Implement RFC 2532 – Associated Type Defaults

src/librustc_error_codes/error_codes/E0399.md

@@ -1,9 +1,11 @@
+#### Note: this error code is no longer emitted by the compiler
+
 You implemented a trait, overriding one or more of its associated types but did
 not reimplement its default methods.
 
 Example of erroneous code:
 
-```compile_fail,E0399
+```
 #![feature(associated_type_defaults)]
 
 pub trait Foo {

src/librustc_infer/traits/project.rs

@@ -1054,25 +1054,40 @@ fn assemble_candidates_from_impls<'cx, 'tcx>(
                     // an error when we confirm the candidate
                     // (which will ultimately lead to `normalize_to_error`
                     // being invoked).
-                    node_item.item.defaultness.has_value()
+                    false
                 } else {
+                    // If we're looking at a trait *impl*, the item is
+                    // specializable if the impl or the item are marked
+                    // `default`.
                     node_item.item.defaultness.is_default()
                         || super::util::impl_is_default(selcx.tcx(), node_item.node.def_id())
                 };
 
-                // Only reveal a specializable default if we're past type-checking
-                // and the obligations is monomorphic, otherwise passes such as
-                // transmute checking and polymorphic MIR optimizations could
-                // get a result which isn't correct for all monomorphizations.
-                if !is_default {
-                    true
-                } else if obligation.param_env.reveal == Reveal::All {
-                    // NOTE(eddyb) inference variables can resolve to parameters, so
-                    // assume `poly_trait_ref` isn't monomorphic, if it contains any.
-                    let poly_trait_ref = selcx.infcx().resolve_vars_if_possible(&poly_trait_ref);
-                    !poly_trait_ref.needs_infer() && !poly_trait_ref.needs_subst()
-                } else {
-                    false
+                match is_default {
+                    // Non-specializable items are always projectable
+                    false => true,
+
+                    // Only reveal a specializable default if we're past type-checking
+                    // and the obligation is monomorphic, otherwise passes such as
+                    // transmute checking and polymorphic MIR optimizations could
+                    // get a result which isn't correct for all monomorphizations.
+                    true if obligation.param_env.reveal == Reveal::All => {
+                        // NOTE(eddyb) inference variables can resolve to parameters, so
+                        // assume `poly_trait_ref` isn't monomorphic, if it contains any.
+                        let poly_trait_ref =
+                            selcx.infcx().resolve_vars_if_possible(&poly_trait_ref);
+                        !poly_trait_ref.needs_infer() && !poly_trait_ref.needs_subst()
+                    }
+
+                    true => {
+                        debug!(
+                            "assemble_candidates_from_impls: not eligible due to default: \
+                             assoc_ty={} predicate={}",
+                            selcx.tcx().def_path_str(node_item.item.def_id),
+                            obligation.predicate,
+                        );
+                        false
+                    }
                 }
             }
             super::VtableParam(..) => {

src/librustc_typeck/check/mod.rs

@@ -1964,7 +1964,6 @@ fn check_impl_items_against_trait<'tcx>(
 
     // Locate trait definition and items
     let trait_def = tcx.trait_def(impl_trait_ref.def_id);
-    let mut overridden_associated_type = None;
 
     let impl_items = || impl_item_refs.iter().map(|iiref| tcx.hir().impl_item(iiref.id));
 
@@ -2046,9 +2045,6 @@ fn check_impl_items_against_trait<'tcx>(
                 hir::ImplItemKind::OpaqueTy(..) | hir::ImplItemKind::TyAlias(_) => {
                     let opt_trait_span = tcx.hir().span_if_local(ty_trait_item.def_id);
                     if ty_trait_item.kind == ty::AssocKind::Type {
-                        if ty_trait_item.defaultness.has_value() {
-                            overridden_associated_type = Some(impl_item);
-                        }
                         compare_ty_impl(
                             tcx,
                             &ty_impl_item,
@@ -2082,8 +2078,6 @@ fn check_impl_items_against_trait<'tcx>(
 
     // Check for missing items from trait
     let mut missing_items = Vec::new();
-    let mut invalidated_items = Vec::new();
-    let associated_type_overridden = overridden_associated_type.is_some();
     for trait_item in tcx.associated_items(impl_trait_ref.def_id).in_definition_order() {
         let is_implemented = trait_def
             .ancestors(tcx, impl_id)
@@ -2094,28 +2088,13 @@ fn check_impl_items_against_trait<'tcx>(
         if !is_implemented && !traits::impl_is_default(tcx, impl_id) {
             if !trait_item.defaultness.has_value() {
                 missing_items.push(*trait_item);
-            } else if associated_type_overridden {
-                invalidated_items.push(trait_item.ident);
             }
         }
     }
 
     if !missing_items.is_empty() {
         missing_items_err(tcx, impl_span, &missing_items, full_impl_span);
     }
-
-    if !invalidated_items.is_empty() {
-        let invalidator = overridden_associated_type.unwrap();
-        struct_span_err!(
-            tcx.sess,
-            invalidator.span,
-            E0399,
-            "the following trait items need to be reimplemented as `{}` was overridden: `{}`",
-            invalidator.ident,
-            invalidated_items.iter().map(|name| name.to_string()).collect::<Vec<_>>().join("`, `")
-        )
-        .emit();
-    }
 }
 
 fn missing_items_err(

src/librustc_typeck/check/wfcheck.rs

@@ -425,10 +425,109 @@ fn check_trait(tcx: TyCtxt<'_>, item: &hir::Item<'_>) {
 
     for_item(tcx, item).with_fcx(|fcx, _| {
         check_where_clauses(tcx, fcx, item.span, trait_def_id, None);
+        check_associated_type_defaults(fcx, trait_def_id);
+
         vec![]
     });
 }
 
+/// Checks all associated type defaults of trait `trait_def_id`.
+///
+/// Assuming the defaults are used, check that all predicates (bounds on the
+/// assoc type and where clauses on the trait) hold.
+fn check_associated_type_defaults(fcx: &FnCtxt<'_, '_>, trait_def_id: DefId) {
+    let tcx = fcx.tcx;
+    let substs = InternalSubsts::identity_for_item(tcx, trait_def_id);
+
+    // For all assoc. types with defaults, build a map from
+    // `<Self as Trait<...>>::Assoc` to the default type.
+    let map = tcx
+        .associated_items(trait_def_id)
+        .in_definition_order()
+        .filter_map(|item| {
+            if item.kind == ty::AssocKind::Type && item.defaultness.has_value() {
+                // `<Self as Trait<...>>::Assoc`
+                let proj = ty::ProjectionTy { substs, item_def_id: item.def_id };
+                let default_ty = tcx.type_of(item.def_id);
+                debug!("assoc. type default mapping: {} -> {}", proj, default_ty);
+                Some((proj, default_ty))
+            } else {
+                None
+            }
+        })
+        .collect::<FxHashMap<_, _>>();
+
+    /// Replaces projections of associated types with their default types.
+    ///
+    /// This does a "shallow substitution", meaning that defaults that refer to
+    /// other defaulted assoc. types will still refer to the projection
+    /// afterwards, not to the other default. For example:
+    ///
+    /// ```compile_fail
+    /// trait Tr {
+    ///     type A: Clone = Vec<Self::B>;
+    ///     type B = u8;
+    /// }
+    /// ```
+    ///
+    /// This will end up replacing the bound `Self::A: Clone` with
+    /// `Vec<Self::B>: Clone`, not with `Vec<u8>: Clone`. If we did a deep
+    /// substitution and ended up with the latter, the trait would be accepted.
+    /// If an `impl` then replaced `B` with something that isn't `Clone`,
+    /// suddenly the default for `A` is no longer valid. The shallow
+    /// substitution forces the trait to add a `B: Clone` bound to be accepted,
+    /// which means that an `impl` can replace any default without breaking
+    /// others.
+    ///
+    /// Note that this isn't needed for soundness: The defaults would still be
+    /// checked in any impl that doesn't override them.
+    struct DefaultNormalizer<'tcx> {
+        tcx: TyCtxt<'tcx>,
+        map: FxHashMap<ty::ProjectionTy<'tcx>, Ty<'tcx>>,
+    }
+
+    impl<'tcx> ty::fold::TypeFolder<'tcx> for DefaultNormalizer<'tcx> {
+        fn tcx<'a>(&'a self) -> TyCtxt<'tcx> {
+            self.tcx
+        }
+
+        fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
+            match t.kind {
+                ty::Projection(proj_ty) => {
+                    if let Some(default) = self.map.get(&proj_ty) {
+                        default
+                    } else {
+                        t.super_fold_with(self)
+                    }
+                }
+                _ => t.super_fold_with(self),
+            }
+        }
+    }
+
+    // Now take all predicates defined on the trait, replace any mention of
+    // the assoc. types with their default, and prove them.
+    // We only consider predicates that directly mention the assoc. type.
+    let mut norm = DefaultNormalizer { tcx, map };
+    let predicates = fcx.tcx.predicates_of(trait_def_id);
+    for &(orig_pred, span) in predicates.predicates.iter() {
+        let pred = orig_pred.fold_with(&mut norm);
+        if pred != orig_pred {
+            // Mentions one of the defaulted assoc. types
+            debug!("default suitability check: proving predicate: {} -> {}", orig_pred, pred);
+            let pred = fcx.normalize_associated_types_in(span, &pred);
+            let cause = traits::ObligationCause::new(
+                span,
+                fcx.body_id,
+                traits::ItemObligation(trait_def_id),
+            );
+            let obligation = traits::Obligation::new(cause, fcx.param_env, pred);
+
+            fcx.register_predicate(obligation);
+        }
+    }
+}
+
 fn check_item_fn(tcx: TyCtxt<'_>, item: &hir::Item<'_>) {
     for_item(tcx, item).with_fcx(|fcx, tcx| {
         let def_id = fcx.tcx.hir().local_def_id(item.hir_id);

src/test/ui/associated-const/defaults-cyclic-fail.rs

@@ -0,0 +1,17 @@
+// build-fail
+
+// Cyclic assoc. const defaults don't error unless *used*
+trait Tr {
+    const A: u8 = Self::B;
+    //~^ ERROR cycle detected when const-evaluating + checking `Tr::A`
+
+    const B: u8 = Self::A;
+}
+
+// This impl is *allowed* unless its assoc. consts are used
+impl Tr for () {}
+
+fn main() {
+    // This triggers the cycle error
+    assert_eq!(<() as Tr>::A, 0);
+}

src/test/ui/associated-const/defaults-cyclic-fail.stderr

@@ -0,0 +1,31 @@
+error[E0391]: cycle detected when const-evaluating + checking `Tr::A`
+  --> $DIR/defaults-cyclic-fail.rs:5:5
+   |
+LL |     const A: u8 = Self::B;
+   |     ^^^^^^^^^^^^^^^^^^^^^^
+   |
+note: ...which requires const-evaluating `Tr::A`...
+  --> $DIR/defaults-cyclic-fail.rs:5:19
+   |
+LL |     const A: u8 = Self::B;
+   |                   ^^^^^^^
+note: ...which requires const-evaluating + checking `Tr::B`...
+  --> $DIR/defaults-cyclic-fail.rs:8:5
+   |
+LL |     const B: u8 = Self::A;
+   |     ^^^^^^^^^^^^^^^^^^^^^^
+note: ...which requires const-evaluating `Tr::B`...
+  --> $DIR/defaults-cyclic-fail.rs:8:19
+   |
+LL |     const B: u8 = Self::A;
+   |                   ^^^^^^^
+   = note: ...which again requires const-evaluating + checking `Tr::A`, completing the cycle
+note: cycle used when const-evaluating `main`
+  --> $DIR/defaults-cyclic-fail.rs:16:16
+   |
+LL |     assert_eq!(<() as Tr>::A, 0);
+   |                ^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0391`.

src/test/ui/associated-const/defaults-cyclic-pass.rs

@@ -0,0 +1,36 @@
+// run-pass
+
+// Cyclic assoc. const defaults don't error unless *used*
+trait Tr {
+    const A: u8 = Self::B;
+    const B: u8 = Self::A;
+}
+
+// This impl is *allowed* unless its assoc. consts are used, matching the
+// behavior without defaults.
+impl Tr for () {}
+
+// Overriding either constant breaks the cycle
+impl Tr for u8 {
+    const A: u8 = 42;
+}
+
+impl Tr for u16 {
+    const B: u8 = 0;
+}
+
+impl Tr for u32 {
+    const A: u8 = 100;
+    const B: u8 = 123;
+}
+
+fn main() {
+    assert_eq!(<u8 as Tr>::A, 42);
+    assert_eq!(<u8 as Tr>::B, 42);
+
+    assert_eq!(<u16 as Tr>::A, 0);
+    assert_eq!(<u16 as Tr>::B, 0);
+
+    assert_eq!(<u32 as Tr>::A, 100);
+    assert_eq!(<u32 as Tr>::B, 123);
+}

src/test/ui/associated-const/defaults-not-assumed-fail.rs

@@ -0,0 +1,45 @@
+// build-fail
+
+trait Tr {
+    const A: u8 = 255;
+
+    // This should not be a constant evaluation error (overflow). The value of
+    // `Self::A` must not be assumed to hold inside the trait.
+    const B: u8 = Self::A + 1;
+    //~^ ERROR any use of this value will cause an error
+}
+
+// An impl that doesn't override any constant will NOT cause a const eval error
+// just because it's defined, but only if the bad constant is used anywhere.
+// This matches the behavior without defaults.
+impl Tr for () {}
+
+// An impl that overrides either constant with a suitable value will be fine.
+impl Tr for u8 {
+    const A: u8 = 254;
+}
+
+impl Tr for u16 {
+    const B: u8 = 0;
+}
+
+impl Tr for u32 {
+    const A: u8 = 254;
+    const B: u8 = 0;
+}
+
+fn main() {
+    assert_eq!(<() as Tr>::A, 255);
+    assert_eq!(<() as Tr>::B, 0);    // causes the error above
+    //~^ ERROR evaluation of constant expression failed
+    //~| ERROR erroneous constant used
+
+    assert_eq!(<u8 as Tr>::A, 254);
+    assert_eq!(<u8 as Tr>::B, 255);
+
+    assert_eq!(<u16 as Tr>::A, 255);
+    assert_eq!(<u16 as Tr>::B, 0);
+
+    assert_eq!(<u32 as Tr>::A, 254);
+    assert_eq!(<u32 as Tr>::B, 0);
+}

src/test/ui/associated-const/defaults-not-assumed-fail.stderr

@@ -0,0 +1,31 @@
+error: any use of this value will cause an error
+  --> $DIR/defaults-not-assumed-fail.rs:8:19
+   |
+LL |     const B: u8 = Self::A + 1;
+   |     --------------^^^^^^^^^^^-
+   |                   |
+   |                   attempt to add with overflow
+   |
+   = note: `#[deny(const_err)]` on by default
+
+error[E0080]: evaluation of constant expression failed
+  --> $DIR/defaults-not-assumed-fail.rs:33:5
+   |
+LL |     assert_eq!(<() as Tr>::B, 0);    // causes the error above
+   |     ^^^^^^^^^^^-------------^^^^^
+   |                |
+   |                referenced constant has errors
+   |
+   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: erroneous constant used
+  --> $DIR/defaults-not-assumed-fail.rs:33:5
+   |
+LL |     assert_eq!(<() as Tr>::B, 0);    // causes the error above
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ referenced constant has errors
+   |
+   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 3 previous errors
+
+For more information about this error, try `rustc --explain E0080`.