Don't assume traits used as type are trait objs

compiler/rustc_hir_analysis/src/hir_ty_lowering/lint.rs

@@ -1,6 +1,6 @@
 use rustc_ast::TraitObjectSyntax;
 use rustc_errors::codes::*;
-use rustc_errors::{Diag, EmissionGuarantee, StashKey};
+use rustc_errors::{Diag, EmissionGuarantee, ErrorGuaranteed, StashKey, Suggestions};
 use rustc_hir as hir;
 use rustc_hir::def::{DefKind, Res};
 use rustc_lint_defs::Applicability;
@@ -15,13 +15,16 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
     ///
     /// *Bare* trait object types are ones that aren't preceded by the keyword `dyn`.
     /// In edition 2021 and onward we emit a hard error for them.
-    pub(super) fn prohibit_or_lint_bare_trait_object_ty(&self, self_ty: &hir::Ty<'_>) {
+    pub(super) fn prohibit_or_lint_bare_trait_object_ty(
+        &self,
+        self_ty: &hir::Ty<'_>,
+    ) -> Option<ErrorGuaranteed> {
         let tcx = self.tcx();
 
         let hir::TyKind::TraitObject([poly_trait_ref, ..], _, TraitObjectSyntax::None) =
             self_ty.kind
         else {
-            return;
+            return None;
         };
 
         let in_path = match tcx.parent_hir_node(self_ty.hir_id) {
@@ -70,8 +73,8 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         }
 
         if self_ty.span.edition().at_least_rust_2021() {
-            let msg = "trait objects must include the `dyn` keyword";
-            let label = "add `dyn` keyword before this trait";
+            let msg = "expected a type, found a trait";
+            let label = "you can add the `dyn` keyword if you want a trait object";
             let mut diag =
                 rustc_errors::struct_span_code_err!(self.dcx(), self_ty.span, E0782, "{}", msg);
             if self_ty.span.can_be_used_for_suggestions()
@@ -83,7 +86,15 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             // Check if the impl trait that we are considering is an impl of a local trait.
             self.maybe_suggest_blanket_trait_impl(self_ty, &mut diag);
             self.maybe_suggest_assoc_ty_bound(self_ty, &mut diag);
-            diag.stash(self_ty.span, StashKey::TraitMissingMethod);
+            if let Some(mut sugg) =
+                tcx.dcx().steal_non_err(self_ty.span, StashKey::AssociatedTypeSuggestion)
+                && let Suggestions::Enabled(ref mut s1) = diag.suggestions
+                && let Suggestions::Enabled(ref mut s2) = sugg.suggestions
+            {
+                s1.append(s2);
+                sugg.cancel();
+            }
+            diag.stash(self_ty.span, StashKey::TraitMissingMethod)
         } else {
             tcx.node_span_lint(BARE_TRAIT_OBJECTS, self_ty.hir_id, self_ty.span, |lint| {
                 lint.primary_message("trait objects without an explicit `dyn` are deprecated");
@@ -96,6 +107,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 }
                 self.maybe_suggest_blanket_trait_impl(self_ty, lint);
             });
+            None
         }
     }
 
@@ -174,41 +186,31 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
         // 1. Independent functions
         // 2. Functions inside trait blocks
         // 3. Functions inside impl blocks
-        let (sig, generics, owner) = match tcx.hir_node_by_def_id(parent_id) {
+        let (sig, generics) = match tcx.hir_node_by_def_id(parent_id) {
             hir::Node::Item(hir::Item { kind: hir::ItemKind::Fn(sig, generics, _), .. }) => {
-                (sig, generics, None)
+                (sig, generics)
             }
             hir::Node::TraitItem(hir::TraitItem {
                 kind: hir::TraitItemKind::Fn(sig, _),
                 generics,
-                owner_id,
                 ..
-            }) => (sig, generics, Some(tcx.parent(owner_id.to_def_id()))),
+            }) => (sig, generics),
             hir::Node::ImplItem(hir::ImplItem {
                 kind: hir::ImplItemKind::Fn(sig, _),
                 generics,
-                owner_id,
                 ..
-            }) => (sig, generics, Some(tcx.parent(owner_id.to_def_id()))),
+            }) => (sig, generics),
             _ => return false,
         };
         let Ok(trait_name) = tcx.sess.source_map().span_to_snippet(self_ty.span) else {
             return false;
         };
         let impl_sugg = vec![(self_ty.span.shrink_to_lo(), "impl ".to_string())];
-        let mut is_downgradable = true;
-
         // Check if trait object is safe for suggesting dynamic dispatch.
         let is_dyn_compatible = match self_ty.kind {
             hir::TyKind::TraitObject(objects, ..) => {
                 objects.iter().all(|(o, _)| match o.trait_ref.path.res {
-                    Res::Def(DefKind::Trait, id) => {
-                        if Some(id) == owner {
-                            // For recursive traits, don't downgrade the error. (#119652)
-                            is_downgradable = false;
-                        }
-                        tcx.is_dyn_compatible(id)
-                    }
+                    Res::Def(DefKind::Trait, id) => tcx.is_dyn_compatible(id),
                     _ => false,
                 })
             }
@@ -255,9 +257,6 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                     suggestion,
                     Applicability::MachineApplicable,
                 );
-            } else if is_downgradable {
-                // We'll emit the dyn-compatibility error already, with a structured suggestion.
-                diag.downgrade_to_delayed_bug();
             }
             return true;
         }
@@ -281,10 +280,6 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
             );
             if !is_dyn_compatible {
                 diag.note(format!("`{trait_name}` it is dyn-incompatible, so it can't be `dyn`"));
-                if is_downgradable {
-                    // We'll emit the dyn-compatibility error already, with a structured suggestion.
-                    diag.downgrade_to_delayed_bug();
-                }
             } else {
                 // No ampersand in suggestion if it's borrowed already
                 let (dyn_str, paren_dyn_str) =

compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs

@@ -2063,13 +2063,18 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 )
             }
             hir::TyKind::TraitObject(bounds, lifetime, repr) => {
-                self.prohibit_or_lint_bare_trait_object_ty(hir_ty);
-
-                let repr = match repr {
-                    TraitObjectSyntax::Dyn | TraitObjectSyntax::None => ty::Dyn,
-                    TraitObjectSyntax::DynStar => ty::DynStar,
-                };
-                self.lower_trait_object_ty(hir_ty.span, hir_ty.hir_id, bounds, lifetime, repr)
+                if let Some(guar) = self.prohibit_or_lint_bare_trait_object_ty(hir_ty) {
+                    // Don't continue with type analysis if the `dyn` keyword is missing
+                    // It generates confusing errors, especially if the user meant to use another
+                    // keyword like `impl`
+                    Ty::new_error(tcx, guar)
+                } else {
+                    let repr = match repr {
+                        TraitObjectSyntax::Dyn | TraitObjectSyntax::None => ty::Dyn,
+                        TraitObjectSyntax::DynStar => ty::DynStar,
+                    };
+                    self.lower_trait_object_ty(hir_ty.span, hir_ty.hir_id, bounds, lifetime, repr)
+                }
             }
             // If we encounter a fully qualified path with RTN generics, then it must have
             // *not* gone through `lower_ty_maybe_return_type_notation`, and therefore

tests/ui/associated-type-bounds/suggest-assoc-ty-bound-on-eq-bound.rs

@@ -2,28 +2,24 @@
 //@ edition: 2021
 
 fn f(_: impl Trait<T = Copy>) {}
-//~^ ERROR trait objects must include the `dyn` keyword
-//~| HELP add `dyn` keyword before this trait
+//~^ ERROR expected a type, found a trait
+//~| HELP you can add the `dyn` keyword if you want a trait object
 //~| HELP you might have meant to write a bound here
-//~| ERROR the trait `Copy` cannot be made into an object
 
 fn g(_: impl Trait<T = std::fmt::Debug + Eq>) {}
-//~^ ERROR trait objects must include the `dyn` keyword
-//~| HELP add `dyn` keyword before this trait
+//~^ ERROR expected a type, found a trait
+//~| HELP you can add the `dyn` keyword if you want a trait object
 //~| HELP you might have meant to write a bound here
-//~| ERROR only auto traits can be used as additional traits in a trait object
-//~| HELP consider creating a new trait
-//~| ERROR the trait `Eq` cannot be made into an object
 
 fn h(_: impl Trait<T<> = 'static + for<'a> Fn(&'a ())>) {}
-//~^ ERROR trait objects must include the `dyn` keyword
-//~| HELP add `dyn` keyword before this trait
+//~^ ERROR expected a type, found a trait
+//~| HELP you can add the `dyn` keyword if you want a trait object
 //~| HELP you might have meant to write a bound here
 
 // Don't suggest assoc ty bound in trait object types, that's not valid:
 type Obj = dyn Trait<T = Clone>;
-//~^ ERROR trait objects must include the `dyn` keyword
-//~| HELP add `dyn` keyword before this trait
+//~^ ERROR expected a type, found a trait
+//~| HELP you can add the `dyn` keyword if you want a trait object
 
 trait Trait { type T; }
 

tests/ui/associated-type-bounds/suggest-assoc-ty-bound-on-eq-bound.stderr

@@ -1,41 +1,10 @@
-error[E0038]: the trait `Copy` cannot be made into an object
-  --> $DIR/suggest-assoc-ty-bound-on-eq-bound.rs:4:20
-   |
-LL | fn f(_: impl Trait<T = Copy>) {}
-   |                    ^^^^^^^^ `Copy` cannot be made into an object
-   |
-   = note: the trait cannot be made into an object because it requires `Self: Sized`
-   = note: for a trait to be "dyn-compatible" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
-
-error[E0225]: only auto traits can be used as additional traits in a trait object
-  --> $DIR/suggest-assoc-ty-bound-on-eq-bound.rs:10:42
-   |
-LL | fn g(_: impl Trait<T = std::fmt::Debug + Eq>) {}
-   |                        ---------------   ^^ additional non-auto trait
-   |                        |
-   |                        first non-auto trait
-   |
-   = help: consider creating a new trait with all of these as supertraits and using that trait here instead: `trait NewTrait: Debug + Eq {}`
-   = note: auto-traits like `Send` and `Sync` are traits that have special properties; for more information on them, visit <https://doc.rust-lang.org/reference/special-types-and-traits.html#auto-traits>
-
-error[E0038]: the trait `Eq` cannot be made into an object
-  --> $DIR/suggest-assoc-ty-bound-on-eq-bound.rs:10:24
-   |
-LL | fn g(_: impl Trait<T = std::fmt::Debug + Eq>) {}
-   |                        ^^^^^^^^^^^^^^^^^^^^ `Eq` cannot be made into an object
-   |
-note: for a trait to be "dyn-compatible" it needs to allow building a vtable to allow the call to be resolvable dynamically; for more information visit <https://doc.rust-lang.org/reference/items/traits.html#object-safety>
-  --> $SRC_DIR/core/src/cmp.rs:LL:COL
-   |
-   = note: the trait cannot be made into an object because it uses `Self` as a type parameter
-
-error[E0782]: trait objects must include the `dyn` keyword
+error[E0782]: expected a type, found a trait
   --> $DIR/suggest-assoc-ty-bound-on-eq-bound.rs:4:24
    |
 LL | fn f(_: impl Trait<T = Copy>) {}
    |                        ^^^^
    |
-help: add `dyn` keyword before this trait
+help: you can add the `dyn` keyword if you want a trait object
    |
 LL | fn f(_: impl Trait<T = dyn Copy>) {}
    |                        +++
@@ -44,13 +13,13 @@ help: you might have meant to write a bound here
 LL | fn f(_: impl Trait<T: Copy>) {}
    |                     ~
 
-error[E0782]: trait objects must include the `dyn` keyword
-  --> $DIR/suggest-assoc-ty-bound-on-eq-bound.rs:10:24
+error[E0782]: expected a type, found a trait
+  --> $DIR/suggest-assoc-ty-bound-on-eq-bound.rs:9:24
    |
 LL | fn g(_: impl Trait<T = std::fmt::Debug + Eq>) {}
    |                        ^^^^^^^^^^^^^^^^^^^^
    |
-help: add `dyn` keyword before this trait
+help: you can add the `dyn` keyword if you want a trait object
    |
 LL | fn g(_: impl Trait<T = dyn std::fmt::Debug + Eq>) {}
    |                        +++
@@ -59,13 +28,13 @@ help: you might have meant to write a bound here
 LL | fn g(_: impl Trait<T: std::fmt::Debug + Eq>) {}
    |                     ~
 
-error[E0782]: trait objects must include the `dyn` keyword
-  --> $DIR/suggest-assoc-ty-bound-on-eq-bound.rs:18:26
+error[E0782]: expected a type, found a trait
+  --> $DIR/suggest-assoc-ty-bound-on-eq-bound.rs:14:26
    |
 LL | fn h(_: impl Trait<T<> = 'static + for<'a> Fn(&'a ())>) {}
    |                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
-help: add `dyn` keyword before this trait
+help: you can add the `dyn` keyword if you want a trait object
    |
 LL | fn h(_: impl Trait<T<> = dyn 'static + for<'a> Fn(&'a ())>) {}
    |                          +++
@@ -74,18 +43,17 @@ help: you might have meant to write a bound here
 LL | fn h(_: impl Trait<T<>: 'static + for<'a> Fn(&'a ())>) {}
    |                       ~
 
-error[E0782]: trait objects must include the `dyn` keyword
-  --> $DIR/suggest-assoc-ty-bound-on-eq-bound.rs:24:26
+error[E0782]: expected a type, found a trait
+  --> $DIR/suggest-assoc-ty-bound-on-eq-bound.rs:20:26
    |
 LL | type Obj = dyn Trait<T = Clone>;
    |                          ^^^^^
    |
-help: add `dyn` keyword before this trait
+help: you can add the `dyn` keyword if you want a trait object
    |
 LL | type Obj = dyn Trait<T = dyn Clone>;
    |                          +++
 
-error: aborting due to 7 previous errors
+error: aborting due to 4 previous errors
 
-Some errors have detailed explanations: E0038, E0225, E0782.
-For more information about an error, try `rustc --explain E0038`.
+For more information about this error, try `rustc --explain E0782`.

tests/ui/const-generics/generic_const_exprs/expected-type-of-closure-body-to-be-a-closure-or-coroutine-ice-113776.rs

@@ -14,10 +14,8 @@ fn f<T>(
          1
      }],
 ) -> impl Iterator<Item = SubAssign> {
-//~^ ERROR the type parameter `Rhs` must be explicitly specified
+//~^ ERROR expected a type, found a trait
 //~| ERROR `()` is not an iterator
-//~| ERROR trait objects must include the `dyn` keyword
-//~| ERROR the type parameter `Rhs` must be explicitly specified [E0393]
 }
 
 pub fn main() {}

tests/ui/const-generics/generic_const_exprs/expected-type-of-closure-body-to-be-a-closure-or-coroutine-ice-113776.stderr

@@ -16,37 +16,6 @@ help: you might be missing a type parameter
 LL | fn f<T, F>(
    |       +++
 
-error[E0393]: the type parameter `Rhs` must be explicitly specified
-  --> $DIR/expected-type-of-closure-body-to-be-a-closure-or-coroutine-ice-113776.rs:16:27
-   |
-LL | ) -> impl Iterator<Item = SubAssign> {
-   |                           ^^^^^^^^^
-  --> $SRC_DIR/core/src/ops/arith.rs:LL:COL
-   |
-   = note: type parameter `Rhs` must be specified for this
-   |
-   = note: because of the default `Self` reference, type parameters must be specified on object types
-help: set the type parameter to the desired type
-   |
-LL | ) -> impl Iterator<Item = SubAssign<Rhs>> {
-   |                                    +++++
-
-error[E0393]: the type parameter `Rhs` must be explicitly specified
-  --> $DIR/expected-type-of-closure-body-to-be-a-closure-or-coroutine-ice-113776.rs:16:27
-   |
-LL | ) -> impl Iterator<Item = SubAssign> {
-   |                           ^^^^^^^^^
-  --> $SRC_DIR/core/src/ops/arith.rs:LL:COL
-   |
-   = note: type parameter `Rhs` must be specified for this
-   |
-   = note: because of the default `Self` reference, type parameters must be specified on object types
-   = note: duplicate diagnostic emitted due to `-Z deduplicate-diagnostics=no`
-help: set the type parameter to the desired type
-   |
-LL | ) -> impl Iterator<Item = SubAssign<Rhs>> {
-   |                                    +++++
-
 error[E0277]: `()` is not an iterator
   --> $DIR/expected-type-of-closure-body-to-be-a-closure-or-coroutine-ice-113776.rs:16:6
    |
@@ -55,13 +24,13 @@ LL | ) -> impl Iterator<Item = SubAssign> {
    |
    = help: the trait `Iterator` is not implemented for `()`
 
-error[E0782]: trait objects must include the `dyn` keyword
+error[E0782]: expected a type, found a trait
   --> $DIR/expected-type-of-closure-body-to-be-a-closure-or-coroutine-ice-113776.rs:16:27
    |
 LL | ) -> impl Iterator<Item = SubAssign> {
    |                           ^^^^^^^^^
    |
-help: add `dyn` keyword before this trait
+help: you can add the `dyn` keyword if you want a trait object
    |
 LL | ) -> impl Iterator<Item = dyn SubAssign> {
    |                           +++
@@ -70,7 +39,7 @@ help: you might have meant to write a bound here
 LL | ) -> impl Iterator<Item: SubAssign> {
    |                        ~
 
-error: aborting due to 5 previous errors
+error: aborting due to 3 previous errors
 
-Some errors have detailed explanations: E0277, E0393, E0412, E0782.
+Some errors have detailed explanations: E0277, E0412, E0782.
 For more information about an error, try `rustc --explain E0277`.

tests/ui/const-generics/not_wf_param_in_rpitit.rs

@@ -1,12 +1,11 @@
 //@ edition:2021
 
-trait Trait<const N: Trait = bar> {
+trait Trait<const N: dyn Trait = bar> {
     //~^ ERROR: cannot find value `bar` in this scope
     //~| ERROR: cycle detected when computing type of `Trait::N`
     //~| ERROR: the trait `Trait` cannot be made into an object
     //~| ERROR: the trait `Trait` cannot be made into an object
     //~| ERROR: the trait `Trait` cannot be made into an object
-    //~| ERROR: trait objects must include the `dyn` keyword
     async fn a() {}
 }