-
 ...feature-gate-dispatch-from-dyn-cell.stderr |  4 +-
 tests/ui/issues/issue-56806.stderr            |  4 +-
 .../could-not-resolve-issue-121503.stderr     |  2 +-
 .../methods/call_method_unknown_referent.rs   | 48 +++++++++
 .../call_method_unknown_referent.stderr       | 29 ++++++
 .../methods/call_method_unknown_referent2.rs  | 24 +++++
 ...y-self-from-method-substs-with-receiver.rs | 64 ++++++++++++
 ...lf-from-method-substs-with-receiver.stderr | 70 +++++++++++++
 ...ary-self-from-method-substs.default.stderr |  4 +-
 tests/ui/self/arbitrary-self-opaque.stderr    |  4 +-
 ...itrary_self_types_generic_over_receiver.rs | 23 +++++
 ...ry_self_types_generic_over_receiver.stderr | 59 +++++++++++
 .../arbitrary_self_types_lifetime_elision.rs  | 27 ++++++
 .../self/arbitrary_self_types_no_generics.rs  | 32 ++++++
 ...self_types_not_allow_call_with_no_deref.rs | 38 ++++++++
 ..._types_not_allow_call_with_no_deref.stderr | 39 ++++++++
 ...arbitrary_self_types_recursive_receiver.rs | 32 ++++++
 ...itrary_self_types_struct_receiver_trait.rs | 31 ++++++
 ...bitrary_self_types_trait_receiver_trait.rs | 25 +++++
 tests/ui/span/issue-27522.stderr              |  4 +-
 ...atible-trait-should-use-where-sized.stderr |  4 +-
 .../effects/auxiliary/minicore.rs             | 10 ++
 tests/ui/traits/issue-78372.stderr            |  4 +-
 .../method_resolution3.current.stderr         |  8 +-
 .../method_resolution3.next.stderr            |  8 +-
 .../method_resolution4.current.stderr         |  8 +-
 .../method_resolution4.next.stderr            |  8 +-
 tests/ui/ufcs/ufcs-explicit-self-bad.stderr   | 12 +--
 39 files changed, 737 insertions(+), 97 deletions(-)
 create mode 100644 tests/ui/methods/call_method_unknown_referent.rs
 create mode 100644 tests/ui/methods/call_method_unknown_referent.stderr
 create mode 100644 tests/ui/methods/call_method_unknown_referent2.rs
 create mode 100644 tests/ui/self/arbitrary-self-from-method-substs-with-receiver.rs
 create mode 100644 tests/ui/self/arbitrary-self-from-method-substs-with-receiver.stderr
 create mode 100644 tests/ui/self/arbitrary_self_types_generic_over_receiver.rs
 create mode 100644 tests/ui/self/arbitrary_self_types_generic_over_receiver.stderr
 create mode 100644 tests/ui/self/arbitrary_self_types_lifetime_elision.rs
 create mode 100644 tests/ui/self/arbitrary_self_types_no_generics.rs
 create mode 100644 tests/ui/self/arbitrary_self_types_not_allow_call_with_no_deref.rs
 create mode 100644 tests/ui/self/arbitrary_self_types_not_allow_call_with_no_deref.stderr
 create mode 100644 tests/ui/self/arbitrary_self_types_recursive_receiver.rs
 create mode 100644 tests/ui/self/arbitrary_self_types_struct_receiver_trait.rs
 create mode 100644 tests/ui/self/arbitrary_self_types_trait_receiver_trait.rs

diff --git a/compiler/rustc_error_codes/src/error_codes/E0307.md b/compiler/rustc_error_codes/src/error_codes/E0307.md
index 0d29d56ea1a75..b9c0493e8d6e6 100644
--- a/compiler/rustc_error_codes/src/error_codes/E0307.md
+++ b/compiler/rustc_error_codes/src/error_codes/E0307.md
@@ -65,8 +65,10 @@ impl Trait for Foo {
 ```
 
 The nightly feature [Arbitrary self types][AST] extends the accepted
-set of receiver types to also include any type that can dereference to
-`Self`:
+set of receiver types to also include any type that implements the
+`Receiver` trait and can follow its chain of `Target` types to `Self`.
+There's a blanket implementation of `Receiver` for `T: Deref`, so any
+type which dereferences to `Self` can be used.
 
 ```
 #![feature(arbitrary_self_types)]
diff --git a/compiler/rustc_hir_analysis/messages.ftl b/compiler/rustc_hir_analysis/messages.ftl
index 32498d9c5ab5b..25feb95d5dfeb 100644
--- a/compiler/rustc_hir_analysis/messages.ftl
+++ b/compiler/rustc_hir_analysis/messages.ftl
@@ -241,10 +241,10 @@ hir_analysis_invalid_generic_receiver_ty_help =
     use a concrete type such as `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
 
 hir_analysis_invalid_receiver_ty = invalid `self` parameter type: `{$receiver_ty}`
-    .note = type of `self` must be `Self` or a type that dereferences to it
+    .note = type of `self` must be `Self` or some type implementing `Receiver`
 
 hir_analysis_invalid_receiver_ty_help =
-    consider changing to `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
+    consider changing to `self`, `&self`, `&mut self`, or a type implementing `Receiver` such as `self: Box<Self>`, `self: Rc<Self>`, or `self: Arc<Self>`
 
 hir_analysis_invalid_union_field =
     field must implement `Copy` or be wrapped in `ManuallyDrop<...>` to be used in a union
diff --git a/compiler/rustc_hir_analysis/src/autoderef.rs b/compiler/rustc_hir_analysis/src/autoderef.rs
index 5a66c31a0cc31..d8e9227a87c86 100644
--- a/compiler/rustc_hir_analysis/src/autoderef.rs
+++ b/compiler/rustc_hir_analysis/src/autoderef.rs
@@ -18,7 +18,6 @@ pub enum AutoderefKind {
     /// A type which must dispatch to a `Deref` implementation.
     Overloaded,
 }
-
 struct AutoderefSnapshot<'tcx> {
     at_start: bool,
     reached_recursion_limit: bool,
@@ -27,6 +26,10 @@ struct AutoderefSnapshot<'tcx> {
     obligations: PredicateObligations<'tcx>,
 }
 
+/// Recursively dereference a type, considering both built-in
+/// dereferences (`*`) and the `Deref` trait.
+/// Although called `Autoderef` it can be configured to use the
+/// `Receiver` trait instead of the `Deref` trait.
 pub struct Autoderef<'a, 'tcx> {
     // Meta infos:
     infcx: &'a InferCtxt<'tcx>,
@@ -39,6 +42,7 @@ pub struct Autoderef<'a, 'tcx> {
 
     // Configurations:
     include_raw_pointers: bool,
+    use_receiver_trait: bool,
     silence_errors: bool,
 }
 
@@ -69,6 +73,10 @@ impl<'a, 'tcx> Iterator for Autoderef<'a, 'tcx> {
         }
 
         // Otherwise, deref if type is derefable:
+        // NOTE: in the case of self.use_receiver_trait = true, you might think it would
+        // be better to skip this clause and use the Overloaded case only, since &T
+        // and &mut T implement Receiver. But built-in derefs apply equally to Receiver
+        // and Deref, and this has benefits for const and the emitted MIR.
         let (kind, new_ty) =
             if let Some(ty) = self.state.cur_ty.builtin_deref(self.include_raw_pointers) {
                 debug_assert_eq!(ty, self.infcx.resolve_vars_if_possible(ty));
@@ -111,7 +119,7 @@ impl<'a, 'tcx> Autoderef<'a, 'tcx> {
         body_def_id: LocalDefId,
         span: Span,
         base_ty: Ty<'tcx>,
-    ) -> Autoderef<'a, 'tcx> {
+    ) -> Self {
         Autoderef {
             infcx,
             span,
@@ -125,6 +133,7 @@ impl<'a, 'tcx> Autoderef<'a, 'tcx> {
                 reached_recursion_limit: false,
             },
             include_raw_pointers: false,
+            use_receiver_trait: false,
             silence_errors: false,
         }
     }
@@ -137,8 +146,13 @@ impl<'a, 'tcx> Autoderef<'a, 'tcx> {
             return None;
         }
 
-        // <ty as Deref>
-        let trait_ref = ty::TraitRef::new(tcx, tcx.lang_items().deref_trait()?, [ty]);
+        // <ty as Deref>, or whatever the equivalent trait is that we've been asked to walk.
+        let (trait_def_id, trait_target_def_id) = if self.use_receiver_trait {
+            (tcx.lang_items().receiver_trait()?, tcx.lang_items().receiver_target()?)
+        } else {
+            (tcx.lang_items().deref_trait()?, tcx.lang_items().deref_target()?)
+        };
+        let trait_ref = ty::TraitRef::new(tcx, trait_def_id, [ty]);
         let cause = traits::ObligationCause::misc(self.span, self.body_id);
         let obligation = traits::Obligation::new(
             tcx,
@@ -151,11 +165,8 @@ impl<'a, 'tcx> Autoderef<'a, 'tcx> {
             return None;
         }
 
-        let (normalized_ty, obligations) = self.structurally_normalize(Ty::new_projection(
-            tcx,
-            tcx.lang_items().deref_target()?,
-            [ty],
-        ))?;
+        let (normalized_ty, obligations) =
+            self.structurally_normalize(Ty::new_projection(tcx, trait_target_def_id, [ty]))?;
         debug!("overloaded_deref_ty({:?}) = ({:?}, {:?})", ty, normalized_ty, obligations);
         self.state.obligations.extend(obligations);
 
@@ -234,6 +245,14 @@ impl<'a, 'tcx> Autoderef<'a, 'tcx> {
         self
     }
 
+    /// Use `core::ops::Receiver` and `core::ops::Receiver::Target` as
+    /// the trait and associated type to iterate, instead of
+    /// `core::ops::Deref` and `core::ops::Deref::Target`
+    pub fn use_receiver_trait(mut self) -> Self {
+        self.use_receiver_trait = true;
+        self
+    }
+
     pub fn silence_errors(mut self) -> Self {
         self.silence_errors = true;
         self
diff --git a/compiler/rustc_hir_analysis/src/check/wfcheck.rs b/compiler/rustc_hir_analysis/src/check/wfcheck.rs
index c9773972d9a24..57264d0bd2afc 100644
--- a/compiler/rustc_hir_analysis/src/check/wfcheck.rs
+++ b/compiler/rustc_hir_analysis/src/check/wfcheck.rs
@@ -1821,13 +1821,18 @@ fn receiver_is_valid<'tcx>(
 
     let mut autoderef = Autoderef::new(infcx, wfcx.param_env, wfcx.body_def_id, span, receiver_ty);
 
+    // The `arbitrary_self_types` feature allows custom smart pointer
+    // types to be method receivers, as identified by following the Receiver<Target=T>
+    // chain.
+    if arbitrary_self_types_enabled.is_some() {
+        autoderef = autoderef.use_receiver_trait();
+    }
+
     // The `arbitrary_self_types_pointers` feature allows raw pointer receivers like `self: *const Self`.
     if arbitrary_self_types_enabled == Some(ArbitrarySelfTypesLevel::WithPointers) {
         autoderef = autoderef.include_raw_pointers();
     }
 
-    let receiver_trait_def_id = tcx.require_lang_item(LangItem::LegacyReceiver, Some(span));
-
     // Keep dereferencing `receiver_ty` until we get to `self_ty`.
     while let Some((potential_self_ty, _)) = autoderef.next() {
         debug!(
@@ -1849,11 +1854,13 @@ fn receiver_is_valid<'tcx>(
         }
 
         // Without `feature(arbitrary_self_types)`, we require that each step in the
-        // deref chain implement `receiver`.
+        // deref chain implement `LegacyReceiver`.
         if arbitrary_self_types_enabled.is_none() {
-            if !receiver_is_implemented(
+            let legacy_receiver_trait_def_id =
+                tcx.require_lang_item(LangItem::LegacyReceiver, Some(span));
+            if !legacy_receiver_is_implemented(
                 wfcx,
-                receiver_trait_def_id,
+                legacy_receiver_trait_def_id,
                 cause.clone(),
                 potential_self_ty,
             ) {
@@ -1866,7 +1873,7 @@ fn receiver_is_valid<'tcx>(
                 cause.clone(),
                 wfcx.param_env,
                 potential_self_ty,
-                receiver_trait_def_id,
+                legacy_receiver_trait_def_id,
             );
         }
     }
@@ -1875,14 +1882,14 @@ fn receiver_is_valid<'tcx>(
     Err(ReceiverValidityError::DoesNotDeref)
 }
 
-fn receiver_is_implemented<'tcx>(
+fn legacy_receiver_is_implemented<'tcx>(
     wfcx: &WfCheckingCtxt<'_, 'tcx>,
-    receiver_trait_def_id: DefId,
+    legacy_receiver_trait_def_id: DefId,
     cause: ObligationCause<'tcx>,
     receiver_ty: Ty<'tcx>,
 ) -> bool {
     let tcx = wfcx.tcx();
-    let trait_ref = ty::TraitRef::new(tcx, receiver_trait_def_id, [receiver_ty]);
+    let trait_ref = ty::TraitRef::new(tcx, legacy_receiver_trait_def_id, [receiver_ty]);
 
     let obligation = Obligation::new(tcx, cause, wfcx.param_env, trait_ref);
 
@@ -1890,7 +1897,7 @@ fn receiver_is_implemented<'tcx>(
         true
     } else {
         debug!(
-            "receiver_is_implemented: type `{:?}` does not implement `Receiver` trait",
+            "receiver_is_implemented: type `{:?}` does not implement `LegacyReceiver` trait",
             receiver_ty
         );
         false
diff --git a/compiler/rustc_hir_typeck/src/method/probe.rs b/compiler/rustc_hir_typeck/src/method/probe.rs
index 039c117c09995..91e65af8df954 100644
--- a/compiler/rustc_hir_typeck/src/method/probe.rs
+++ b/compiler/rustc_hir_typeck/src/method/probe.rs
@@ -366,6 +366,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
                         autoderefs: 0,
                         from_unsafe_deref: false,
                         unsize: false,
+                        reachable_via_deref: true,
                     }]),
                     opt_bad_ty: None,
                     reached_recursion_limit: false,
@@ -516,47 +517,93 @@ fn method_autoderef_steps<'tcx>(
     let (ref infcx, goal, inference_vars) = tcx.infer_ctxt().build_with_canonical(DUMMY_SP, &goal);
     let ParamEnvAnd { param_env, value: self_ty } = goal;
 
-    let mut autoderef =
+    // If arbitrary self types is not enabled, we follow the chain of
+    // `Deref<Target=T>`. If arbitrary self types is enabled, we instead
+    // follow the chain of `Receiver<Target=T>`, but we also record whether
+    // such types are reachable by following the (potentially shorter)
+    // chain of `Deref<Target=T>`. We will use the first list when finding
+    // potentially relevant function implementations (e.g. relevant impl blocks)
+    // but the second list when determining types that the receiver may be
+    // converted to, in order to find out which of those methods might actually
+    // be callable.
+    let mut autoderef_via_deref =
         Autoderef::new(infcx, param_env, hir::def_id::CRATE_DEF_ID, DUMMY_SP, self_ty)
             .include_raw_pointers()
             .silence_errors();
-    let mut reached_raw_pointer = false;
-    let mut steps: Vec<_> = autoderef
-        .by_ref()
-        .map(|(ty, d)| {
-            let step = CandidateStep {
-                self_ty: infcx.make_query_response_ignoring_pending_obligations(inference_vars, ty),
-                autoderefs: d,
-                from_unsafe_deref: reached_raw_pointer,
-                unsize: false,
-            };
-            if let ty::RawPtr(_, _) = ty.kind() {
-                // all the subsequent steps will be from_unsafe_deref
-                reached_raw_pointer = true;
-            }
-            step
-        })
-        .collect();
 
-    let final_ty = autoderef.final_ty(true);
+    let mut reached_raw_pointer = false;
+    let arbitrary_self_types_enabled =
+        tcx.features().arbitrary_self_types() || tcx.features().arbitrary_self_types_pointers();
+    let (mut steps, reached_recursion_limit): (Vec<_>, bool) = if arbitrary_self_types_enabled {
+        let reachable_via_deref =
+            autoderef_via_deref.by_ref().map(|_| true).chain(std::iter::repeat(false));
+
+        let mut autoderef_via_receiver =
+            Autoderef::new(infcx, param_env, hir::def_id::CRATE_DEF_ID, DUMMY_SP, self_ty)
+                .include_raw_pointers()
+                .use_receiver_trait()
+                .silence_errors();
+        let steps = autoderef_via_receiver
+            .by_ref()
+            .zip(reachable_via_deref)
+            .map(|((ty, d), reachable_via_deref)| {
+                let step = CandidateStep {
+                    self_ty: infcx
+                        .make_query_response_ignoring_pending_obligations(inference_vars, ty),
+                    autoderefs: d,
+                    from_unsafe_deref: reached_raw_pointer,
+                    unsize: false,
+                    reachable_via_deref,
+                };
+                if ty.is_unsafe_ptr() {
+                    // all the subsequent steps will be from_unsafe_deref
+                    reached_raw_pointer = true;
+                }
+                step
+            })
+            .collect();
+        (steps, autoderef_via_receiver.reached_recursion_limit())
+    } else {
+        let steps = autoderef_via_deref
+            .by_ref()
+            .map(|(ty, d)| {
+                let step = CandidateStep {
+                    self_ty: infcx
+                        .make_query_response_ignoring_pending_obligations(inference_vars, ty),
+                    autoderefs: d,
+                    from_unsafe_deref: reached_raw_pointer,
+                    unsize: false,
+                    reachable_via_deref: true,
+                };
+                if ty.is_unsafe_ptr() {
+                    // all the subsequent steps will be from_unsafe_deref
+                    reached_raw_pointer = true;
+                }
+                step
+            })
+            .collect();
+        (steps, autoderef_via_deref.reached_recursion_limit())
+    };
+    let final_ty = autoderef_via_deref.final_ty(true);
     let opt_bad_ty = match final_ty.kind() {
         ty::Infer(ty::TyVar(_)) | ty::Error(_) => Some(MethodAutoderefBadTy {
             reached_raw_pointer,
             ty: infcx.make_query_response_ignoring_pending_obligations(inference_vars, final_ty),
         }),
         ty::Array(elem_ty, _) => {
-            let dereferences = steps.len() - 1;
-
+            let autoderefs = steps.iter().filter(|s| s.reachable_via_deref).count() - 1;
             steps.push(CandidateStep {
                 self_ty: infcx.make_query_response_ignoring_pending_obligations(
                     inference_vars,
                     Ty::new_slice(infcx.tcx, *elem_ty),
                 ),
-                autoderefs: dereferences,
+                autoderefs,
                 // this could be from an unsafe deref if we had
                 // a *mut/const [T; N]
                 from_unsafe_deref: reached_raw_pointer,
                 unsize: true,
+                reachable_via_deref: true, // this is always the final type from
+                                           // autoderef_via_deref
             });
 
             None
@@ -569,7 +616,7 @@ fn method_autoderef_steps<'tcx>(
     MethodAutoderefStepsResult {
         steps: tcx.arena.alloc_from_iter(steps),
         opt_bad_ty: opt_bad_ty.map(|ty| &*tcx.arena.alloc(ty)),
-        reached_recursion_limit: autoderef.reached_recursion_limit(),
+        reached_recursion_limit,
     }
 }
 
@@ -1065,6 +1112,10 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
     ) -> Option<PickResult<'tcx>> {
         self.steps
             .iter()
+            // At this point we're considering the types to which the receiver can be converted,
+            // so we want to follow the `Deref` chain not the `Receiver` chain. Filter out
+            // steps which can only be reached by following the (longer) `Receiver` chain.
+            .filter(|step| step.reachable_via_deref)
             .filter(|step| {
                 debug!("pick_all_method: step={:?}", step);
                 // skip types that are from a type error or that would require dereferencing
diff --git a/compiler/rustc_middle/src/traits/query.rs b/compiler/rustc_middle/src/traits/query.rs
index eeed5118592bc..f049da95f29ae 100644
--- a/compiler/rustc_middle/src/traits/query.rs
+++ b/compiler/rustc_middle/src/traits/query.rs
@@ -149,11 +149,21 @@ pub struct CandidateStep<'tcx> {
     /// `foo.by_raw_ptr()` will work and `foo.by_ref()` won't.
     pub from_unsafe_deref: bool,
     pub unsize: bool,
+    /// We will generate CandidateSteps which are reachable via a chain
+    /// of following `Receiver`. The first 'n' of those will be reachable
+    /// by following a chain of 'Deref' instead (since there's a blanket
+    /// implementation of Receiver for Deref).
+    /// We use the entire set of steps when identifying method candidates
+    /// (e.g. identifying relevant `impl` blocks) but only those that are
+    /// reachable via Deref when examining what the receiver type can
+    /// be converted into by autodereffing.
+    pub reachable_via_deref: bool,
 }
 
 #[derive(Copy, Clone, Debug, HashStable)]
 pub struct MethodAutoderefStepsResult<'tcx> {
-    /// The valid autoderef steps that could be found.
+    /// The valid autoderef steps that could be found by following a chain
+    /// of `Receiver<Target=T>` or `Deref<Target=T>` trait implementations.
     pub steps: &'tcx [CandidateStep<'tcx>],
     /// If Some(T), a type autoderef reported an error on.
     pub opt_bad_ty: Option<&'tcx MethodAutoderefBadTy<'tcx>>,
diff --git a/tests/ui/async-await/inference_var_self_argument.stderr b/tests/ui/async-await/inference_var_self_argument.stderr
index 7b7b3dbc757f1..a33c5f7b07dca 100644
--- a/tests/ui/async-await/inference_var_self_argument.stderr
+++ b/tests/ui/async-await/inference_var_self_argument.stderr
@@ -4,8 +4,8 @@ error[E0307]: invalid `self` parameter type: `&dyn Foo`
 LL |     async fn foo(self: &dyn Foo) {
    |                        ^^^^^^^^
    |
-   = note: type of `self` must be `Self` or a type that dereferences to it
-   = help: consider changing to `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
+   = note: type of `self` must be `Self` or some type implementing `Receiver`
+   = help: consider changing to `self`, `&self`, `&mut self`, or a type implementing `Receiver` such as `self: Box<Self>`, `self: Rc<Self>`, or `self: Arc<Self>`
 
 error[E0038]: the trait `Foo` cannot be made into an object
   --> $DIR/inference_var_self_argument.rs:5:5
diff --git a/tests/ui/async-await/issue-66312.stderr b/tests/ui/async-await/issue-66312.stderr
index c95ae1147df36..f4db949a5f430 100644
--- a/tests/ui/async-await/issue-66312.stderr
+++ b/tests/ui/async-await/issue-66312.stderr
@@ -4,8 +4,8 @@ error[E0307]: invalid `self` parameter type: `T`
 LL |     fn is_some(self: T);
    |                      ^
    |
-   = note: type of `self` must be `Self` or a type that dereferences to it
-   = help: consider changing to `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
+   = note: type of `self` must be `Self` or some type implementing `Receiver`
+   = help: consider changing to `self`, `&self`, `&mut self`, or a type implementing `Receiver` such as `self: Box<Self>`, `self: Rc<Self>`, or `self: Arc<Self>`
 
 error[E0308]: mismatched types
   --> $DIR/issue-66312.rs:9:8
diff --git a/tests/ui/feature-gates/feature-gate-arbitrary-self-types-pointers.stderr b/tests/ui/feature-gates/feature-gate-arbitrary-self-types-pointers.stderr
index 3bb93cf2ea0b1..0c5b8a4d3b685 100644
--- a/tests/ui/feature-gates/feature-gate-arbitrary-self-types-pointers.stderr
+++ b/tests/ui/feature-gates/feature-gate-arbitrary-self-types-pointers.stderr
@@ -7,7 +7,7 @@ LL |     fn foo(self: *const Self) {}
    = note: see issue #44874 <https://github.com/rust-lang/rust/issues/44874> for more information
    = help: add `#![feature(arbitrary_self_types_pointers)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
-   = help: consider changing to `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
+   = help: consider changing to `self`, `&self`, `&mut self`, or a type implementing `Receiver` such as `self: Box<Self>`, `self: Rc<Self>`, or `self: Arc<Self>`
 
 error[E0658]: `*mut Bar` cannot be used as the type of `self` without the `arbitrary_self_types_pointers` feature
   --> $DIR/feature-gate-arbitrary-self-types-pointers.rs:12:18
@@ -18,7 +18,7 @@ LL |     fn bar(self: *mut Self) {}
    = note: see issue #44874 <https://github.com/rust-lang/rust/issues/44874> for more information
    = help: add `#![feature(arbitrary_self_types_pointers)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
-   = help: consider changing to `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
+   = help: consider changing to `self`, `&self`, `&mut self`, or a type implementing `Receiver` such as `self: Box<Self>`, `self: Rc<Self>`, or `self: Arc<Self>`
 
 error[E0658]: `*const Self` cannot be used as the type of `self` without the `arbitrary_self_types_pointers` feature
   --> $DIR/feature-gate-arbitrary-self-types-pointers.rs:2:18
@@ -29,7 +29,7 @@ LL |     fn foo(self: *const Self);
    = note: see issue #44874 <https://github.com/rust-lang/rust/issues/44874> for more information
    = help: add `#![feature(arbitrary_self_types_pointers)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
-   = help: consider changing to `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
+   = help: consider changing to `self`, `&self`, `&mut self`, or a type implementing `Receiver` such as `self: Box<Self>`, `self: Rc<Self>`, or `self: Arc<Self>`
 
 error: aborting due to 3 previous errors
 
diff --git a/tests/ui/feature-gates/feature-gate-arbitrary-self-types.stderr b/tests/ui/feature-gates/feature-gate-arbitrary-self-types.stderr
index 7f0e02c91f8f8..3ffba533d63fc 100644
--- a/tests/ui/feature-gates/feature-gate-arbitrary-self-types.stderr
+++ b/tests/ui/feature-gates/feature-gate-arbitrary-self-types.stderr
@@ -7,7 +7,7 @@ LL |     fn foo(self: Ptr<Self>) {}
    = note: see issue #44874 <https://github.com/rust-lang/rust/issues/44874> for more information
    = help: add `#![feature(arbitrary_self_types)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
-   = help: consider changing to `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
+   = help: consider changing to `self`, `&self`, `&mut self`, or a type implementing `Receiver` such as `self: Box<Self>`, `self: Rc<Self>`, or `self: Arc<Self>`
 
 error[E0658]: `Box<Ptr<Bar>>` cannot be used as the type of `self` without the `arbitrary_self_types` feature
   --> $DIR/feature-gate-arbitrary-self-types.rs:26:18
@@ -18,7 +18,7 @@ LL |     fn bar(self: Box<Ptr<Self>>) {}
    = note: see issue #44874 <https://github.com/rust-lang/rust/issues/44874> for more information
    = help: add `#![feature(arbitrary_self_types)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
-   = help: consider changing to `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
+   = help: consider changing to `self`, `&self`, `&mut self`, or a type implementing `Receiver` such as `self: Box<Self>`, `self: Rc<Self>`, or `self: Arc<Self>`
 
 error[E0658]: `Ptr<Self>` cannot be used as the type of `self` without the `arbitrary_self_types` feature
   --> $DIR/feature-gate-arbitrary-self-types.rs:16:18
@@ -29,7 +29,7 @@ LL |     fn foo(self: Ptr<Self>);
    = note: see issue #44874 <https://github.com/rust-lang/rust/issues/44874> for more information
    = help: add `#![feature(arbitrary_self_types)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
-   = help: consider changing to `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
+   = help: consider changing to `self`, `&self`, `&mut self`, or a type implementing `Receiver` such as `self: Box<Self>`, `self: Rc<Self>`, or `self: Arc<Self>`
 
 error: aborting due to 3 previous errors
 
diff --git a/tests/ui/feature-gates/feature-gate-arbitrary_self_types-raw-pointer.stderr b/tests/ui/feature-gates/feature-gate-arbitrary_self_types-raw-pointer.stderr
index 856e059533139..3eb87adb33adc 100644
--- a/tests/ui/feature-gates/feature-gate-arbitrary_self_types-raw-pointer.stderr
+++ b/tests/ui/feature-gates/feature-gate-arbitrary_self_types-raw-pointer.stderr
@@ -7,7 +7,7 @@ LL |     fn foo(self: *const Self) {}
    = note: see issue #44874 <https://github.com/rust-lang/rust/issues/44874> for more information
    = help: add `#![feature(arbitrary_self_types_pointers)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
-   = help: consider changing to `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
+   = help: consider changing to `self`, `&self`, `&mut self`, or a type implementing `Receiver` such as `self: Box<Self>`, `self: Rc<Self>`, or `self: Arc<Self>`
 
 error[E0658]: `*const ()` cannot be used as the type of `self` without the `arbitrary_self_types_pointers` feature
   --> $DIR/feature-gate-arbitrary_self_types-raw-pointer.rs:14:18
@@ -18,7 +18,7 @@ LL |     fn bar(self: *const Self) {}
    = note: see issue #44874 <https://github.com/rust-lang/rust/issues/44874> for more information
    = help: add `#![feature(arbitrary_self_types_pointers)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
-   = help: consider changing to `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
+   = help: consider changing to `self`, `&self`, `&mut self`, or a type implementing `Receiver` such as `self: Box<Self>`, `self: Rc<Self>`, or `self: Arc<Self>`
 
 error[E0658]: `*const Self` cannot be used as the type of `self` without the `arbitrary_self_types_pointers` feature
   --> $DIR/feature-gate-arbitrary_self_types-raw-pointer.rs:9:18
@@ -29,7 +29,7 @@ LL |     fn bar(self: *const Self);
    = note: see issue #44874 <https://github.com/rust-lang/rust/issues/44874> for more information
    = help: add `#![feature(arbitrary_self_types_pointers)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
-   = help: consider changing to `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
+   = help: consider changing to `self`, `&self`, `&mut self`, or a type implementing `Receiver` such as `self: Box<Self>`, `self: Rc<Self>`, or `self: Arc<Self>`
 
 error: aborting due to 3 previous errors
 
diff --git a/tests/ui/feature-gates/feature-gate-dispatch-from-dyn-cell.stderr b/tests/ui/feature-gates/feature-gate-dispatch-from-dyn-cell.stderr
index 2150effc3b74d..eb9e51a04c394 100644
--- a/tests/ui/feature-gates/feature-gate-dispatch-from-dyn-cell.stderr
+++ b/tests/ui/feature-gates/feature-gate-dispatch-from-dyn-cell.stderr
@@ -4,8 +4,8 @@ error[E0307]: invalid `self` parameter type: `Cell<&Self>`
 LL |     fn cell(self: Cell<&Self>);
    |                   ^^^^^^^^^^^
    |
-   = note: type of `self` must be `Self` or a type that dereferences to it
-   = help: consider changing to `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
+   = note: type of `self` must be `Self` or some type implementing `Receiver`
+   = help: consider changing to `self`, `&self`, `&mut self`, or a type implementing `Receiver` such as `self: Box<Self>`, `self: Rc<Self>`, or `self: Arc<Self>`
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/issues/issue-56806.stderr b/tests/ui/issues/issue-56806.stderr
index ec50d863758db..4b0a59fe12def 100644
--- a/tests/ui/issues/issue-56806.stderr
+++ b/tests/ui/issues/issue-56806.stderr
@@ -4,8 +4,8 @@ error[E0307]: invalid `self` parameter type: `Box<(dyn Trait + 'static)>`
 LL |     fn dyn_instead_of_self(self: Box<dyn Trait>);
    |                                  ^^^^^^^^^^^^^^
    |
-   = note: type of `self` must be `Self` or a type that dereferences to it
-   = help: consider changing to `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
+   = note: type of `self` must be `Self` or some type implementing `Receiver`
+   = help: consider changing to `self`, `&self`, `&mut self`, or a type implementing `Receiver` such as `self: Box<Self>`, `self: Rc<Self>`, or `self: Arc<Self>`
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/lifetimes/could-not-resolve-issue-121503.stderr b/tests/ui/lifetimes/could-not-resolve-issue-121503.stderr
index 3babf63347c35..46804642af88e 100644
--- a/tests/ui/lifetimes/could-not-resolve-issue-121503.stderr
+++ b/tests/ui/lifetimes/could-not-resolve-issue-121503.stderr
@@ -7,7 +7,7 @@ LL |     async fn box_ref_Struct(self: Box<Self, impl FnMut(&mut Self)>) -> &u32
    = note: see issue #44874 <https://github.com/rust-lang/rust/issues/44874> for more information
    = help: add `#![feature(arbitrary_self_types)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
-   = help: consider changing to `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
+   = help: consider changing to `self`, `&self`, `&mut self`, or a type implementing `Receiver` such as `self: Box<Self>`, `self: Rc<Self>`, or `self: Arc<Self>`
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/methods/call_method_unknown_referent.rs b/tests/ui/methods/call_method_unknown_referent.rs
new file mode 100644
index 0000000000000..b01e2d80f7f80
--- /dev/null
+++ b/tests/ui/methods/call_method_unknown_referent.rs
@@ -0,0 +1,48 @@
+//@ edition: 2018
+
+#![feature(arbitrary_self_types)]
+
+// tests that the referent type of a reference must be known to call methods on it
+
+struct SmartPtr<T>(T);
+
+impl<T> core::ops::Receiver for SmartPtr<T> {
+    type Target = T;
+}
+
+impl<T> SmartPtr<T> {
+    fn foo(&self) {}
+}
+
+fn main() {
+    let val = 1_u32;
+    let ptr = &val;
+    let _a: i32 = (ptr as &_).read();
+    //~^ ERROR type annotations needed
+
+    // Same again, but with a smart pointer type
+    let val2 = 1_u32;
+    let rc = std::rc::Rc::new(val2);
+    let _b = (rc as std::rc::Rc<_>).read();
+    //~^ ERROR type annotations needed
+
+    // Same again, but with a smart pointer type
+    let ptr = SmartPtr(val);
+
+    // We can call unambiguous outer-type methods on this
+    (ptr as SmartPtr<_>).foo();
+    // ... but we can't follow the Receiver chain to the inner type
+    // because we end up with _.
+
+    // Because SmartPtr implements Receiver, it's arguable which of the
+    // following two diagnostics we'd want in this case:
+    // (a) "type annotations needed" (because the inner type is _)
+    // (b) "no method named `read` found for struct `SmartPtr`"
+    //     (ignoring the fact that there might have been methods on the
+    //      inner type, had it not been _)
+    // At present we produce error type (b), which is necessary because
+    // our resolution logic needs to be able to call methods such as foo()
+    // on the outer type even if the inner type is ambiguous.
+    let _c = (ptr as SmartPtr<_>).read();
+    //~^ ERROR no method named `read` found for struct `SmartPtr`
+}
diff --git a/tests/ui/methods/call_method_unknown_referent.stderr b/tests/ui/methods/call_method_unknown_referent.stderr
new file mode 100644
index 0000000000000..748b02b52b577
--- /dev/null
+++ b/tests/ui/methods/call_method_unknown_referent.stderr
@@ -0,0 +1,29 @@
+error[E0282]: type annotations needed
+  --> $DIR/call_method_unknown_referent.rs:20:31
+   |
+LL |     let _a: i32 = (ptr as &_).read();
+   |                               ^^^^ cannot infer type
+
+error[E0282]: type annotations needed
+  --> $DIR/call_method_unknown_referent.rs:26:37
+   |
+LL |     let _b = (rc as std::rc::Rc<_>).read();
+   |                                     ^^^^ cannot infer type
+
+error[E0599]: no method named `read` found for struct `SmartPtr` in the current scope
+  --> $DIR/call_method_unknown_referent.rs:46:35
+   |
+LL | struct SmartPtr<T>(T);
+   | ------------------ method `read` not found for this struct
+...
+LL |     let _c = (ptr as SmartPtr<_>).read();
+   |                                   ^^^^ method not found in `SmartPtr<_>`
+   |
+   = help: items from traits can only be used if the trait is implemented and in scope
+   = note: the following trait defines an item `read`, perhaps you need to implement it:
+           candidate #1: `std::io::Read`
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0282, E0599.
+For more information about an error, try `rustc --explain E0282`.
diff --git a/tests/ui/methods/call_method_unknown_referent2.rs b/tests/ui/methods/call_method_unknown_referent2.rs
new file mode 100644
index 0000000000000..b1615bd8d4414
--- /dev/null
+++ b/tests/ui/methods/call_method_unknown_referent2.rs
@@ -0,0 +1,24 @@
+//@ edition: 2018
+//@ run-pass
+
+#![feature(arbitrary_self_types)]
+
+// tests that the referent type of a reference must be known to call methods on it
+
+struct SmartPtr<T>(T);
+
+impl<T> core::ops::Receiver for SmartPtr<T> {
+    type Target = T;
+}
+
+impl<T> SmartPtr<T> {
+    fn foo(&self) -> usize { 3 }
+}
+
+fn main() {
+    let val = 1_u32;
+    let ptr = SmartPtr(val);
+    // Ensure calls to outer methods work even if inner methods can't be
+    // resolved due to the type variable
+    assert_eq!((ptr as SmartPtr<_>).foo(), 3);
+}
diff --git a/tests/ui/self/arbitrary-self-from-method-substs-with-receiver.rs b/tests/ui/self/arbitrary-self-from-method-substs-with-receiver.rs
new file mode 100644
index 0000000000000..495d261c549ca
--- /dev/null
+++ b/tests/ui/self/arbitrary-self-from-method-substs-with-receiver.rs
@@ -0,0 +1,64 @@
+#![feature(arbitrary_self_types)]
+
+use std::ops::{Receiver, Deref};
+
+struct SmartPtr<'a, T: ?Sized>(&'a T);
+
+impl<'a, T: ?Sized> Deref for SmartPtr<'a, T> {
+    type Target = T;
+    fn deref(&self) -> &Self::Target {
+        self.0
+    }
+}
+
+impl<'a, T: ?Sized> Clone for SmartPtr<'a, T> {
+    fn clone(&self) -> Self {
+        Self(self.0)
+    }
+}
+
+impl<'a, T: ?Sized> Copy for SmartPtr<'a, T> {
+}
+
+struct Foo(u32);
+impl Foo {
+    fn a<R: Receiver<Target=Self>>(self: R) -> u32 {
+        //~^ ERROR invalid generic `self` parameter type: `R`
+        2
+    }
+    fn b<R: Deref<Target=Self>>(self: R) -> u32 {
+        //~^ ERROR invalid generic `self` parameter type: `R`
+        self.0
+    }
+    fn c(self: impl Receiver<Target=Self>) -> u32 {
+        //~^ ERROR invalid generic `self` parameter type: `impl Receiver<Target = Self>`
+        3
+    }
+    fn d(self: impl Deref<Target=Self>) -> u32 {
+        //~^ ERROR invalid generic `self` parameter type: `impl Deref<Target = Self>`
+        self.0
+    }
+}
+
+fn main() {
+    let foo = Foo(1);
+    assert_eq!((&foo).a::<&Foo>(), 2);
+    assert_eq!((&foo).b::<&Foo>(), 1);
+    assert_eq!((&foo).a(), 2);
+    assert_eq!((&foo).b(), 1);
+    assert_eq!((&foo).c(), 3);
+    assert_eq!((&foo).d(), 1);
+    assert_eq!(foo.a::<&Foo>(), 2);
+    //~^ ERROR mismatched types
+    assert_eq!(foo.b::<&Foo>(), 1);
+    //~^ ERROR mismatched types
+    let smart_ptr = SmartPtr(&foo);
+    assert_eq!(smart_ptr.a(), 2);
+    assert_eq!(smart_ptr.b(), 1);
+    assert_eq!(smart_ptr.c(), 3);
+    assert_eq!(smart_ptr.d(), 1);
+    assert_eq!(smart_ptr.a::<&Foo>(), 2);
+    //~^ ERROR mismatched types
+    assert_eq!(smart_ptr.b::<&Foo>(), 1);
+    //~^ ERROR mismatched types
+}
diff --git a/tests/ui/self/arbitrary-self-from-method-substs-with-receiver.stderr b/tests/ui/self/arbitrary-self-from-method-substs-with-receiver.stderr
new file mode 100644
index 0000000000000..9af2a08f3712d
--- /dev/null
+++ b/tests/ui/self/arbitrary-self-from-method-substs-with-receiver.stderr
@@ -0,0 +1,70 @@
+error[E0801]: invalid generic `self` parameter type: `R`
+  --> $DIR/arbitrary-self-from-method-substs-with-receiver.rs:25:42
+   |
+LL |     fn a<R: Receiver<Target=Self>>(self: R) -> u32 {
+   |                                          ^
+   |
+   = note: type of `self` must not be a method generic parameter type
+   = help: use a concrete type such as `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
+
+error[E0801]: invalid generic `self` parameter type: `R`
+  --> $DIR/arbitrary-self-from-method-substs-with-receiver.rs:29:39
+   |
+LL |     fn b<R: Deref<Target=Self>>(self: R) -> u32 {
+   |                                       ^
+   |
+   = note: type of `self` must not be a method generic parameter type
+   = help: use a concrete type such as `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
+
+error[E0801]: invalid generic `self` parameter type: `impl Receiver<Target = Self>`
+  --> $DIR/arbitrary-self-from-method-substs-with-receiver.rs:33:16
+   |
+LL |     fn c(self: impl Receiver<Target=Self>) -> u32 {
+   |                ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: type of `self` must not be a method generic parameter type
+   = help: use a concrete type such as `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
+
+error[E0801]: invalid generic `self` parameter type: `impl Deref<Target = Self>`
+  --> $DIR/arbitrary-self-from-method-substs-with-receiver.rs:37:16
+   |
+LL |     fn d(self: impl Deref<Target=Self>) -> u32 {
+   |                ^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: type of `self` must not be a method generic parameter type
+   = help: use a concrete type such as `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
+
+error[E0308]: mismatched types
+  --> $DIR/arbitrary-self-from-method-substs-with-receiver.rs:51:16
+   |
+LL |     assert_eq!(foo.a::<&Foo>(), 2);
+   |                ^^^ expected `&Foo`, found `Foo`
+
+error[E0308]: mismatched types
+  --> $DIR/arbitrary-self-from-method-substs-with-receiver.rs:53:16
+   |
+LL |     assert_eq!(foo.b::<&Foo>(), 1);
+   |                ^^^ expected `&Foo`, found `Foo`
+
+error[E0308]: mismatched types
+  --> $DIR/arbitrary-self-from-method-substs-with-receiver.rs:60:16
+   |
+LL |     assert_eq!(smart_ptr.a::<&Foo>(), 2);
+   |                ^^^^^^^^^ expected `&Foo`, found `SmartPtr<'_, Foo>`
+   |
+   = note: expected reference `&Foo`
+                 found struct `SmartPtr<'_, Foo, >`
+
+error[E0308]: mismatched types
+  --> $DIR/arbitrary-self-from-method-substs-with-receiver.rs:62:16
+   |
+LL |     assert_eq!(smart_ptr.b::<&Foo>(), 1);
+   |                ^^^^^^^^^ expected `&Foo`, found `SmartPtr<'_, Foo>`
+   |
+   = note: expected reference `&Foo`
+                 found struct `SmartPtr<'_, Foo, >`
+
+error: aborting due to 8 previous errors
+
+Some errors have detailed explanations: E0308, E0801.
+For more information about an error, try `rustc --explain E0308`.
diff --git a/tests/ui/self/arbitrary-self-from-method-substs.default.stderr b/tests/ui/self/arbitrary-self-from-method-substs.default.stderr
index 5dc3a0b023472..7cf9c9a3afd4d 100644
--- a/tests/ui/self/arbitrary-self-from-method-substs.default.stderr
+++ b/tests/ui/self/arbitrary-self-from-method-substs.default.stderr
@@ -61,7 +61,7 @@ LL |     fn get6<FR: FindReceiver>(self: FR::Receiver, other: FR) -> u32 {
    = note: see issue #44874 <https://github.com/rust-lang/rust/issues/44874> for more information
    = help: add `#![feature(arbitrary_self_types)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
-   = help: consider changing to `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
+   = help: consider changing to `self`, `&self`, `&mut self`, or a type implementing `Receiver` such as `self: Box<Self>`, `self: Rc<Self>`, or `self: Arc<Self>`
 
 error[E0658]: `R` cannot be used as the type of `self` without the `arbitrary_self_types` feature
   --> $DIR/arbitrary-self-from-method-substs.rs:61:18
@@ -72,7 +72,7 @@ LL |     fn get(self: R) {}
    = note: see issue #44874 <https://github.com/rust-lang/rust/issues/44874> for more information
    = help: add `#![feature(arbitrary_self_types)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
-   = help: consider changing to `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
+   = help: consider changing to `self`, `&self`, `&mut self`, or a type implementing `Receiver` such as `self: Box<Self>`, `self: Rc<Self>`, or `self: Arc<Self>`
 
 error[E0271]: type mismatch resolving `<Silly as FindReceiver>::Receiver == Foo`
   --> $DIR/arbitrary-self-from-method-substs.rs:92:9
diff --git a/tests/ui/self/arbitrary-self-opaque.stderr b/tests/ui/self/arbitrary-self-opaque.stderr
index c75165d9f8e27..0469aca27dc81 100644
--- a/tests/ui/self/arbitrary-self-opaque.stderr
+++ b/tests/ui/self/arbitrary-self-opaque.stderr
@@ -4,8 +4,8 @@ error[E0307]: invalid `self` parameter type: `Bar`
 LL |     fn foo(self: Bar) {}
    |                  ^^^
    |
-   = note: type of `self` must be `Self` or a type that dereferences to it
-   = help: consider changing to `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
+   = note: type of `self` must be `Self` or some type implementing `Receiver`
+   = help: consider changing to `self`, `&self`, `&mut self`, or a type implementing `Receiver` such as `self: Box<Self>`, `self: Rc<Self>`, or `self: Arc<Self>`
 
 error: item does not constrain `Bar::{opaque#0}`, but has it in its signature
   --> $DIR/arbitrary-self-opaque.rs:7:8
diff --git a/tests/ui/self/arbitrary_self_types_generic_over_receiver.rs b/tests/ui/self/arbitrary_self_types_generic_over_receiver.rs
new file mode 100644
index 0000000000000..8ccda9368addf
--- /dev/null
+++ b/tests/ui/self/arbitrary_self_types_generic_over_receiver.rs
@@ -0,0 +1,23 @@
+#![feature(arbitrary_self_types)]
+
+use std::ops::{Receiver, Deref};
+
+struct Foo(u32);
+impl Foo {
+    fn a(self: impl Receiver<Target=Self>) -> u32 {
+        //~^ ERROR invalid generic `self` parameter type: `impl Receiver<Target = Self>`
+        3
+    }
+    fn b(self: impl Deref<Target=Self>) -> u32 {
+        //~^ ERROR invalid generic `self` parameter type: `impl Deref<Target = Self>`
+        self.0
+    }
+}
+
+fn main() {
+    let foo = Foo(1);
+    foo.a();
+    //~^ ERROR the trait bound
+    foo.b();
+    //~^ ERROR the trait bound
+}
diff --git a/tests/ui/self/arbitrary_self_types_generic_over_receiver.stderr b/tests/ui/self/arbitrary_self_types_generic_over_receiver.stderr
new file mode 100644
index 0000000000000..2da3925341e2f
--- /dev/null
+++ b/tests/ui/self/arbitrary_self_types_generic_over_receiver.stderr
@@ -0,0 +1,59 @@
+error[E0801]: invalid generic `self` parameter type: `impl Receiver<Target = Self>`
+  --> $DIR/arbitrary_self_types_generic_over_receiver.rs:7:16
+   |
+LL |     fn a(self: impl Receiver<Target=Self>) -> u32 {
+   |                ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: type of `self` must not be a method generic parameter type
+   = help: use a concrete type such as `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
+
+error[E0801]: invalid generic `self` parameter type: `impl Deref<Target = Self>`
+  --> $DIR/arbitrary_self_types_generic_over_receiver.rs:11:16
+   |
+LL |     fn b(self: impl Deref<Target=Self>) -> u32 {
+   |                ^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: type of `self` must not be a method generic parameter type
+   = help: use a concrete type such as `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
+
+error[E0277]: the trait bound `Foo: std::ops::Receiver` is not satisfied
+  --> $DIR/arbitrary_self_types_generic_over_receiver.rs:19:9
+   |
+LL |     foo.a();
+   |         ^ the trait `std::ops::Receiver` is not implemented for `Foo`
+   |
+   = note: required for `Foo` to implement `std::ops::Receiver`
+note: required by a bound in `Foo::a`
+  --> $DIR/arbitrary_self_types_generic_over_receiver.rs:7:21
+   |
+LL |     fn a(self: impl Receiver<Target=Self>) -> u32 {
+   |                     ^^^^^^^^^^^^^^^^^^^^^ required by this bound in `Foo::a`
+help: consider borrowing here
+   |
+LL |     &foo.a();
+   |     +
+LL |     &mut foo.a();
+   |     ++++
+
+error[E0277]: the trait bound `Foo: Deref` is not satisfied
+  --> $DIR/arbitrary_self_types_generic_over_receiver.rs:21:9
+   |
+LL |     foo.b();
+   |         ^ the trait `Deref` is not implemented for `Foo`
+   |
+note: required by a bound in `Foo::b`
+  --> $DIR/arbitrary_self_types_generic_over_receiver.rs:11:21
+   |
+LL |     fn b(self: impl Deref<Target=Self>) -> u32 {
+   |                     ^^^^^^^^^^^^^^^^^^ required by this bound in `Foo::b`
+help: consider borrowing here
+   |
+LL |     &foo.b();
+   |     +
+LL |     &mut foo.b();
+   |     ++++
+
+error: aborting due to 4 previous errors
+
+Some errors have detailed explanations: E0277, E0801.
+For more information about an error, try `rustc --explain E0277`.
diff --git a/tests/ui/self/arbitrary_self_types_lifetime_elision.rs b/tests/ui/self/arbitrary_self_types_lifetime_elision.rs
new file mode 100644
index 0000000000000..fd645c1013b64
--- /dev/null
+++ b/tests/ui/self/arbitrary_self_types_lifetime_elision.rs
@@ -0,0 +1,27 @@
+//@ run-pass
+
+#![feature(arbitrary_self_types)]
+
+#[derive(Clone)]
+struct SmartPtr<'a, T: ?Sized>(&'a T);
+
+impl<'a, T: ?Sized> std::ops::Receiver for SmartPtr<'a, T> {
+    type Target = T;
+}
+
+#[derive(Clone)]
+struct MyType;
+
+impl MyType {
+    fn m(self: SmartPtr<Self>) {}
+    fn n(self: SmartPtr<'_, Self>) {}
+    fn o<'a>(self: SmartPtr<'a, Self>) {}
+}
+
+fn main() {
+    let a = MyType;
+    let ptr = SmartPtr(&a);
+    ptr.clone().m();
+    ptr.clone().n();
+    ptr.o();
+}
diff --git a/tests/ui/self/arbitrary_self_types_no_generics.rs b/tests/ui/self/arbitrary_self_types_no_generics.rs
new file mode 100644
index 0000000000000..8a6f57418829e
--- /dev/null
+++ b/tests/ui/self/arbitrary_self_types_no_generics.rs
@@ -0,0 +1,32 @@
+//@ run-pass
+
+#![feature(arbitrary_self_types)]
+
+pub struct A;
+
+impl A {
+    pub fn f(self: B) -> i32 { 1 }
+}
+
+pub struct B(A);
+
+impl core::ops::Receiver for B {
+    type Target = A;
+}
+
+struct C;
+
+struct D;
+
+impl C {
+    fn weird(self: D) -> i32 { 3 }
+}
+
+impl core::ops::Receiver for D {
+    type Target = C;
+}
+
+fn main() {
+    assert_eq!(B(A).f(), 1);
+    assert_eq!(D.weird(), 3);
+}
diff --git a/tests/ui/self/arbitrary_self_types_not_allow_call_with_no_deref.rs b/tests/ui/self/arbitrary_self_types_not_allow_call_with_no_deref.rs
new file mode 100644
index 0000000000000..26e48f69d23ea
--- /dev/null
+++ b/tests/ui/self/arbitrary_self_types_not_allow_call_with_no_deref.rs
@@ -0,0 +1,38 @@
+#![feature(arbitrary_self_types)]
+
+use std::rc::Rc;
+
+struct Foo;
+
+struct CppRef<T>(T);
+
+impl<T> std::ops::Receiver for CppRef<T> {
+    type Target = T;
+}
+
+impl Foo{
+    fn frobnicate_self(self) {}
+    fn frobnicate_ref(&self) {}
+    fn frobnicate_cpp_ref(self: CppRef<Self>) {}
+}
+
+fn main() {
+    let foo_rc = Rc::new(Foo);
+
+    // this compiles fine, and desugars to `Foo::frobnicate_ref(&*foo_rc)`
+    foo_rc.frobnicate_ref();
+
+    let foo_cpp_ref = CppRef(Foo);
+
+    // should not compile because it would desugar to `Foo::frobnicate_ref(&*foo_cpp_ref)`
+    // and you can't deref a CppRef
+    foo_cpp_ref.frobnicate_ref();
+    //~^ ERROR no method named
+
+    foo_cpp_ref.frobnicate_self(); // would desugar to `Foo::frobnicate_self(*foo_cpp_ref)`
+    //~^ ERROR no method named
+
+    // should compile, because we're not dereffing the CppRef
+    // desugars to `Foo::frobnicate_cpp_ref(foo_cpp_ref)`
+    foo_cpp_ref.frobnicate_cpp_ref();
+}
diff --git a/tests/ui/self/arbitrary_self_types_not_allow_call_with_no_deref.stderr b/tests/ui/self/arbitrary_self_types_not_allow_call_with_no_deref.stderr
new file mode 100644
index 0000000000000..4c0ab88493e94
--- /dev/null
+++ b/tests/ui/self/arbitrary_self_types_not_allow_call_with_no_deref.stderr
@@ -0,0 +1,39 @@
+error[E0599]: no method named `frobnicate_ref` found for struct `CppRef` in the current scope
+  --> $DIR/arbitrary_self_types_not_allow_call_with_no_deref.rs:29:17
+   |
+LL | struct CppRef<T>(T);
+   | ---------------- method `frobnicate_ref` not found for this struct
+...
+LL |     foo_cpp_ref.frobnicate_ref();
+   |                 ^^^^^^^^^^^^^^
+   |
+help: one of the expressions' fields has a method of the same name
+   |
+LL |     foo_cpp_ref.0.frobnicate_ref();
+   |                 ++
+help: there is a method `frobnicate_cpp_ref` with a similar name
+   |
+LL |     foo_cpp_ref.frobnicate_cpp_ref();
+   |                 ~~~~~~~~~~~~~~~~~~
+
+error[E0599]: no method named `frobnicate_self` found for struct `CppRef` in the current scope
+  --> $DIR/arbitrary_self_types_not_allow_call_with_no_deref.rs:32:17
+   |
+LL | struct CppRef<T>(T);
+   | ---------------- method `frobnicate_self` not found for this struct
+...
+LL |     foo_cpp_ref.frobnicate_self(); // would desugar to `Foo::frobnicate_self(*foo_cpp_ref)`
+   |                 ^^^^^^^^^^^^^^^
+   |
+help: one of the expressions' fields has a method of the same name
+   |
+LL |     foo_cpp_ref.0.frobnicate_self(); // would desugar to `Foo::frobnicate_self(*foo_cpp_ref)`
+   |                 ++
+help: there is a method `frobnicate_cpp_ref` with a similar name
+   |
+LL |     foo_cpp_ref.frobnicate_cpp_ref(); // would desugar to `Foo::frobnicate_self(*foo_cpp_ref)`
+   |                 ~~~~~~~~~~~~~~~~~~
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0599`.
diff --git a/tests/ui/self/arbitrary_self_types_recursive_receiver.rs b/tests/ui/self/arbitrary_self_types_recursive_receiver.rs
new file mode 100644
index 0000000000000..f3e7f96d7c4de
--- /dev/null
+++ b/tests/ui/self/arbitrary_self_types_recursive_receiver.rs
@@ -0,0 +1,32 @@
+//@ run-pass
+#![feature(arbitrary_self_types)]
+
+struct MyNonNull<T>(*const T);
+
+impl<T> std::ops::Receiver for MyNonNull<T> {
+    type Target = T;
+}
+
+#[allow(dead_code)]
+impl<T> MyNonNull<T> {
+    fn foo<U>(&self) -> *const U {
+        self.cast::<U>().bar()
+    }
+    fn cast<U>(&self) -> MyNonNull<U> {
+        MyNonNull(self.0 as *const U)
+    }
+    fn bar(&self) -> *const T {
+        self.0
+    }
+}
+
+#[repr(transparent)]
+struct Foo(usize);
+#[repr(transparent)]
+struct Bar(usize);
+
+fn main() {
+    let a = Foo(3);
+    let ptr = MyNonNull(&a);
+    let _bar_ptr: *const Bar = ptr.foo();
+}
diff --git a/tests/ui/self/arbitrary_self_types_struct_receiver_trait.rs b/tests/ui/self/arbitrary_self_types_struct_receiver_trait.rs
new file mode 100644
index 0000000000000..cebf0ea7cba93
--- /dev/null
+++ b/tests/ui/self/arbitrary_self_types_struct_receiver_trait.rs
@@ -0,0 +1,31 @@
+//@ run-pass
+#![feature(arbitrary_self_types)]
+
+use std::ops::Receiver;
+
+struct SmartPtr<T>(T);
+
+impl<T> Receiver for SmartPtr<T> {
+    type Target = T;
+}
+
+struct Foo {
+    x: i32,
+    y: i32,
+}
+
+impl Foo {
+    fn x(self: &SmartPtr<Self>) -> i32 {
+        self.0.x
+    }
+
+    fn y(self: SmartPtr<Self>) -> i32 {
+        self.0.y
+    }
+}
+
+fn main() {
+    let foo = SmartPtr(Foo {x: 3, y: 4});
+    assert_eq!(3, foo.x());
+    assert_eq!(4, foo.y());
+}
diff --git a/tests/ui/self/arbitrary_self_types_trait_receiver_trait.rs b/tests/ui/self/arbitrary_self_types_trait_receiver_trait.rs
new file mode 100644
index 0000000000000..9c0402da0befe
--- /dev/null
+++ b/tests/ui/self/arbitrary_self_types_trait_receiver_trait.rs
@@ -0,0 +1,25 @@
+//@ run-pass
+#![feature(arbitrary_self_types)]
+#![allow(unused_allocation)]
+
+struct SmartPtr<T: ?Sized>(T);
+
+impl<T: ?Sized> std::ops::Receiver for SmartPtr<T> {
+    type Target = T;
+}
+
+trait Trait {
+    fn trait_method<'a>(self: &'a Box<SmartPtr<Self>>) -> &'a [i32];
+}
+
+impl Trait for Vec<i32> {
+    fn trait_method<'a>(self: &'a Box<SmartPtr<Self>>) -> &'a [i32] {
+        &(**self).0
+    }
+}
+
+fn main() {
+    let v = vec![1, 2, 3];
+
+    assert_eq!(&[1, 2, 3], Box::new(SmartPtr(v)).trait_method());
+}
diff --git a/tests/ui/span/issue-27522.stderr b/tests/ui/span/issue-27522.stderr
index c57a100bbe227..04904b0ddc1c5 100644
--- a/tests/ui/span/issue-27522.stderr
+++ b/tests/ui/span/issue-27522.stderr
@@ -4,8 +4,8 @@ error[E0307]: invalid `self` parameter type: `&SomeType`
 LL |     fn handler(self: &SomeType);
    |                      ^^^^^^^^^
    |
-   = note: type of `self` must be `Self` or a type that dereferences to it
-   = help: consider changing to `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
+   = note: type of `self` must be `Self` or some type implementing `Receiver`
+   = help: consider changing to `self`, `&self`, `&mut self`, or a type implementing `Receiver` such as `self: Box<Self>`, `self: Rc<Self>`, or `self: Arc<Self>`
 
 error: aborting due to 1 previous error
 
diff --git a/tests/ui/suggestions/dyn-incompatible-trait-should-use-where-sized.stderr b/tests/ui/suggestions/dyn-incompatible-trait-should-use-where-sized.stderr
index beafd7c2ab00f..eb9f9196a7234 100644
--- a/tests/ui/suggestions/dyn-incompatible-trait-should-use-where-sized.stderr
+++ b/tests/ui/suggestions/dyn-incompatible-trait-should-use-where-sized.stderr
@@ -32,8 +32,8 @@ error[E0307]: invalid `self` parameter type: `()`
 LL |     fn bar(self: ()) {}
    |                  ^^
    |
-   = note: type of `self` must be `Self` or a type that dereferences to it
-   = help: consider changing to `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
+   = note: type of `self` must be `Self` or some type implementing `Receiver`
+   = help: consider changing to `self`, `&self`, `&mut self`, or a type implementing `Receiver` such as `self: Box<Self>`, `self: Rc<Self>`, or `self: Arc<Self>`
 
 error: aborting due to 2 previous errors
 
diff --git a/tests/ui/traits/const-traits/effects/auxiliary/minicore.rs b/tests/ui/traits/const-traits/effects/auxiliary/minicore.rs
index aaa61e21155f4..e606d896e93d0 100644
--- a/tests/ui/traits/const-traits/effects/auxiliary/minicore.rs
+++ b/tests/ui/traits/const-traits/effects/auxiliary/minicore.rs
@@ -110,6 +110,16 @@ impl<T: ?Sized> LegacyReceiver for &T {}
 
 impl<T: ?Sized> LegacyReceiver for &mut T {}
 
+#[lang = "receiver"]
+pub trait Receiver {
+    #[lang = "receiver_target"]
+    type Target: ?Sized;
+}
+
+impl<T: Deref + ?Sized> Receiver for T {
+    type Target = <T as Deref>::Target;
+}
+
 #[lang = "destruct"]
 #[const_trait]
 pub trait Destruct {}
diff --git a/tests/ui/traits/issue-78372.stderr b/tests/ui/traits/issue-78372.stderr
index 86234d15a5d4b..1c58915111f2c 100644
--- a/tests/ui/traits/issue-78372.stderr
+++ b/tests/ui/traits/issue-78372.stderr
@@ -61,8 +61,8 @@ error[E0307]: invalid `self` parameter type: `Smaht<Self, T>`
 LL |     fn foo(self: Smaht<Self, T>);
    |                  ^^^^^^^^^^^^^^
    |
-   = note: type of `self` must be `Self` or a type that dereferences to it
-   = help: consider changing to `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
+   = note: type of `self` must be `Self` or some type implementing `Receiver`
+   = help: consider changing to `self`, `&self`, `&mut self`, or a type implementing `Receiver` such as `self: Box<Self>`, `self: Rc<Self>`, or `self: Arc<Self>`
 
 error[E0378]: the trait `DispatchFromDyn` may only be implemented for a coercion between structures
   --> $DIR/issue-78372.rs:3:1
diff --git a/tests/ui/type-alias-impl-trait/method_resolution3.current.stderr b/tests/ui/type-alias-impl-trait/method_resolution3.current.stderr
index 09efd7a9e7e2d..c6aa0e811f36c 100644
--- a/tests/ui/type-alias-impl-trait/method_resolution3.current.stderr
+++ b/tests/ui/type-alias-impl-trait/method_resolution3.current.stderr
@@ -4,8 +4,8 @@ error[E0307]: invalid `self` parameter type: `Bar<u32>`
 LL |     fn bar(self: Bar<u32>) {
    |                  ^^^^^^^^
    |
-   = note: type of `self` must be `Self` or a type that dereferences to it
-   = help: consider changing to `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
+   = note: type of `self` must be `Self` or some type implementing `Receiver`
+   = help: consider changing to `self`, `&self`, `&mut self`, or a type implementing `Receiver` such as `self: Box<Self>`, `self: Rc<Self>`, or `self: Arc<Self>`
 
 error[E0307]: invalid `self` parameter type: `&Bar<u32>`
   --> $DIR/method_resolution3.rs:20:18
@@ -13,8 +13,8 @@ error[E0307]: invalid `self` parameter type: `&Bar<u32>`
 LL |     fn baz(self: &Bar<u32>) {
    |                  ^^^^^^^^^
    |
-   = note: type of `self` must be `Self` or a type that dereferences to it
-   = help: consider changing to `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
+   = note: type of `self` must be `Self` or some type implementing `Receiver`
+   = help: consider changing to `self`, `&self`, `&mut self`, or a type implementing `Receiver` such as `self: Box<Self>`, `self: Rc<Self>`, or `self: Arc<Self>`
 
 error: aborting due to 2 previous errors
 
diff --git a/tests/ui/type-alias-impl-trait/method_resolution3.next.stderr b/tests/ui/type-alias-impl-trait/method_resolution3.next.stderr
index 09efd7a9e7e2d..c6aa0e811f36c 100644
--- a/tests/ui/type-alias-impl-trait/method_resolution3.next.stderr
+++ b/tests/ui/type-alias-impl-trait/method_resolution3.next.stderr
@@ -4,8 +4,8 @@ error[E0307]: invalid `self` parameter type: `Bar<u32>`
 LL |     fn bar(self: Bar<u32>) {
    |                  ^^^^^^^^
    |
-   = note: type of `self` must be `Self` or a type that dereferences to it
-   = help: consider changing to `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
+   = note: type of `self` must be `Self` or some type implementing `Receiver`
+   = help: consider changing to `self`, `&self`, `&mut self`, or a type implementing `Receiver` such as `self: Box<Self>`, `self: Rc<Self>`, or `self: Arc<Self>`
 
 error[E0307]: invalid `self` parameter type: `&Bar<u32>`
   --> $DIR/method_resolution3.rs:20:18
@@ -13,8 +13,8 @@ error[E0307]: invalid `self` parameter type: `&Bar<u32>`
 LL |     fn baz(self: &Bar<u32>) {
    |                  ^^^^^^^^^
    |
-   = note: type of `self` must be `Self` or a type that dereferences to it
-   = help: consider changing to `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
+   = note: type of `self` must be `Self` or some type implementing `Receiver`
+   = help: consider changing to `self`, `&self`, `&mut self`, or a type implementing `Receiver` such as `self: Box<Self>`, `self: Rc<Self>`, or `self: Arc<Self>`
 
 error: aborting due to 2 previous errors
 
diff --git a/tests/ui/type-alias-impl-trait/method_resolution4.current.stderr b/tests/ui/type-alias-impl-trait/method_resolution4.current.stderr
index 8ffdb21f2517b..e4c4d121733b6 100644
--- a/tests/ui/type-alias-impl-trait/method_resolution4.current.stderr
+++ b/tests/ui/type-alias-impl-trait/method_resolution4.current.stderr
@@ -4,8 +4,8 @@ error[E0307]: invalid `self` parameter type: `Bar<Foo>`
 LL |     fn foo(self: Bar<Foo>) {
    |                  ^^^^^^^^
    |
-   = note: type of `self` must be `Self` or a type that dereferences to it
-   = help: consider changing to `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
+   = note: type of `self` must be `Self` or some type implementing `Receiver`
+   = help: consider changing to `self`, `&self`, `&mut self`, or a type implementing `Receiver` such as `self: Box<Self>`, `self: Rc<Self>`, or `self: Arc<Self>`
 
 error[E0307]: invalid `self` parameter type: `&Bar<Foo>`
   --> $DIR/method_resolution4.rs:31:20
@@ -13,8 +13,8 @@ error[E0307]: invalid `self` parameter type: `&Bar<Foo>`
 LL |     fn foomp(self: &Bar<Foo>) {
    |                    ^^^^^^^^^
    |
-   = note: type of `self` must be `Self` or a type that dereferences to it
-   = help: consider changing to `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
+   = note: type of `self` must be `Self` or some type implementing `Receiver`
+   = help: consider changing to `self`, `&self`, `&mut self`, or a type implementing `Receiver` such as `self: Box<Self>`, `self: Rc<Self>`, or `self: Arc<Self>`
 
 error: aborting due to 2 previous errors
 
diff --git a/tests/ui/type-alias-impl-trait/method_resolution4.next.stderr b/tests/ui/type-alias-impl-trait/method_resolution4.next.stderr
index 8ffdb21f2517b..e4c4d121733b6 100644
--- a/tests/ui/type-alias-impl-trait/method_resolution4.next.stderr
+++ b/tests/ui/type-alias-impl-trait/method_resolution4.next.stderr
@@ -4,8 +4,8 @@ error[E0307]: invalid `self` parameter type: `Bar<Foo>`
 LL |     fn foo(self: Bar<Foo>) {
    |                  ^^^^^^^^
    |
-   = note: type of `self` must be `Self` or a type that dereferences to it
-   = help: consider changing to `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
+   = note: type of `self` must be `Self` or some type implementing `Receiver`
+   = help: consider changing to `self`, `&self`, `&mut self`, or a type implementing `Receiver` such as `self: Box<Self>`, `self: Rc<Self>`, or `self: Arc<Self>`
 
 error[E0307]: invalid `self` parameter type: `&Bar<Foo>`
   --> $DIR/method_resolution4.rs:31:20
@@ -13,8 +13,8 @@ error[E0307]: invalid `self` parameter type: `&Bar<Foo>`
 LL |     fn foomp(self: &Bar<Foo>) {
    |                    ^^^^^^^^^
    |
-   = note: type of `self` must be `Self` or a type that dereferences to it
-   = help: consider changing to `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
+   = note: type of `self` must be `Self` or some type implementing `Receiver`
+   = help: consider changing to `self`, `&self`, `&mut self`, or a type implementing `Receiver` such as `self: Box<Self>`, `self: Rc<Self>`, or `self: Arc<Self>`
 
 error: aborting due to 2 previous errors
 
diff --git a/tests/ui/ufcs/ufcs-explicit-self-bad.stderr b/tests/ui/ufcs/ufcs-explicit-self-bad.stderr
index 2a8c4edbdb5f3..36bdc714e050b 100644
--- a/tests/ui/ufcs/ufcs-explicit-self-bad.stderr
+++ b/tests/ui/ufcs/ufcs-explicit-self-bad.stderr
@@ -22,8 +22,8 @@ error[E0307]: invalid `self` parameter type: `isize`
 LL |     fn foo(self: isize, x: isize) -> isize {
    |                  ^^^^^
    |
-   = note: type of `self` must be `Self` or a type that dereferences to it
-   = help: consider changing to `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
+   = note: type of `self` must be `Self` or some type implementing `Receiver`
+   = help: consider changing to `self`, `&self`, `&mut self`, or a type implementing `Receiver` such as `self: Box<Self>`, `self: Rc<Self>`, or `self: Arc<Self>`
 
 error[E0307]: invalid `self` parameter type: `Bar<isize>`
   --> $DIR/ufcs-explicit-self-bad.rs:19:18
@@ -31,8 +31,8 @@ error[E0307]: invalid `self` parameter type: `Bar<isize>`
 LL |     fn foo(self: Bar<isize>, x: isize) -> isize {
    |                  ^^^^^^^^^^
    |
-   = note: type of `self` must be `Self` or a type that dereferences to it
-   = help: consider changing to `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
+   = note: type of `self` must be `Self` or some type implementing `Receiver`
+   = help: consider changing to `self`, `&self`, `&mut self`, or a type implementing `Receiver` such as `self: Box<Self>`, `self: Rc<Self>`, or `self: Arc<Self>`
 
 error[E0307]: invalid `self` parameter type: `&Bar<usize>`
   --> $DIR/ufcs-explicit-self-bad.rs:23:18
@@ -40,8 +40,8 @@ error[E0307]: invalid `self` parameter type: `&Bar<usize>`
 LL |     fn bar(self: &Bar<usize>, x: isize) -> isize {
    |                  ^^^^^^^^^^^
    |
-   = note: type of `self` must be `Self` or a type that dereferences to it
-   = help: consider changing to `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
+   = note: type of `self` must be `Self` or some type implementing `Receiver`
+   = help: consider changing to `self`, `&self`, `&mut self`, or a type implementing `Receiver` such as `self: Box<Self>`, `self: Rc<Self>`, or `self: Arc<Self>`
 
 error[E0308]: mismatched `self` parameter type
   --> $DIR/ufcs-explicit-self-bad.rs:37:21

From 7f7c964e475d36789de0ce1473f696b8b1fc5434 Mon Sep 17 00:00:00 2001
From: Adrian Taylor <adetaylor@chromium.org>
Date: Mon, 11 Nov 2024 16:37:56 +0000
Subject: [PATCH 2/6] Arbitrary self types v2: pick diags to stack.

This commit makes no (intentional) functional change.

Previously, the picking process maintained two lists of extra
information useful for diagnostics:

* any unstable candidates which might have been picked
* any unsatisfied predicates

Previously, these were dealt with quite differently - the former list
was passed around as a function parameter; the latter lived in a RefCell
in the ProbeCtxt.

With this change we increase consistency by keeping them together in a
new PickDiagHints structure, passed as a parameter, with no need for
interior mutability.

The lifecycle of each of these lists remains fairly complex, so it's
explained with new comments in pick_core.

A further cleanup here would be to package the widely-used tuple
  (ty::Predicate<'tcx>,
   Option<ty::Predicate<'tcx>>,
   Option<ObligationCause<'tcx>>)
into a named struct for UnsatisfiedPredicate. This seems worth doing but
it turns out that this tuple is used in dozens of places, so if we're
going to do this we should do it as a separate PR to avoid constant
rebase trouble.
---
 compiler/rustc_hir_typeck/src/method/probe.rs | 162 +++++++++---------
 1 file changed, 85 insertions(+), 77 deletions(-)

diff --git a/compiler/rustc_hir_typeck/src/method/probe.rs b/compiler/rustc_hir_typeck/src/method/probe.rs
index 91e65af8df954..c92f00a54dd2b 100644
--- a/compiler/rustc_hir_typeck/src/method/probe.rs
+++ b/compiler/rustc_hir_typeck/src/method/probe.rs
@@ -79,12 +79,6 @@ pub(crate) struct ProbeContext<'a, 'tcx> {
     /// used for error reporting
     static_candidates: RefCell<Vec<CandidateSource>>,
 
-    /// Collects near misses when trait bounds for type parameters are unsatisfied and is only used
-    /// for error reporting
-    unsatisfied_predicates: RefCell<
-        Vec<(ty::Predicate<'tcx>, Option<ty::Predicate<'tcx>>, Option<ObligationCause<'tcx>>)>,
-    >,
-
     scope_expr_id: HirId,
 
     /// Is this probe being done for a diagnostic? This will skip some error reporting
@@ -162,6 +156,21 @@ impl AutorefOrPtrAdjustment {
     }
 }
 
+/// Extra information required only for error reporting.
+#[derive(Debug)]
+struct PickDiagHints<'a, 'tcx> {
+    /// Unstable candidates alongside the stable ones.
+    unstable_candidates: Option<Vec<(Candidate<'tcx>, Symbol)>>,
+
+    /// Collects near misses when trait bounds for type parameters are unsatisfied and is only used
+    /// for error reporting
+    unsatisfied_predicates: &'a mut Vec<(
+        ty::Predicate<'tcx>,
+        Option<ty::Predicate<'tcx>>,
+        Option<ObligationCause<'tcx>>,
+    )>,
+}
+
 #[derive(Debug, Clone)]
 pub(crate) struct Pick<'tcx> {
     pub item: ty::AssocItem,
@@ -647,7 +656,6 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
             private_candidates: Vec::new(),
             private_candidate: Cell::new(None),
             static_candidates: RefCell::new(Vec::new()),
-            unsatisfied_predicates: RefCell::new(Vec::new()),
             scope_expr_id,
             is_suggestion,
         }
@@ -660,7 +668,6 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
         self.private_candidates.clear();
         self.private_candidate.set(None);
         self.static_candidates.borrow_mut().clear();
-        self.unsatisfied_predicates.borrow_mut().clear();
     }
 
     /// When we're looking up a method by path (UFCS), we relate the receiver
@@ -1036,7 +1043,9 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
     fn pick(mut self) -> PickResult<'tcx> {
         assert!(self.method_name.is_some());
 
-        if let Some(r) = self.pick_core() {
+        let mut unsatisfied_predicates = Vec::new();
+
+        if let Some(r) = self.pick_core(&mut unsatisfied_predicates) {
             return r;
         }
 
@@ -1056,7 +1065,6 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
 
         let static_candidates = std::mem::take(self.static_candidates.get_mut());
         let private_candidate = self.private_candidate.take();
-        let unsatisfied_predicates = std::mem::take(self.unsatisfied_predicates.get_mut());
 
         // things failed, so lets look at all traits, for diagnostic purposes now:
         self.reset();
@@ -1066,7 +1074,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
 
         self.assemble_extension_candidates_for_all_traits();
 
-        let out_of_scope_traits = match self.pick_core() {
+        let out_of_scope_traits = match self.pick_core(&mut Vec::new()) {
             Some(Ok(p)) => vec![p.item.container_id(self.tcx)],
             Some(Err(MethodError::Ambiguity(v))) => v
                 .into_iter()
@@ -1101,14 +1109,40 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
         }))
     }
 
-    fn pick_core(&self) -> Option<PickResult<'tcx>> {
+    fn pick_core(
+        &self,
+        unsatisfied_predicates: &mut Vec<(
+            ty::Predicate<'tcx>,
+            Option<ty::Predicate<'tcx>>,
+            Option<ObligationCause<'tcx>>,
+        )>,
+    ) -> Option<PickResult<'tcx>> {
         // Pick stable methods only first, and consider unstable candidates if not found.
-        self.pick_all_method(Some(&mut vec![])).or_else(|| self.pick_all_method(None))
+        self.pick_all_method(&mut PickDiagHints {
+            // This first cycle, maintain a list of unstable candidates which
+            // we encounter. This will end up in the Pick for diagnostics.
+            unstable_candidates: Some(Vec::new()),
+            // Contribute to the list of unsatisfied predicates which may
+            // also be used for diagnostics.
+            unsatisfied_predicates,
+        })
+        .or_else(|| {
+            self.pick_all_method(&mut PickDiagHints {
+                // On the second search, don't provide a special list of unstable
+                // candidates. This indicates to the picking code that it should
+                // in fact include such unstable candidates in the actual
+                // search.
+                unstable_candidates: None,
+                // And there's no need to duplicate ourselves in the
+                // unsatisifed predicates list. Provide a throwaway list.
+                unsatisfied_predicates: &mut Vec::new(),
+            })
+        })
     }
 
-    fn pick_all_method(
+    fn pick_all_method<'b>(
         &self,
-        mut unstable_candidates: Option<&mut Vec<(Candidate<'tcx>, Symbol)>>,
+        pick_diag_hints: &mut PickDiagHints<'b, 'tcx>,
     ) -> Option<PickResult<'tcx>> {
         self.steps
             .iter()
@@ -1133,37 +1167,19 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
                     .unwrap_or_else(|_| {
                         span_bug!(self.span, "{:?} was applicable but now isn't?", step.self_ty)
                     });
-                self.pick_by_value_method(step, self_ty, unstable_candidates.as_deref_mut())
-                    .or_else(|| {
-                        self.pick_autorefd_method(
-                            step,
-                            self_ty,
-                            hir::Mutability::Not,
-                            unstable_candidates.as_deref_mut(),
-                        )
+                self.pick_by_value_method(step, self_ty, pick_diag_hints).or_else(|| {
+                    self.pick_autorefd_method(step, self_ty, hir::Mutability::Not, pick_diag_hints)
                         .or_else(|| {
                             self.pick_autorefd_method(
                                 step,
                                 self_ty,
                                 hir::Mutability::Mut,
-                                unstable_candidates.as_deref_mut(),
+                                pick_diag_hints,
                             )
                         })
-                        .or_else(|| {
-                            self.pick_const_ptr_method(
-                                step,
-                                self_ty,
-                                unstable_candidates.as_deref_mut(),
-                            )
-                        })
-                        .or_else(|| {
-                            self.pick_reborrow_pin_method(
-                                step,
-                                self_ty,
-                                unstable_candidates.as_deref_mut(),
-                            )
-                        })
-                    })
+                        .or_else(|| self.pick_const_ptr_method(step, self_ty, pick_diag_hints))
+                        .or_else(|| self.pick_reborrow_pin_method(step, self_ty, pick_diag_hints))
+                })
             })
     }
 
@@ -1177,13 +1193,13 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
         &self,
         step: &CandidateStep<'tcx>,
         self_ty: Ty<'tcx>,
-        unstable_candidates: Option<&mut Vec<(Candidate<'tcx>, Symbol)>>,
+        pick_diag_hints: &mut PickDiagHints<'_, 'tcx>,
     ) -> Option<PickResult<'tcx>> {
         if step.unsize {
             return None;
         }
 
-        self.pick_method(self_ty, unstable_candidates).map(|r| {
+        self.pick_method(self_ty, pick_diag_hints).map(|r| {
             r.map(|mut pick| {
                 pick.autoderefs = step.autoderefs;
 
@@ -1221,7 +1237,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
         step: &CandidateStep<'tcx>,
         self_ty: Ty<'tcx>,
         mutbl: hir::Mutability,
-        unstable_candidates: Option<&mut Vec<(Candidate<'tcx>, Symbol)>>,
+        pick_diag_hints: &mut PickDiagHints<'_, 'tcx>,
     ) -> Option<PickResult<'tcx>> {
         let tcx = self.tcx;
 
@@ -1229,7 +1245,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
         let region = tcx.lifetimes.re_erased;
 
         let autoref_ty = Ty::new_ref(tcx, region, self_ty, mutbl);
-        self.pick_method(autoref_ty, unstable_candidates).map(|r| {
+        self.pick_method(autoref_ty, pick_diag_hints).map(|r| {
             r.map(|mut pick| {
                 pick.autoderefs = step.autoderefs;
                 pick.autoref_or_ptr_adjustment =
@@ -1240,12 +1256,12 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
     }
 
     /// Looks for applicable methods if we reborrow a `Pin<&mut T>` as a `Pin<&T>`.
-    #[instrument(level = "debug", skip(self, step, unstable_candidates))]
+    #[instrument(level = "debug", skip(self, step, pick_diag_hints))]
     fn pick_reborrow_pin_method(
         &self,
         step: &CandidateStep<'tcx>,
         self_ty: Ty<'tcx>,
-        unstable_candidates: Option<&mut Vec<(Candidate<'tcx>, Symbol)>>,
+        pick_diag_hints: &mut PickDiagHints<'_, 'tcx>,
     ) -> Option<PickResult<'tcx>> {
         if !self.tcx.features().pin_ergonomics() {
             return None;
@@ -1266,7 +1282,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
 
         let region = self.tcx.lifetimes.re_erased;
         let autopin_ty = Ty::new_pinned_ref(self.tcx, region, inner_ty, hir::Mutability::Not);
-        self.pick_method(autopin_ty, unstable_candidates).map(|r| {
+        self.pick_method(autopin_ty, pick_diag_hints).map(|r| {
             r.map(|mut pick| {
                 pick.autoderefs = step.autoderefs;
                 pick.autoref_or_ptr_adjustment =
@@ -1283,7 +1299,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
         &self,
         step: &CandidateStep<'tcx>,
         self_ty: Ty<'tcx>,
-        unstable_candidates: Option<&mut Vec<(Candidate<'tcx>, Symbol)>>,
+        pick_diag_hints: &mut PickDiagHints<'_, 'tcx>,
     ) -> Option<PickResult<'tcx>> {
         // Don't convert an unsized reference to ptr
         if step.unsize {
@@ -1295,7 +1311,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
         };
 
         let const_ptr_ty = Ty::new_imm_ptr(self.tcx, ty);
-        self.pick_method(const_ptr_ty, unstable_candidates).map(|r| {
+        self.pick_method(const_ptr_ty, pick_diag_hints).map(|r| {
             r.map(|mut pick| {
                 pick.autoderefs = step.autoderefs;
                 pick.autoref_or_ptr_adjustment = Some(AutorefOrPtrAdjustment::ToConstPtr);
@@ -1307,22 +1323,15 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
     fn pick_method(
         &self,
         self_ty: Ty<'tcx>,
-        mut unstable_candidates: Option<&mut Vec<(Candidate<'tcx>, Symbol)>>,
+        pick_diag_hints: &mut PickDiagHints<'_, 'tcx>,
     ) -> Option<PickResult<'tcx>> {
         debug!("pick_method(self_ty={})", self.ty_to_string(self_ty));
 
-        let mut possibly_unsatisfied_predicates = Vec::new();
-
         for (kind, candidates) in
             [("inherent", &self.inherent_candidates), ("extension", &self.extension_candidates)]
         {
             debug!("searching {} candidates", kind);
-            let res = self.consider_candidates(
-                self_ty,
-                candidates,
-                &mut possibly_unsatisfied_predicates,
-                unstable_candidates.as_deref_mut(),
-            );
+            let res = self.consider_candidates(self_ty, candidates, pick_diag_hints);
             if let Some(pick) = res {
                 return Some(pick);
             }
@@ -1330,17 +1339,14 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
 
         if self.private_candidate.get().is_none() {
             if let Some(Ok(pick)) =
-                self.consider_candidates(self_ty, &self.private_candidates, &mut vec![], None)
+                self.consider_candidates(self_ty, &self.private_candidates, &mut PickDiagHints {
+                    unstable_candidates: None,
+                    unsatisfied_predicates: &mut vec![],
+                })
             {
                 self.private_candidate.set(Some((pick.item.kind.as_def_kind(), pick.item.def_id)));
             }
         }
-
-        // `pick_method` may be called twice for the same self_ty if no stable methods
-        // match. Only extend once.
-        if unstable_candidates.is_some() {
-            self.unsatisfied_predicates.borrow_mut().extend(possibly_unsatisfied_predicates);
-        }
         None
     }
 
@@ -1348,17 +1354,19 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
         &self,
         self_ty: Ty<'tcx>,
         candidates: &[Candidate<'tcx>],
-        possibly_unsatisfied_predicates: &mut Vec<(
-            ty::Predicate<'tcx>,
-            Option<ty::Predicate<'tcx>>,
-            Option<ObligationCause<'tcx>>,
-        )>,
-        mut unstable_candidates: Option<&mut Vec<(Candidate<'tcx>, Symbol)>>,
+        pick_diag_hints: &mut PickDiagHints<'_, 'tcx>,
     ) -> Option<PickResult<'tcx>> {
         let mut applicable_candidates: Vec<_> = candidates
             .iter()
             .map(|probe| {
-                (probe, self.consider_probe(self_ty, probe, possibly_unsatisfied_predicates))
+                (
+                    probe,
+                    self.consider_probe(
+                        self_ty,
+                        probe,
+                        &mut pick_diag_hints.unsatisfied_predicates,
+                    ),
+                )
             })
             .filter(|&(_, status)| status != ProbeResult::NoMatch)
             .collect();
@@ -1373,7 +1381,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
             }
         }
 
-        if let Some(uc) = &mut unstable_candidates {
+        if let Some(uc) = &mut pick_diag_hints.unstable_candidates {
             applicable_candidates.retain(|&(candidate, _)| {
                 if let stability::EvalResult::Deny { feature, .. } =
                     self.tcx.eval_stability(candidate.item.def_id, None, self.span, None)
@@ -1391,10 +1399,10 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
         }
 
         applicable_candidates.pop().map(|(probe, status)| match status {
-            ProbeResult::Match => {
-                Ok(probe
-                    .to_unadjusted_pick(self_ty, unstable_candidates.cloned().unwrap_or_default()))
-            }
+            ProbeResult::Match => Ok(probe.to_unadjusted_pick(
+                self_ty,
+                pick_diag_hints.unstable_candidates.clone().unwrap_or_default(),
+            )),
             ProbeResult::NoMatch | ProbeResult::BadReturnType => Err(MethodError::BadReturnType),
         })
     }
@@ -1859,7 +1867,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
                     pcx.method_name = Some(method_name);
                     pcx.assemble_inherent_candidates();
                     pcx.assemble_extension_candidates_for_all_traits();
-                    pcx.pick_core().and_then(|pick| pick.ok()).map(|pick| pick.item)
+                    pcx.pick_core(&mut Vec::new()).and_then(|pick| pick.ok()).map(|pick| pick.item)
                 })
                 .collect();
 

From 2707f5578da273284770298be9202c0cdd6621b3 Mon Sep 17 00:00:00 2001
From: Adrian Taylor <adetaylor@chromium.org>
Date: Fri, 12 Jul 2024 14:26:35 +0000
Subject: [PATCH 3/6] Arbitrary self types v2: deshadowing probe

This is the first part of a series of commits which impact the
"deshadowing detection" in the arbitrary self types v2 RFC.

This commit should not have any functional changes, but may impact
performance. Subsequent commits add back the performance, and add error
checking to this new code such that it has a functional effect.

Rust prioritizes method candidates in this order:
1. By value;
2. By reference;
3. By mutable reference;
4. By const ptr.
5. By reborrowed pin.

Previously, if a suitable candidate was found in one of these earlier
categories, Rust wouldn't even move onto probing the other categories.

As part of the arbitrary self types work, we're going to need to change
that - even if we choose a method from one of the earlier categories, we
will sometimes need to probe later categories to search for methods that
we may be shadowing.

This commit adds those extra searches for shadowing, but it does not yet
produce an error when such shadowing problems are found. That will come
in a subsequent commit, by filling out the 'check_for_shadowing'
method.

This commit contains a naive approach to detecting these shadowing
problems, which shows what we've functionally looking to do. However,
it's too slow. The performance of this approach was explored in this
PR:
https://github.com/rust-lang/rust/pull/127812#issuecomment-2236911900
Subsequent commits will improve the speed of the search.
---
 compiler/rustc_hir_typeck/src/method/probe.rs | 124 ++++++++++++++++--
 1 file changed, 114 insertions(+), 10 deletions(-)

diff --git a/compiler/rustc_hir_typeck/src/method/probe.rs b/compiler/rustc_hir_typeck/src/method/probe.rs
index c92f00a54dd2b..66ff71f729cbf 100644
--- a/compiler/rustc_hir_typeck/src/method/probe.rs
+++ b/compiler/rustc_hir_typeck/src/method/probe.rs
@@ -1144,6 +1144,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
         &self,
         pick_diag_hints: &mut PickDiagHints<'b, 'tcx>,
     ) -> Option<PickResult<'tcx>> {
+        let track_unstable_candidates = pick_diag_hints.unstable_candidates.is_some();
         self.steps
             .iter()
             // At this point we're considering the types to which the receiver can be converted,
@@ -1167,22 +1168,125 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
                     .unwrap_or_else(|_| {
                         span_bug!(self.span, "{:?} was applicable but now isn't?", step.self_ty)
                     });
-                self.pick_by_value_method(step, self_ty, pick_diag_hints).or_else(|| {
-                    self.pick_autorefd_method(step, self_ty, hir::Mutability::Not, pick_diag_hints)
-                        .or_else(|| {
-                            self.pick_autorefd_method(
+
+                let by_value_pick = self.pick_by_value_method(step, self_ty, pick_diag_hints);
+
+                // Check for shadowing of a by-reference method by a by-value method (see comments on check_for_shadowing)
+                if let Some(by_value_pick) = by_value_pick {
+                    if let Ok(by_value_pick) = by_value_pick.as_ref() {
+                        if by_value_pick.kind == PickKind::InherentImplPick {
+                            if let Err(e) = self.check_for_shadowed_autorefd_method(
+                                by_value_pick,
+                                step,
+                                self_ty,
+                                hir::Mutability::Not,
+                                track_unstable_candidates,
+                            ) {
+                                return Some(Err(e));
+                            }
+                            if let Err(e) = self.check_for_shadowed_autorefd_method(
+                                by_value_pick,
                                 step,
                                 self_ty,
                                 hir::Mutability::Mut,
-                                pick_diag_hints,
-                            )
-                        })
-                        .or_else(|| self.pick_const_ptr_method(step, self_ty, pick_diag_hints))
-                        .or_else(|| self.pick_reborrow_pin_method(step, self_ty, pick_diag_hints))
-                })
+                                track_unstable_candidates,
+                            ) {
+                                return Some(Err(e));
+                            }
+                        }
+                    }
+                    return Some(by_value_pick);
+                }
+
+                let autoref_pick =
+                    self.pick_autorefd_method(step, self_ty, hir::Mutability::Not, pick_diag_hints);
+                // Check for shadowing of a by-mut-ref method by a by-reference method (see comments on check_for_shadowing)
+                if let Some(autoref_pick) = autoref_pick {
+                    if let Ok(autoref_pick) = autoref_pick.as_ref() {
+                        // Check we're not shadowing others
+                        if autoref_pick.kind == PickKind::InherentImplPick {
+                            if let Err(e) = self.check_for_shadowed_autorefd_method(
+                                autoref_pick,
+                                step,
+                                self_ty,
+                                hir::Mutability::Mut,
+                                track_unstable_candidates,
+                            ) {
+                                return Some(Err(e));
+                            }
+                        }
+                    }
+                    return Some(autoref_pick);
+                }
+
+                // Note that no shadowing errors are produced from here on,
+                // as we consider const ptr methods.
+                // We allow new methods that take *mut T to shadow
+                // methods which took *const T, so there is no entry in
+                // this list for the results of `pick_const_ptr_method`.
+                // The reason is that the standard pointer cast method
+                // (on a mutable pointer) always already shadows the
+                // cast method (on a const pointer). So, if we added
+                // `pick_const_ptr_method` to this method, the anti-
+                // shadowing algorithm would always complain about
+                // the conflict between *const::cast and *mut::cast.
+                // In practice therefore this does constrain us:
+                // we cannot add new
+                //   self: *mut Self
+                // methods to types such as NonNull or anything else
+                // which implements Receiver, because this might in future
+                // shadow existing methods taking
+                //   self: *const NonNull<Self>
+                // in the pointee. In practice, methods taking raw pointers
+                // are rare, and it seems that it should be easily possible
+                // to avoid such compatibility breaks.
+                // We also don't check for reborrowed pin methods which
+                // may be shadowed; these also seem unlikely to occur.
+                self.pick_autorefd_method(step, self_ty, hir::Mutability::Mut, pick_diag_hints)
+                    .or_else(|| self.pick_const_ptr_method(step, self_ty, pick_diag_hints))
+                    .or_else(|| self.pick_reborrow_pin_method(step, self_ty, pick_diag_hints))
             })
     }
 
+    /// Check for cases where arbitrary self types allows shadowing
+    /// of methods that might be a compatibility break. Specifically,
+    /// we have something like:
+    /// ```ignore (illustrative)
+    /// struct A;
+    /// impl A {
+    ///   fn foo(self: &NonNull<A>) {}
+    ///      // note this is by reference
+    /// }
+    /// ```
+    /// then we've come along and added this method to `NonNull`:
+    /// ```ignore (illustrative)
+    ///   fn foo(self)  // note this is by value
+    /// ```
+    /// Report an error in this case.
+    fn check_for_shadowed_autorefd_method(
+        &self,
+        _possible_shadower: &Pick<'tcx>,
+        step: &CandidateStep<'tcx>,
+        self_ty: Ty<'tcx>,
+        mutbl: hir::Mutability,
+        track_unstable_candidates: bool,
+    ) -> Result<(), MethodError<'tcx>> {
+        // We don't want to remember any of the diagnostic hints from this
+        // shadow search, but we do need to provide Some/None for the
+        // unstable_candidates in order to reflect the behavior of the
+        // main search.
+        let mut pick_diag_hints = PickDiagHints {
+            unstable_candidates: if track_unstable_candidates { Some(Vec::new()) } else { None },
+            unsatisfied_predicates: &mut Vec::new(),
+        };
+        let _potentially_shadowed_pick =
+            self.pick_autorefd_method(step, self_ty, mutbl, &mut pick_diag_hints);
+
+        // At the moment, this function does no checks. A future
+        // commit will fill out the body here.
+        Ok(())
+    }
+
     /// For each type `T` in the step list, this attempts to find a method where
     /// the (transformed) self type is exactly `T`. We do however do one
     /// transformation on the adjustment: if we are passing a region pointer in,

From 48e1df87e1422a845f05a8f6c5fc4773758cc9d9 Mon Sep 17 00:00:00 2001
From: Adrian Taylor <adetaylor@chromium.org>
Date: Mon, 11 Nov 2024 13:47:51 +0000
Subject: [PATCH 4/6] Arbitrary self types v2: deshadow quicker
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

A previous commit added a search for certain types of "shadowing"
situation where one method (in an outer smart pointer type, typically)
might hide or shadow the method in the pointee.

Performance investigation showed that the naïve approach is too slow -
this commit speeds it up, while being functionally the same.

This still does not actually cause the deshadowing check to emit any
errors; that comes in a subsequent commit which is where all the tests
live.
---
 compiler/rustc_hir_typeck/src/demand.rs       |   2 +-
 compiler/rustc_hir_typeck/src/method/probe.rs | 210 +++++++++++++++---
 2 files changed, 174 insertions(+), 38 deletions(-)

diff --git a/compiler/rustc_hir_typeck/src/demand.rs b/compiler/rustc_hir_typeck/src/demand.rs
index 3399a9fe88078..d0272651c0809 100644
--- a/compiler/rustc_hir_typeck/src/demand.rs
+++ b/compiler/rustc_hir_typeck/src/demand.rs
@@ -917,7 +917,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             [candidate] => format!(
                 "the method of the same name on {} `{}`",
                 match candidate.kind {
-                    probe::CandidateKind::InherentImplCandidate(_) => "the inherent impl for",
+                    probe::CandidateKind::InherentImplCandidate { .. } => "the inherent impl for",
                     _ => "trait",
                 },
                 self.tcx.def_path_str(candidate.item.container_id(self.tcx))
diff --git a/compiler/rustc_hir_typeck/src/method/probe.rs b/compiler/rustc_hir_typeck/src/method/probe.rs
index 66ff71f729cbf..7524a1faaa394 100644
--- a/compiler/rustc_hir_typeck/src/method/probe.rs
+++ b/compiler/rustc_hir_typeck/src/method/probe.rs
@@ -103,7 +103,7 @@ pub(crate) struct Candidate<'tcx> {
 
 #[derive(Debug, Clone)]
 pub(crate) enum CandidateKind<'tcx> {
-    InherentImplCandidate(DefId),
+    InherentImplCandidate { impl_def_id: DefId, receiver_steps: usize },
     ObjectCandidate(ty::PolyTraitRef<'tcx>),
     TraitCandidate(ty::PolyTraitRef<'tcx>),
     WhereClauseCandidate(ty::PolyTraitRef<'tcx>),
@@ -171,6 +171,37 @@ struct PickDiagHints<'a, 'tcx> {
     )>,
 }
 
+/// Criteria to apply when searching for a given Pick. This is used during
+/// the search for potentially shadowed methods to ensure we don't search
+/// more candidates than strictly necessary.
+#[derive(Debug)]
+struct PickConstraintsForShadowed {
+    autoderefs: usize,
+    receiver_steps: Option<usize>,
+    def_id: DefId,
+}
+
+impl PickConstraintsForShadowed {
+    fn may_shadow_based_on_autoderefs(&self, autoderefs: usize) -> bool {
+        autoderefs == self.autoderefs
+    }
+
+    fn candidate_may_shadow(&self, candidate: &Candidate<'_>) -> bool {
+        // An item never shadows itself
+        candidate.item.def_id != self.def_id
+            // and we're only concerned about inherent impls doing the shadowing.
+            // Shadowing can only occur if the shadowed is further along
+            // the Receiver dereferencing chain than the shadowed.
+            && match candidate.kind {
+                CandidateKind::InherentImplCandidate { receiver_steps, .. } => match self.receiver_steps {
+                    Some(shadowed_receiver_steps) => receiver_steps > shadowed_receiver_steps,
+                    _ => false
+                },
+                _ => false
+            }
+    }
+}
+
 #[derive(Debug, Clone)]
 pub(crate) struct Pick<'tcx> {
     pub item: ty::AssocItem,
@@ -190,6 +221,11 @@ pub(crate) struct Pick<'tcx> {
 
     /// Unstable candidates alongside the stable ones.
     unstable_candidates: Vec<(Candidate<'tcx>, Symbol)>,
+
+    /// Number of jumps along the `Receiver::Target` chain we followed
+    /// to identify this method. Used only for deshadowing errors.
+    /// Only applies for inherent impls.
+    pub receiver_steps: Option<usize>,
 }
 
 #[derive(Clone, Debug, PartialEq, Eq)]
@@ -706,12 +742,16 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
 
     fn assemble_inherent_candidates(&mut self) {
         for step in self.steps.iter() {
-            self.assemble_probe(&step.self_ty);
+            self.assemble_probe(&step.self_ty, step.autoderefs);
         }
     }
 
     #[instrument(level = "debug", skip(self))]
-    fn assemble_probe(&mut self, self_ty: &Canonical<'tcx, QueryResponse<'tcx, Ty<'tcx>>>) {
+    fn assemble_probe(
+        &mut self,
+        self_ty: &Canonical<'tcx, QueryResponse<'tcx, Ty<'tcx>>>,
+        receiver_steps: usize,
+    ) {
         let raw_self_ty = self_ty.value.value;
         match *raw_self_ty.kind() {
             ty::Dynamic(data, ..) if let Some(p) = data.principal() => {
@@ -736,22 +776,31 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
                     self.fcx.instantiate_canonical(self.span, self_ty);
 
                 self.assemble_inherent_candidates_from_object(generalized_self_ty);
-                self.assemble_inherent_impl_candidates_for_type(p.def_id());
+                self.assemble_inherent_impl_candidates_for_type(p.def_id(), receiver_steps);
                 if self.tcx.has_attr(p.def_id(), sym::rustc_has_incoherent_inherent_impls) {
-                    self.assemble_inherent_candidates_for_incoherent_ty(raw_self_ty);
+                    self.assemble_inherent_candidates_for_incoherent_ty(
+                        raw_self_ty,
+                        receiver_steps,
+                    );
                 }
             }
             ty::Adt(def, _) => {
                 let def_id = def.did();
-                self.assemble_inherent_impl_candidates_for_type(def_id);
+                self.assemble_inherent_impl_candidates_for_type(def_id, receiver_steps);
                 if self.tcx.has_attr(def_id, sym::rustc_has_incoherent_inherent_impls) {
-                    self.assemble_inherent_candidates_for_incoherent_ty(raw_self_ty);
+                    self.assemble_inherent_candidates_for_incoherent_ty(
+                        raw_self_ty,
+                        receiver_steps,
+                    );
                 }
             }
             ty::Foreign(did) => {
-                self.assemble_inherent_impl_candidates_for_type(did);
+                self.assemble_inherent_impl_candidates_for_type(did, receiver_steps);
                 if self.tcx.has_attr(did, sym::rustc_has_incoherent_inherent_impls) {
-                    self.assemble_inherent_candidates_for_incoherent_ty(raw_self_ty);
+                    self.assemble_inherent_candidates_for_incoherent_ty(
+                        raw_self_ty,
+                        receiver_steps,
+                    );
                 }
             }
             ty::Param(p) => {
@@ -768,29 +817,35 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
             | ty::RawPtr(_, _)
             | ty::Ref(..)
             | ty::Never
-            | ty::Tuple(..) => self.assemble_inherent_candidates_for_incoherent_ty(raw_self_ty),
+            | ty::Tuple(..) => {
+                self.assemble_inherent_candidates_for_incoherent_ty(raw_self_ty, receiver_steps)
+            }
             _ => {}
         }
     }
 
-    fn assemble_inherent_candidates_for_incoherent_ty(&mut self, self_ty: Ty<'tcx>) {
+    fn assemble_inherent_candidates_for_incoherent_ty(
+        &mut self,
+        self_ty: Ty<'tcx>,
+        receiver_steps: usize,
+    ) {
         let Some(simp) = simplify_type(self.tcx, self_ty, TreatParams::InstantiateWithInfer) else {
             bug!("unexpected incoherent type: {:?}", self_ty)
         };
         for &impl_def_id in self.tcx.incoherent_impls(simp).into_iter() {
-            self.assemble_inherent_impl_probe(impl_def_id);
+            self.assemble_inherent_impl_probe(impl_def_id, receiver_steps);
         }
     }
 
-    fn assemble_inherent_impl_candidates_for_type(&mut self, def_id: DefId) {
+    fn assemble_inherent_impl_candidates_for_type(&mut self, def_id: DefId, receiver_steps: usize) {
         let impl_def_ids = self.tcx.at(self.span).inherent_impls(def_id).into_iter();
         for &impl_def_id in impl_def_ids {
-            self.assemble_inherent_impl_probe(impl_def_id);
+            self.assemble_inherent_impl_probe(impl_def_id, receiver_steps);
         }
     }
 
     #[instrument(level = "debug", skip(self))]
-    fn assemble_inherent_impl_probe(&mut self, impl_def_id: DefId) {
+    fn assemble_inherent_impl_probe(&mut self, impl_def_id: DefId, receiver_steps: usize) {
         if !self.impl_dups.insert(impl_def_id) {
             return; // already visited
         }
@@ -804,7 +859,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
             self.push_candidate(
                 Candidate {
                     item,
-                    kind: InherentImplCandidate(impl_def_id),
+                    kind: InherentImplCandidate { impl_def_id, receiver_steps },
                     import_ids: smallvec![],
                 },
                 true,
@@ -1198,8 +1253,13 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
                     return Some(by_value_pick);
                 }
 
-                let autoref_pick =
-                    self.pick_autorefd_method(step, self_ty, hir::Mutability::Not, pick_diag_hints);
+                let autoref_pick = self.pick_autorefd_method(
+                    step,
+                    self_ty,
+                    hir::Mutability::Not,
+                    pick_diag_hints,
+                    None,
+                );
                 // Check for shadowing of a by-mut-ref method by a by-reference method (see comments on check_for_shadowing)
                 if let Some(autoref_pick) = autoref_pick {
                     if let Ok(autoref_pick) = autoref_pick.as_ref() {
@@ -1242,9 +1302,15 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
                 // to avoid such compatibility breaks.
                 // We also don't check for reborrowed pin methods which
                 // may be shadowed; these also seem unlikely to occur.
-                self.pick_autorefd_method(step, self_ty, hir::Mutability::Mut, pick_diag_hints)
-                    .or_else(|| self.pick_const_ptr_method(step, self_ty, pick_diag_hints))
-                    .or_else(|| self.pick_reborrow_pin_method(step, self_ty, pick_diag_hints))
+                self.pick_autorefd_method(
+                    step,
+                    self_ty,
+                    hir::Mutability::Mut,
+                    pick_diag_hints,
+                    None,
+                )
+                .or_else(|| self.pick_const_ptr_method(step, self_ty, pick_diag_hints))
+                .or_else(|| self.pick_reborrow_pin_method(step, self_ty, pick_diag_hints))
             })
     }
 
@@ -1265,7 +1331,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
     /// Report an error in this case.
     fn check_for_shadowed_autorefd_method(
         &self,
-        _possible_shadower: &Pick<'tcx>,
+        possible_shadower: &Pick<'tcx>,
         step: &CandidateStep<'tcx>,
         self_ty: Ty<'tcx>,
         mutbl: hir::Mutability,
@@ -1279,8 +1345,54 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
             unstable_candidates: if track_unstable_candidates { Some(Vec::new()) } else { None },
             unsatisfied_predicates: &mut Vec::new(),
         };
-        let _potentially_shadowed_pick =
-            self.pick_autorefd_method(step, self_ty, mutbl, &mut pick_diag_hints);
+        // Set criteria for how we find methods possibly shadowed by 'possible_shadower'
+        let pick_constraints = PickConstraintsForShadowed {
+            // It's the same `self` type...
+            autoderefs: possible_shadower.autoderefs,
+            // ... but the method was found in an impl block determined
+            // by searching further along the Receiver chain than the other,
+            // showing that it's a smart pointer type causing the problem...
+            receiver_steps: possible_shadower.receiver_steps,
+            // ... and they don't end up pointing to the same item in the
+            // first place (could happen with things like blanket impls for T)
+            def_id: possible_shadower.item.def_id,
+        };
+        // A note on the autoderefs above. Within pick_by_value_method, an extra
+        // autoderef may be applied in order to reborrow a reference with
+        // a different lifetime. That seems as though it would break the
+        // logic of these constraints, since the number of autoderefs could
+        // no longer be used to identify the fundamental type of the receiver.
+        // However, this extra autoderef is applied only to by-value calls
+        // where the receiver is already a reference. So this situation would
+        // only occur in cases where the shadowing looks like this:
+        // ```
+        // struct A;
+        // impl A {
+        //   fn foo(self: &&NonNull<A>) {}
+        //      // note this is by DOUBLE reference
+        // }
+        // ```
+        // then we've come along and added this method to `NonNull`:
+        // ```
+        //   fn foo(&self)  // note this is by single reference
+        // ```
+        // and the call is:
+        // ```
+        // let bar = NonNull<Foo>;
+        // let bar = &foo;
+        // bar.foo();
+        // ```
+        // In these circumstances, the logic is wrong, and we wouldn't spot
+        // the shadowing, because the autoderef-based maths wouldn't line up.
+        // This is a niche case and we can live without generating an error
+        // in the case of such shadowing.
+        let _potentially_shadowed_pick = self.pick_autorefd_method(
+            step,
+            self_ty,
+            mutbl,
+            &mut pick_diag_hints,
+            Some(&pick_constraints),
+        );
 
         // At the moment, this function does no checks. A future
         // commit will fill out the body here.
@@ -1303,7 +1415,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
             return None;
         }
 
-        self.pick_method(self_ty, pick_diag_hints).map(|r| {
+        self.pick_method(self_ty, pick_diag_hints, None).map(|r| {
             r.map(|mut pick| {
                 pick.autoderefs = step.autoderefs;
 
@@ -1342,14 +1454,21 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
         self_ty: Ty<'tcx>,
         mutbl: hir::Mutability,
         pick_diag_hints: &mut PickDiagHints<'_, 'tcx>,
+        pick_constraints: Option<&PickConstraintsForShadowed>,
     ) -> Option<PickResult<'tcx>> {
         let tcx = self.tcx;
 
+        if let Some(pick_constraints) = pick_constraints {
+            if !pick_constraints.may_shadow_based_on_autoderefs(step.autoderefs) {
+                return None;
+            }
+        }
+
         // In general, during probing we erase regions.
         let region = tcx.lifetimes.re_erased;
 
         let autoref_ty = Ty::new_ref(tcx, region, self_ty, mutbl);
-        self.pick_method(autoref_ty, pick_diag_hints).map(|r| {
+        self.pick_method(autoref_ty, pick_diag_hints, pick_constraints).map(|r| {
             r.map(|mut pick| {
                 pick.autoderefs = step.autoderefs;
                 pick.autoref_or_ptr_adjustment =
@@ -1386,7 +1505,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
 
         let region = self.tcx.lifetimes.re_erased;
         let autopin_ty = Ty::new_pinned_ref(self.tcx, region, inner_ty, hir::Mutability::Not);
-        self.pick_method(autopin_ty, pick_diag_hints).map(|r| {
+        self.pick_method(autopin_ty, pick_diag_hints, None).map(|r| {
             r.map(|mut pick| {
                 pick.autoderefs = step.autoderefs;
                 pick.autoref_or_ptr_adjustment =
@@ -1415,7 +1534,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
         };
 
         let const_ptr_ty = Ty::new_imm_ptr(self.tcx, ty);
-        self.pick_method(const_ptr_ty, pick_diag_hints).map(|r| {
+        self.pick_method(const_ptr_ty, pick_diag_hints, None).map(|r| {
             r.map(|mut pick| {
                 pick.autoderefs = step.autoderefs;
                 pick.autoref_or_ptr_adjustment = Some(AutorefOrPtrAdjustment::ToConstPtr);
@@ -1428,6 +1547,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
         &self,
         self_ty: Ty<'tcx>,
         pick_diag_hints: &mut PickDiagHints<'_, 'tcx>,
+        pick_constraints: Option<&PickConstraintsForShadowed>,
     ) -> Option<PickResult<'tcx>> {
         debug!("pick_method(self_ty={})", self.ty_to_string(self_ty));
 
@@ -1435,19 +1555,23 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
             [("inherent", &self.inherent_candidates), ("extension", &self.extension_candidates)]
         {
             debug!("searching {} candidates", kind);
-            let res = self.consider_candidates(self_ty, candidates, pick_diag_hints);
+            let res =
+                self.consider_candidates(self_ty, candidates, pick_diag_hints, pick_constraints);
             if let Some(pick) = res {
                 return Some(pick);
             }
         }
 
         if self.private_candidate.get().is_none() {
-            if let Some(Ok(pick)) =
-                self.consider_candidates(self_ty, &self.private_candidates, &mut PickDiagHints {
+            if let Some(Ok(pick)) = self.consider_candidates(
+                self_ty,
+                &self.private_candidates,
+                &mut PickDiagHints {
                     unstable_candidates: None,
                     unsatisfied_predicates: &mut vec![],
-                })
-            {
+                },
+                None,
+            ) {
                 self.private_candidate.set(Some((pick.item.kind.as_def_kind(), pick.item.def_id)));
             }
         }
@@ -1459,9 +1583,15 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
         self_ty: Ty<'tcx>,
         candidates: &[Candidate<'tcx>],
         pick_diag_hints: &mut PickDiagHints<'_, 'tcx>,
+        pick_constraints: Option<&PickConstraintsForShadowed>,
     ) -> Option<PickResult<'tcx>> {
         let mut applicable_candidates: Vec<_> = candidates
             .iter()
+            .filter(|candidate| {
+                pick_constraints
+                    .map(|pick_constraints| pick_constraints.candidate_may_shadow(&candidate))
+                    .unwrap_or(true)
+            })
             .map(|probe| {
                 (
                     probe,
@@ -1535,6 +1665,7 @@ impl<'tcx> Pick<'tcx> {
             autoref_or_ptr_adjustment: _,
             self_ty,
             unstable_candidates: _,
+            receiver_steps: _,
         } = *self;
         self_ty != other.self_ty || def_id != other.item.def_id
     }
@@ -1610,7 +1741,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
     /// so do not use to make a decision that may lead to a successful compilation.
     fn candidate_source(&self, candidate: &Candidate<'tcx>, self_ty: Ty<'tcx>) -> CandidateSource {
         match candidate.kind {
-            InherentImplCandidate(_) => {
+            InherentImplCandidate { .. } => {
                 CandidateSource::Impl(candidate.item.container_id(self.tcx))
             }
             ObjectCandidate(_) | WhereClauseCandidate(_) => {
@@ -1664,7 +1795,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
             let (mut xform_self_ty, mut xform_ret_ty);
 
             match probe.kind {
-                InherentImplCandidate(impl_def_id) => {
+                InherentImplCandidate { impl_def_id, .. } => {
                     let impl_args = self.fresh_args_for_item(self.span, impl_def_id);
                     let impl_ty = self.tcx.type_of(impl_def_id).instantiate(self.tcx, impl_args);
                     (xform_self_ty, xform_ret_ty) =
@@ -1856,7 +1987,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
                 // We don't normalize the other candidates for perf/backwards-compat reasons...
                 // but `self.return_type` is only set on the diagnostic-path, so we
                 // should be okay doing it here.
-                if !matches!(probe.kind, InherentImplCandidate(_)) {
+                if !matches!(probe.kind, InherentImplCandidate { .. }) {
                     xform_ret_ty = ocx.normalize(&cause, self.param_env, xform_ret_ty);
                 }
 
@@ -1934,6 +2065,7 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
             autoref_or_ptr_adjustment: None,
             self_ty,
             unstable_candidates: vec![],
+            receiver_steps: None,
         })
     }
 
@@ -2205,7 +2337,7 @@ impl<'tcx> Candidate<'tcx> {
         Pick {
             item: self.item,
             kind: match self.kind {
-                InherentImplCandidate(_) => InherentImplPick,
+                InherentImplCandidate { .. } => InherentImplPick,
                 ObjectCandidate(_) => ObjectPick,
                 TraitCandidate(_) => TraitPick,
                 WhereClauseCandidate(trait_ref) => {
@@ -2227,6 +2359,10 @@ impl<'tcx> Candidate<'tcx> {
             autoref_or_ptr_adjustment: None,
             self_ty,
             unstable_candidates,
+            receiver_steps: match self.kind {
+                InherentImplCandidate { receiver_steps, .. } => Some(receiver_steps),
+                _ => None,
+            },
         }
     }
 }

From a269b312314e5c3fb96188ff8e797603bc669f63 Mon Sep 17 00:00:00 2001
From: Adrian Taylor <adetaylor@chromium.org>
Date: Thu, 1 Aug 2024 16:56:04 +0000
Subject: [PATCH 5/6] Arbitrary self types v2: detect shadowing problems.

This builds on the previous commits by actually adding checks for cases
where a new method shadows an older method.
---
 compiler/rustc_hir_typeck/src/method/probe.rs |  23 +++-
 .../arbitrary_self_types_by_value_reborrow.rs |  69 ++++++++++
 ...trary_self_types_shadowing_val_constptr.rs |  33 +++++
 .../self/arbitrary_self_types_unshadowing.rs  |  55 ++++++++
 .../arbitrary_self_types_unshadowing.stderr   | 105 +++++++++++++++
 .../arbitrary_self_types_unshadowing_ptrs.rs  |  61 +++++++++
 ...bitrary_self_types_unshadowing_ptrs.stderr | 122 ++++++++++++++++++
 tests/ui/self/conflicting_inner.rs            |  38 ++++++
 tests/ui/self/conflicting_inner2.rs           |  63 +++++++++
 9 files changed, 565 insertions(+), 4 deletions(-)
 create mode 100644 tests/ui/self/arbitrary_self_types_by_value_reborrow.rs
 create mode 100644 tests/ui/self/arbitrary_self_types_shadowing_val_constptr.rs
 create mode 100644 tests/ui/self/arbitrary_self_types_unshadowing.rs
 create mode 100644 tests/ui/self/arbitrary_self_types_unshadowing.stderr
 create mode 100644 tests/ui/self/arbitrary_self_types_unshadowing_ptrs.rs
 create mode 100644 tests/ui/self/arbitrary_self_types_unshadowing_ptrs.stderr
 create mode 100644 tests/ui/self/conflicting_inner.rs
 create mode 100644 tests/ui/self/conflicting_inner2.rs

diff --git a/compiler/rustc_hir_typeck/src/method/probe.rs b/compiler/rustc_hir_typeck/src/method/probe.rs
index 7524a1faaa394..3b377076545da 100644
--- a/compiler/rustc_hir_typeck/src/method/probe.rs
+++ b/compiler/rustc_hir_typeck/src/method/probe.rs
@@ -1386,16 +1386,22 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
         // the shadowing, because the autoderef-based maths wouldn't line up.
         // This is a niche case and we can live without generating an error
         // in the case of such shadowing.
-        let _potentially_shadowed_pick = self.pick_autorefd_method(
+        let potentially_shadowed_pick = self.pick_autorefd_method(
             step,
             self_ty,
             mutbl,
             &mut pick_diag_hints,
             Some(&pick_constraints),
         );
-
-        // At the moment, this function does no checks. A future
-        // commit will fill out the body here.
+        // Look for actual pairs of shadower/shadowed which are
+        // the sort of shadowing case we want to avoid. Specifically...
+        if let Some(Ok(possible_shadowed)) = potentially_shadowed_pick.as_ref() {
+            let sources = [possible_shadower, possible_shadowed]
+                .into_iter()
+                .map(|p| self.candidate_source_from_pick(p))
+                .collect();
+            return Err(MethodError::Ambiguity(sources));
+        }
         Ok(())
     }
 
@@ -1771,6 +1777,15 @@ impl<'a, 'tcx> ProbeContext<'a, 'tcx> {
         }
     }
 
+    fn candidate_source_from_pick(&self, pick: &Pick<'tcx>) -> CandidateSource {
+        match pick.kind {
+            InherentImplPick => CandidateSource::Impl(pick.item.container_id(self.tcx)),
+            ObjectPick | WhereClausePick(_) | TraitPick => {
+                CandidateSource::Trait(pick.item.container_id(self.tcx))
+            }
+        }
+    }
+
     #[instrument(level = "trace", skip(self, possibly_unsatisfied_predicates), ret)]
     fn consider_probe(
         &self,
diff --git a/tests/ui/self/arbitrary_self_types_by_value_reborrow.rs b/tests/ui/self/arbitrary_self_types_by_value_reborrow.rs
new file mode 100644
index 0000000000000..de4db1b9afec7
--- /dev/null
+++ b/tests/ui/self/arbitrary_self_types_by_value_reborrow.rs
@@ -0,0 +1,69 @@
+//@ run-pass
+
+#![feature(arbitrary_self_types)]
+#![allow(dead_code)]
+
+// With arbitrary self types v2, we show an error if there are
+// multiple contenders for a method call in an inner and outer type.
+// The goal is to avoid any possibility of confusion by a new
+// 'shadowing' method calling a 'shadowed' method.
+// However, there are niche circumstances where this
+// algorithm doesn't quite work, due to reborrows to get a different
+// lifetime. The test below explicitly tests those circumstances to ensure
+// the behavior is as expected, even if it's not 100% desirable. They're
+// very niche circumstances.
+
+#[derive(Debug, PartialEq)]
+enum Callee {
+    INNER,
+    OUTER
+}
+
+struct MyNonNull<T>(T);
+
+impl<T> std::ops::Receiver for MyNonNull<T> {
+    type Target = T;
+}
+
+struct A;
+impl A {
+    fn foo(self: MyNonNull<A>) -> Callee {
+        Callee::INNER
+    }
+
+    fn bar(self: &MyNonNull<A>) -> Callee {
+        Callee::INNER
+    }
+
+    fn baz(self: &&MyNonNull<A>) -> Callee {
+        // note this is by DOUBLE reference
+        Callee::INNER
+    }
+}
+
+impl<T> MyNonNull<T> {
+    fn foo(&self) -> Callee{
+        Callee::OUTER
+    }
+
+    fn bar(&self) -> Callee{
+        Callee::OUTER
+    }
+
+    fn baz(&self) -> Callee{
+        Callee::OUTER
+    }
+}
+
+fn main() {
+    // The normal deshadowing case. Does not compile.
+    // assert_eq!(MyNonNull(A).foo(), Callee::INNER);
+
+    // Similarly, does not compile.
+    //assert_eq!(MyNonNull(A).bar(), Callee::INNER);
+
+    // The double-reference case.
+    // We call the newly-added outer type method.
+    // Not ideal but very niche so we accept it.
+    assert_eq!(MyNonNull(A).baz(), Callee::OUTER);
+}
diff --git a/tests/ui/self/arbitrary_self_types_shadowing_val_constptr.rs b/tests/ui/self/arbitrary_self_types_shadowing_val_constptr.rs
new file mode 100644
index 0000000000000..2b718cb0454ce
--- /dev/null
+++ b/tests/ui/self/arbitrary_self_types_shadowing_val_constptr.rs
@@ -0,0 +1,33 @@
+//@ run-pass
+
+#![feature(arbitrary_self_types)]
+#![feature(arbitrary_self_types_pointers)]
+
+pub struct A;
+
+impl A {
+    pub fn f(self: *const MyNonNull<Self>) -> i32 { 1 }
+}
+
+pub struct MyNonNull<T>(T);
+
+impl<T> core::ops::Receiver for MyNonNull<T> {
+    type Target = T;
+}
+
+impl<T> MyNonNull<T> {
+    // Imagine this a NEW method in B<T> shadowing an EXISTING
+    // method in A.
+    pub fn f(self: *mut Self) -> i32 {
+        2
+    }
+}
+
+fn main() {
+    let mut b = MyNonNull(A);
+    let b = &mut b;
+    let b = b as *mut MyNonNull<A>;
+    // We actually allow the shadowing in the case of const vs mut raw
+    // pointer receivers.
+    assert_eq!(b.f(), 2);
+}
diff --git a/tests/ui/self/arbitrary_self_types_unshadowing.rs b/tests/ui/self/arbitrary_self_types_unshadowing.rs
new file mode 100644
index 0000000000000..cd195654cc195
--- /dev/null
+++ b/tests/ui/self/arbitrary_self_types_unshadowing.rs
@@ -0,0 +1,55 @@
+#![feature(arbitrary_self_types)]
+
+pub struct A;
+
+// The receiver of the potentially shadowed method
+// precisely matches that of the shadower
+impl A {
+    pub fn f(self: Wrapper<Self>) -> i32 { 1 }
+    pub fn g(self: &Wrapper<Self>) -> i32 { 2 }
+    pub fn h(self: &mut Wrapper<Self>) -> i32 { 3 }
+}
+
+// The receiver of the potentially shadowed method is a reference
+pub struct B;
+
+impl B {
+    pub fn f(self: &Wrapper<Self>) -> i32 { 9 }
+}
+
+// The receiver of the potentially shadowed method is a mut reference
+
+pub struct C;
+
+impl C {
+    pub fn f(self: &mut Wrapper<Self>) -> i32 { 10 }
+    pub fn g(self: &mut Wrapper<Self>) -> i32 { 11 }
+}
+
+pub struct Wrapper<T>(T);
+
+impl<T> core::ops::Receiver for Wrapper<T> {
+    type Target = T;
+}
+
+impl<T> Wrapper<T> {
+    pub fn f(self) -> i32 { 5 }
+    pub fn g(&self) -> i32 { 6 }
+    pub fn h(&mut self) -> i32 { 7 }
+}
+
+fn main() {
+    assert_eq!(Wrapper(A).f(), 1);
+    //~^ ERROR: multiple applicable items in scope
+    assert_eq!(Wrapper(A).g(), 2);
+    //~^ ERROR: multiple applicable items in scope
+    assert_eq!(Wrapper(A).h(), 3);
+    //~^ ERROR: multiple applicable items in scope
+    let a = Wrapper(A);
+    assert_eq!(Wrapper(B).f(), 9);
+    //~^ ERROR: multiple applicable items in scope
+    assert_eq!(Wrapper(C).f(), 10);
+    //~^ ERROR: multiple applicable items in scope
+    assert_eq!(Wrapper(C).g(), 11);
+    //~^ ERROR: multiple applicable items in scope
+}
diff --git a/tests/ui/self/arbitrary_self_types_unshadowing.stderr b/tests/ui/self/arbitrary_self_types_unshadowing.stderr
new file mode 100644
index 0000000000000..3a126c495c94d
--- /dev/null
+++ b/tests/ui/self/arbitrary_self_types_unshadowing.stderr
@@ -0,0 +1,105 @@
+error[E0034]: multiple applicable items in scope
+  --> $DIR/arbitrary_self_types_unshadowing.rs:42:27
+   |
+LL |     assert_eq!(Wrapper(A).f(), 1);
+   |                           ^ multiple `f` found
+   |
+note: candidate #1 is defined in an impl for the type `A`
+  --> $DIR/arbitrary_self_types_unshadowing.rs:8:5
+   |
+LL |     pub fn f(self: Wrapper<Self>) -> i32 { 1 }
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: candidate #2 is defined in an impl for the type `Wrapper<T>`
+  --> $DIR/arbitrary_self_types_unshadowing.rs:36:5
+   |
+LL |     pub fn f(self) -> i32 { 5 }
+   |     ^^^^^^^^^^^^^^^^^^^^^
+
+error[E0034]: multiple applicable items in scope
+  --> $DIR/arbitrary_self_types_unshadowing.rs:44:27
+   |
+LL |     assert_eq!(Wrapper(A).g(), 2);
+   |                           ^ multiple `g` found
+   |
+note: candidate #1 is defined in an impl for the type `A`
+  --> $DIR/arbitrary_self_types_unshadowing.rs:9:5
+   |
+LL |     pub fn g(self: &Wrapper<Self>) -> i32 { 2 }
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: candidate #2 is defined in an impl for the type `Wrapper<T>`
+  --> $DIR/arbitrary_self_types_unshadowing.rs:37:5
+   |
+LL |     pub fn g(&self) -> i32 { 6 }
+   |     ^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0034]: multiple applicable items in scope
+  --> $DIR/arbitrary_self_types_unshadowing.rs:46:27
+   |
+LL |     assert_eq!(Wrapper(A).h(), 3);
+   |                           ^ multiple `h` found
+   |
+note: candidate #1 is defined in an impl for the type `A`
+  --> $DIR/arbitrary_self_types_unshadowing.rs:10:5
+   |
+LL |     pub fn h(self: &mut Wrapper<Self>) -> i32 { 3 }
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: candidate #2 is defined in an impl for the type `Wrapper<T>`
+  --> $DIR/arbitrary_self_types_unshadowing.rs:38:5
+   |
+LL |     pub fn h(&mut self) -> i32 { 7 }
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0034]: multiple applicable items in scope
+  --> $DIR/arbitrary_self_types_unshadowing.rs:49:27
+   |
+LL |     assert_eq!(Wrapper(B).f(), 9);
+   |                           ^ multiple `f` found
+   |
+note: candidate #1 is defined in an impl for the type `B`
+  --> $DIR/arbitrary_self_types_unshadowing.rs:17:5
+   |
+LL |     pub fn f(self: &Wrapper<Self>) -> i32 { 9 }
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: candidate #2 is defined in an impl for the type `Wrapper<T>`
+  --> $DIR/arbitrary_self_types_unshadowing.rs:36:5
+   |
+LL |     pub fn f(self) -> i32 { 5 }
+   |     ^^^^^^^^^^^^^^^^^^^^^
+
+error[E0034]: multiple applicable items in scope
+  --> $DIR/arbitrary_self_types_unshadowing.rs:51:27
+   |
+LL |     assert_eq!(Wrapper(C).f(), 10);
+   |                           ^ multiple `f` found
+   |
+note: candidate #1 is defined in an impl for the type `C`
+  --> $DIR/arbitrary_self_types_unshadowing.rs:25:5
+   |
+LL |     pub fn f(self: &mut Wrapper<Self>) -> i32 { 10 }
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: candidate #2 is defined in an impl for the type `Wrapper<T>`
+  --> $DIR/arbitrary_self_types_unshadowing.rs:36:5
+   |
+LL |     pub fn f(self) -> i32 { 5 }
+   |     ^^^^^^^^^^^^^^^^^^^^^
+
+error[E0034]: multiple applicable items in scope
+  --> $DIR/arbitrary_self_types_unshadowing.rs:53:27
+   |
+LL |     assert_eq!(Wrapper(C).g(), 11);
+   |                           ^ multiple `g` found
+   |
+note: candidate #1 is defined in an impl for the type `C`
+  --> $DIR/arbitrary_self_types_unshadowing.rs:26:5
+   |
+LL |     pub fn g(self: &mut Wrapper<Self>) -> i32 { 11 }
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: candidate #2 is defined in an impl for the type `Wrapper<T>`
+  --> $DIR/arbitrary_self_types_unshadowing.rs:37:5
+   |
+LL |     pub fn g(&self) -> i32 { 6 }
+   |     ^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 6 previous errors
+
+For more information about this error, try `rustc --explain E0034`.
diff --git a/tests/ui/self/arbitrary_self_types_unshadowing_ptrs.rs b/tests/ui/self/arbitrary_self_types_unshadowing_ptrs.rs
new file mode 100644
index 0000000000000..62553c2622a7e
--- /dev/null
+++ b/tests/ui/self/arbitrary_self_types_unshadowing_ptrs.rs
@@ -0,0 +1,61 @@
+#![feature(arbitrary_self_types_pointers)]
+#![feature(arbitrary_self_types)]
+
+pub struct A;
+
+// The receiver of the potentially shadowed method
+// precisely matches that of the shadower
+impl A {
+    pub fn f(self: Wrapper<Self>) -> i32 { 1 }
+    pub fn g(self: &Wrapper<Self>) -> i32 { 2 }
+    pub fn h(self: &mut Wrapper<Self>) -> i32 { 3 }
+    pub fn i(self: *const Wrapper<Self>) -> i32 { 4 }
+}
+
+// The receiver of the potentially shadowed method is a reference
+pub struct B;
+
+impl B {
+    pub fn f(self: &Wrapper<Self>) -> i32 { 9 }
+}
+
+// The receiver of the potentially shadowed method is a mut reference
+
+pub struct C;
+
+impl C {
+    pub fn f(self: &mut Wrapper<Self>) -> i32 { 10 }
+    pub fn g(self: &mut Wrapper<Self>) -> i32 { 11 }
+}
+
+pub struct Wrapper<T>(T);
+
+impl<T> core::ops::Receiver for Wrapper<T> {
+    type Target = T;
+}
+
+impl<T> Wrapper<T> {
+    pub fn f(self) -> i32 { 5 }
+    pub fn g(&self) -> i32 { 6 }
+    pub fn h(&mut self) -> i32 { 7 }
+    pub fn i(self: *const Self) -> i32 { 8 }
+}
+
+fn main() {
+    assert_eq!(Wrapper(A).f(), 1);
+    //~^ ERROR: multiple applicable items in scope
+    assert_eq!(Wrapper(A).g(), 2);
+    //~^ ERROR: multiple applicable items in scope
+    assert_eq!(Wrapper(A).h(), 3);
+    //~^ ERROR: multiple applicable items in scope
+    let a = Wrapper(A);
+    let a_ptr = &a as *const Wrapper<A>;
+    assert_eq!(a_ptr.i(), 4);
+    //~^ ERROR: multiple applicable items in scope
+    assert_eq!(Wrapper(B).f(), 9);
+    //~^ ERROR: multiple applicable items in scope
+    assert_eq!(Wrapper(C).f(), 10);
+    //~^ ERROR: multiple applicable items in scope
+    assert_eq!(Wrapper(C).g(), 11);
+    //~^ ERROR: multiple applicable items in scope
+}
diff --git a/tests/ui/self/arbitrary_self_types_unshadowing_ptrs.stderr b/tests/ui/self/arbitrary_self_types_unshadowing_ptrs.stderr
new file mode 100644
index 0000000000000..6d453aed0f06d
--- /dev/null
+++ b/tests/ui/self/arbitrary_self_types_unshadowing_ptrs.stderr
@@ -0,0 +1,122 @@
+error[E0034]: multiple applicable items in scope
+  --> $DIR/arbitrary_self_types_unshadowing_ptrs.rs:45:27
+   |
+LL |     assert_eq!(Wrapper(A).f(), 1);
+   |                           ^ multiple `f` found
+   |
+note: candidate #1 is defined in an impl for the type `A`
+  --> $DIR/arbitrary_self_types_unshadowing_ptrs.rs:9:5
+   |
+LL |     pub fn f(self: Wrapper<Self>) -> i32 { 1 }
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: candidate #2 is defined in an impl for the type `Wrapper<T>`
+  --> $DIR/arbitrary_self_types_unshadowing_ptrs.rs:38:5
+   |
+LL |     pub fn f(self) -> i32 { 5 }
+   |     ^^^^^^^^^^^^^^^^^^^^^
+
+error[E0034]: multiple applicable items in scope
+  --> $DIR/arbitrary_self_types_unshadowing_ptrs.rs:47:27
+   |
+LL |     assert_eq!(Wrapper(A).g(), 2);
+   |                           ^ multiple `g` found
+   |
+note: candidate #1 is defined in an impl for the type `A`
+  --> $DIR/arbitrary_self_types_unshadowing_ptrs.rs:10:5
+   |
+LL |     pub fn g(self: &Wrapper<Self>) -> i32 { 2 }
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: candidate #2 is defined in an impl for the type `Wrapper<T>`
+  --> $DIR/arbitrary_self_types_unshadowing_ptrs.rs:39:5
+   |
+LL |     pub fn g(&self) -> i32 { 6 }
+   |     ^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0034]: multiple applicable items in scope
+  --> $DIR/arbitrary_self_types_unshadowing_ptrs.rs:49:27
+   |
+LL |     assert_eq!(Wrapper(A).h(), 3);
+   |                           ^ multiple `h` found
+   |
+note: candidate #1 is defined in an impl for the type `A`
+  --> $DIR/arbitrary_self_types_unshadowing_ptrs.rs:11:5
+   |
+LL |     pub fn h(self: &mut Wrapper<Self>) -> i32 { 3 }
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: candidate #2 is defined in an impl for the type `Wrapper<T>`
+  --> $DIR/arbitrary_self_types_unshadowing_ptrs.rs:40:5
+   |
+LL |     pub fn h(&mut self) -> i32 { 7 }
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0034]: multiple applicable items in scope
+  --> $DIR/arbitrary_self_types_unshadowing_ptrs.rs:53:22
+   |
+LL |     assert_eq!(a_ptr.i(), 4);
+   |                      ^ multiple `i` found
+   |
+note: candidate #1 is defined in an impl for the type `A`
+  --> $DIR/arbitrary_self_types_unshadowing_ptrs.rs:12:5
+   |
+LL |     pub fn i(self: *const Wrapper<Self>) -> i32 { 4 }
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: candidate #2 is defined in an impl for the type `Wrapper<T>`
+  --> $DIR/arbitrary_self_types_unshadowing_ptrs.rs:41:5
+   |
+LL |     pub fn i(self: *const Self) -> i32 { 8 }
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+error[E0034]: multiple applicable items in scope
+  --> $DIR/arbitrary_self_types_unshadowing_ptrs.rs:55:27
+   |
+LL |     assert_eq!(Wrapper(B).f(), 9);
+   |                           ^ multiple `f` found
+   |
+note: candidate #1 is defined in an impl for the type `B`
+  --> $DIR/arbitrary_self_types_unshadowing_ptrs.rs:19:5
+   |
+LL |     pub fn f(self: &Wrapper<Self>) -> i32 { 9 }
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: candidate #2 is defined in an impl for the type `Wrapper<T>`
+  --> $DIR/arbitrary_self_types_unshadowing_ptrs.rs:38:5
+   |
+LL |     pub fn f(self) -> i32 { 5 }
+   |     ^^^^^^^^^^^^^^^^^^^^^
+
+error[E0034]: multiple applicable items in scope
+  --> $DIR/arbitrary_self_types_unshadowing_ptrs.rs:57:27
+   |
+LL |     assert_eq!(Wrapper(C).f(), 10);
+   |                           ^ multiple `f` found
+   |
+note: candidate #1 is defined in an impl for the type `C`
+  --> $DIR/arbitrary_self_types_unshadowing_ptrs.rs:27:5
+   |
+LL |     pub fn f(self: &mut Wrapper<Self>) -> i32 { 10 }
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: candidate #2 is defined in an impl for the type `Wrapper<T>`
+  --> $DIR/arbitrary_self_types_unshadowing_ptrs.rs:38:5
+   |
+LL |     pub fn f(self) -> i32 { 5 }
+   |     ^^^^^^^^^^^^^^^^^^^^^
+
+error[E0034]: multiple applicable items in scope
+  --> $DIR/arbitrary_self_types_unshadowing_ptrs.rs:59:27
+   |
+LL |     assert_eq!(Wrapper(C).g(), 11);
+   |                           ^ multiple `g` found
+   |
+note: candidate #1 is defined in an impl for the type `C`
+  --> $DIR/arbitrary_self_types_unshadowing_ptrs.rs:28:5
+   |
+LL |     pub fn g(self: &mut Wrapper<Self>) -> i32 { 11 }
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+note: candidate #2 is defined in an impl for the type `Wrapper<T>`
+  --> $DIR/arbitrary_self_types_unshadowing_ptrs.rs:39:5
+   |
+LL |     pub fn g(&self) -> i32 { 6 }
+   |     ^^^^^^^^^^^^^^^^^^^^^^
+
+error: aborting due to 7 previous errors
+
+For more information about this error, try `rustc --explain E0034`.
diff --git a/tests/ui/self/conflicting_inner.rs b/tests/ui/self/conflicting_inner.rs
new file mode 100644
index 0000000000000..1a7037dce7393
--- /dev/null
+++ b/tests/ui/self/conflicting_inner.rs
@@ -0,0 +1,38 @@
+//@ run-pass
+//@ revisions: default feature
+#![cfg_attr(feature, feature(arbitrary_self_types))]
+
+// This test aims to be like the IndexVec within rustc, and conflicts
+// over its into_iter().
+
+#[allow(dead_code)]
+trait Foo {
+    fn foo(self) -> usize;
+}
+
+struct IndexVec<T>(T);
+
+impl<T> std::ops::Deref for IndexVec<T> {
+    type Target = T;
+
+    fn deref(&self) -> &Self::Target {
+        &self.0
+    }
+}
+
+impl<'a, T> Foo for &'a IndexVec<T> {
+    fn foo(self) -> usize {
+        2
+    }
+}
+
+impl<T> IndexVec<T> {
+    fn foo(self) -> usize {
+        1
+    }
+}
+
+fn main() {
+    let ivec = IndexVec(0usize);
+    assert_eq!(ivec.foo(), 1);
+}
diff --git a/tests/ui/self/conflicting_inner2.rs b/tests/ui/self/conflicting_inner2.rs
new file mode 100644
index 0000000000000..e6a6b5ef793e3
--- /dev/null
+++ b/tests/ui/self/conflicting_inner2.rs
@@ -0,0 +1,63 @@
+//@ run-pass
+//@ revisions: default feature
+#![cfg_attr(feature, feature(arbitrary_self_types))]
+
+use std::pin::Pin;
+use std::ops::DerefMut;
+use std::marker::Unpin;
+
+struct TryChunks;
+
+impl TryChunks {
+    #[allow(dead_code)]
+    fn take(self: std::pin::Pin<&mut Self>) -> usize {
+        1
+    }
+}
+
+#[allow(dead_code)]
+trait Stream {
+    fn poll_next(self: std::pin::Pin<&mut Self>);
+}
+
+#[allow(dead_code)]
+trait StreamExt: Stream {
+    #[allow(dead_code)]
+    fn take(self) -> usize where Self: Sized
+    {
+        2
+    }
+}
+
+impl<T: ?Sized> StreamExt for T where T: Stream {}
+
+impl Stream for TryChunks {
+    fn poll_next(self: std::pin::Pin<&mut Self>) {
+        assert_eq!(self.take(), 1);
+    }
+}
+
+#[allow(dead_code)]
+impl<S: ?Sized + Stream + Unpin> Stream for &mut S {
+    #[allow(dead_code)]
+    fn poll_next(mut self: Pin<&mut Self>)  {
+        S::poll_next(Pin::new(&mut **self))
+    }
+}
+
+#[allow(dead_code)]
+impl<P> Stream for Pin<P>
+where
+    P: DerefMut + Unpin,
+    P::Target: Stream,
+{
+    #[allow(dead_code)]
+    fn poll_next(self: Pin<&mut Self>) {
+        self.get_mut().as_mut().poll_next()
+    }
+}
+
+fn main() {
+    let mut item = Box::pin(TryChunks);
+    item.as_mut().poll_next();
+}

From 337af8a37014a476d8deb081c3f209c7d56d5171 Mon Sep 17 00:00:00 2001
From: Adrian Taylor <adetaylor@chromium.org>
Date: Thu, 21 Dec 2023 16:19:40 +0000
Subject: [PATCH 6/6] Arbitrary self types v2: generics test.

There's some discussion on the RFC about whether generic receivers should be
allowed, but in the end the conclusion was that they should be blocked
(at least for some definition of 'generic'). This blocking landed in
an earlier PR; this commit adds additional tests to ensure the
interaction with the rest of the Arbitrary Self Types v2 feature is as
expected. This test may be a little duplicative but it seems better
to land it than not.
---
 .../arbitrary_self_types_generic_receiver.rs  | 50 +++++++++++++++++++
 ...bitrary_self_types_generic_receiver.stderr | 48 ++++++++++++++++++
 2 files changed, 98 insertions(+)
 create mode 100644 tests/ui/self/arbitrary_self_types_generic_receiver.rs
 create mode 100644 tests/ui/self/arbitrary_self_types_generic_receiver.stderr

diff --git a/tests/ui/self/arbitrary_self_types_generic_receiver.rs b/tests/ui/self/arbitrary_self_types_generic_receiver.rs
new file mode 100644
index 0000000000000..0739fb778b605
--- /dev/null
+++ b/tests/ui/self/arbitrary_self_types_generic_receiver.rs
@@ -0,0 +1,50 @@
+#![feature(arbitrary_self_types)]
+
+struct PtrA<T>(T);
+
+impl<T> core::ops::Receiver for PtrA<T> {
+    type Target = T;
+}
+
+struct PtrB<T>(T);
+
+trait SomePtr: core::ops::Receiver<Target=<Self as SomePtr>::SomeTarget> {
+    type SomeTarget;
+}
+
+impl<T> SomePtr for PtrB<T> {
+    type SomeTarget = T;
+}
+
+impl<T> core::ops::Receiver for PtrB<T> {
+    type Target = T;
+}
+
+struct Content;
+
+impl Content {
+    fn a<R: core::ops::Receiver<Target=Self>>(self: &R) {}
+    //~^ ERROR invalid generic
+    fn b<R: core::ops::Receiver<Target=Self>>(self: &mut R) {}
+    //~^ ERROR invalid generic
+    fn c<R: core::ops::Receiver<Target=Self>>(self: R) {}
+    //~^ ERROR invalid generic
+    fn d<R: SomePtr<SomeTarget=Self>>(self: R) {}
+    //~^ ERROR invalid generic
+    fn e(self: impl SomePtr<SomeTarget=Self>) {}
+    //~^ ERROR invalid generic
+}
+
+fn main() {
+    PtrA(Content).a();
+    PtrA(Content).b();
+    PtrA(Content).c();
+    std::rc::Rc::new(Content).a();
+    std::rc::Rc::new(Content).b();
+    std::rc::Rc::new(Content).c();
+    PtrB(Content).a();
+    PtrB(Content).b();
+    PtrB(Content).c();
+    PtrB(Content).d();
+    PtrB(Content).e();
+}
diff --git a/tests/ui/self/arbitrary_self_types_generic_receiver.stderr b/tests/ui/self/arbitrary_self_types_generic_receiver.stderr
new file mode 100644
index 0000000000000..788c55ea2f1c3
--- /dev/null
+++ b/tests/ui/self/arbitrary_self_types_generic_receiver.stderr
@@ -0,0 +1,48 @@
+error[E0801]: invalid generic `self` parameter type: `&R`
+  --> $DIR/arbitrary_self_types_generic_receiver.rs:26:53
+   |
+LL |     fn a<R: core::ops::Receiver<Target=Self>>(self: &R) {}
+   |                                                     ^^
+   |
+   = note: type of `self` must not be a method generic parameter type
+   = help: use a concrete type such as `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
+
+error[E0801]: invalid generic `self` parameter type: `&mut R`
+  --> $DIR/arbitrary_self_types_generic_receiver.rs:28:53
+   |
+LL |     fn b<R: core::ops::Receiver<Target=Self>>(self: &mut R) {}
+   |                                                     ^^^^^^
+   |
+   = note: type of `self` must not be a method generic parameter type
+   = help: use a concrete type such as `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
+
+error[E0801]: invalid generic `self` parameter type: `R`
+  --> $DIR/arbitrary_self_types_generic_receiver.rs:30:53
+   |
+LL |     fn c<R: core::ops::Receiver<Target=Self>>(self: R) {}
+   |                                                     ^
+   |
+   = note: type of `self` must not be a method generic parameter type
+   = help: use a concrete type such as `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
+
+error[E0801]: invalid generic `self` parameter type: `R`
+  --> $DIR/arbitrary_self_types_generic_receiver.rs:32:45
+   |
+LL |     fn d<R: SomePtr<SomeTarget=Self>>(self: R) {}
+   |                                             ^
+   |
+   = note: type of `self` must not be a method generic parameter type
+   = help: use a concrete type such as `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
+
+error[E0801]: invalid generic `self` parameter type: `impl SomePtr<SomeTarget = Self>`
+  --> $DIR/arbitrary_self_types_generic_receiver.rs:34:16
+   |
+LL |     fn e(self: impl SomePtr<SomeTarget=Self>) {}
+   |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: type of `self` must not be a method generic parameter type
+   = help: use a concrete type such as `self`, `&self`, `&mut self`, `self: Box<Self>`, `self: Rc<Self>`, `self: Arc<Self>`, or `self: Pin<P>` (where P is one of the previous types except `Self`)
+
+error: aborting due to 5 previous errors
+
+For more information about this error, try `rustc --explain E0801`.