Properly generate multiple candidates for trait upcasting coercion.

compiler/rustc_middle/src/traits/select.rs

@@ -135,6 +135,11 @@ pub enum SelectionCandidate<'tcx> {
     /// `rustc_infer::traits::util::supertraits`.
     ObjectCandidate(usize),
 
+    /// Perform trait upcasting coercion of `dyn Trait` to a supertrait of `Trait`.
+    /// The index is the position in the iterator returned by
+    /// `rustc_infer::traits::util::supertraits`.
+    TraitUpcastingUnsizeCandidate(usize),
+
     BuiltinObjectCandidate,
 
     BuiltinUnsizeCandidate,

compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs

@@ -690,19 +690,50 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
         debug!(?source, ?target, "assemble_candidates_for_unsizing");
 
-        let may_apply = match (source.kind(), target.kind()) {
+        match (source.kind(), target.kind()) {
             // Trait+Kx+'a -> Trait+Ky+'b (upcasts).
             (&ty::Dynamic(ref data_a, ..), &ty::Dynamic(ref data_b, ..)) => {
-                // See `confirm_builtin_unsize_candidate` for more info.
+                // Upcast coercions permit several things:
+                //
+                // 1. Dropping auto traits, e.g., `Foo + Send` to `Foo`
+                // 2. Tightening the region bound, e.g., `Foo + 'a` to `Foo + 'b` if `'a: 'b`
+                // 3. Tightening trait to its super traits, eg. `Foo` to `Bar` if `Foo: Bar`
+                //
+                // Note that neither of the first two of these changes requires any
+                // change at runtime. The third needs to change pointer metadata at runtime.
+                //
+                // We always perform upcasting coercions when we can because of reason
+                // #2 (region bounds).
                 let auto_traits_compatible = data_b
                     .auto_traits()
                     // All of a's auto traits need to be in b's auto traits.
                     .all(|b| data_a.auto_traits().any(|a| a == b));
-                auto_traits_compatible
+                if auto_traits_compatible {
+                    let principal_def_id_a = data_a.principal_def_id();
+                    let principal_def_id_b = data_b.principal_def_id();
+                    if principal_def_id_a == principal_def_id_b {
+                        // no cyclic
+                        candidates.vec.push(BuiltinUnsizeCandidate);
+                    } else if principal_def_id_a.is_some() && principal_def_id_b.is_some() {
+                        // not casual unsizing, now check whether this is trait upcasting coercion.
+                        let principal_a = data_a.principal().unwrap();
+                        let target_trait_did = principal_def_id_b.unwrap();
+                        let source_trait_ref = principal_a.with_self_ty(self.tcx(), source);
+                        for (idx, upcast_trait_ref) in
+                            util::supertraits(self.tcx(), source_trait_ref).enumerate()
+                        {
+                            if upcast_trait_ref.def_id() == target_trait_did {
+                                candidates.vec.push(TraitUpcastingUnsizeCandidate(idx));
+                            }
+                        }
+                    }
+                }
             }
 
             // `T` -> `Trait`
-            (_, &ty::Dynamic(..)) => true,
+            (_, &ty::Dynamic(..)) => {
+                candidates.vec.push(BuiltinUnsizeCandidate);
+            }
 
             // Ambiguous handling is below `T` -> `Trait`, because inference
             // variables can still implement `Unsize<Trait>` and nested
@@ -710,26 +741,29 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             (&ty::Infer(ty::TyVar(_)), _) | (_, &ty::Infer(ty::TyVar(_))) => {
                 debug!("assemble_candidates_for_unsizing: ambiguous");
                 candidates.ambiguous = true;
-                false
             }
 
             // `[T; n]` -> `[T]`
-            (&ty::Array(..), &ty::Slice(_)) => true,
+            (&ty::Array(..), &ty::Slice(_)) => {
+                candidates.vec.push(BuiltinUnsizeCandidate);
+            }
 
             // `Struct<T>` -> `Struct<U>`
             (&ty::Adt(def_id_a, _), &ty::Adt(def_id_b, _)) if def_id_a.is_struct() => {
-                def_id_a == def_id_b
+                if def_id_a == def_id_b {
+                    candidates.vec.push(BuiltinUnsizeCandidate);
+                }
             }
 
             // `(.., T)` -> `(.., U)`
-            (&ty::Tuple(tys_a), &ty::Tuple(tys_b)) => tys_a.len() == tys_b.len(),
+            (&ty::Tuple(tys_a), &ty::Tuple(tys_b)) => {
+                if tys_a.len() == tys_b.len() {
+                    candidates.vec.push(BuiltinUnsizeCandidate);
+                }
+            }
 
-            _ => false,
+            _ => {}
         };
-
-        if may_apply {
-            candidates.vec.push(BuiltinUnsizeCandidate);
-        }
     }
 
     fn assemble_candidates_for_trait_alias(

compiler/rustc_trait_selection/src/traits/select/confirmation.rs

@@ -118,6 +118,11 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 let data = self.confirm_builtin_unsize_candidate(obligation)?;
                 Ok(ImplSource::Builtin(data))
             }
+
+            TraitUpcastingUnsizeCandidate(idx) => {
+                let data = self.confirm_trait_upcasting_unsize_candidate(obligation, idx)?;
+                Ok(ImplSource::Builtin(data))
+            }
         }
     }
 
@@ -685,6 +690,81 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
             .map_err(|e| OutputTypeParameterMismatch(expected_trait_ref, obligation_trait_ref, e))
     }
 
+    fn confirm_trait_upcasting_unsize_candidate(
+        &mut self,
+        obligation: &TraitObligation<'tcx>,
+        idx: usize,
+    ) -> Result<ImplSourceBuiltinData<PredicateObligation<'tcx>>, SelectionError<'tcx>> {
+        let tcx = self.tcx();
+
+        // `assemble_candidates_for_unsizing` should ensure there are no late-bound
+        // regions here. See the comment there for more details.
+        let source = self.infcx.shallow_resolve(obligation.self_ty().no_bound_vars().unwrap());
+        let target = obligation.predicate.skip_binder().trait_ref.substs.type_at(1);
+        let target = self.infcx.shallow_resolve(target);
+
+        debug!(?source, ?target, "confirm_trait_upcasting_unsize_candidate");
+
+        let mut nested = vec![];
+        match (source.kind(), target.kind()) {
+            // TraitA+Kx+'a -> TraitB+Ky+'b (trait upcasting coercion).
+            (&ty::Dynamic(ref data_a, r_a), &ty::Dynamic(ref data_b, r_b)) => {
+                // See `assemble_candidates_for_unsizing` for more info.
+                // We already checked the compatiblity of auto traits within `assemble_candidates_for_unsizing`.
+                let principal_a = data_a.principal().unwrap();
+                let source_trait_ref = principal_a.with_self_ty(tcx, source);
+                let target_trait_ref = util::supertraits(tcx, source_trait_ref).nth(idx).unwrap();
+                assert_eq!(data_b.principal_def_id(), Some(target_trait_ref.def_id()));
+                let existential_predicate = target_trait_ref.map_bound(|trait_ref| {
+                    ty::ExistentialPredicate::Trait(ty::ExistentialTraitRef::erase_self_ty(
+                        tcx, trait_ref,
+                    ))
+                });
+                let iter = Some(existential_predicate)
+                    .into_iter()
+                    .chain(
+                        data_a
+                            .projection_bounds()
+                            .map(|b| b.map_bound(ty::ExistentialPredicate::Projection)),
+                    )
+                    .chain(
+                        data_b
+                            .auto_traits()
+                            .map(ty::ExistentialPredicate::AutoTrait)
+                            .map(ty::Binder::dummy),
+                    );
+                let existential_predicates = tcx.mk_poly_existential_predicates(iter);
+                let source_trait = tcx.mk_dynamic(existential_predicates, r_b);
+
+                // Require that the traits involved in this upcast are **equal**;
+                // only the **lifetime bound** is changed.
+                let InferOk { obligations, .. } = self
+                    .infcx
+                    .at(&obligation.cause, obligation.param_env)
+                    .sup(target, source_trait)
+                    .map_err(|_| Unimplemented)?;
+                nested.extend(obligations);
+
+                // Register one obligation for 'a: 'b.
+                let cause = ObligationCause::new(
+                    obligation.cause.span,
+                    obligation.cause.body_id,
+                    ObjectCastObligation(target),
+                );
+                let outlives = ty::OutlivesPredicate(r_a, r_b);
+                nested.push(Obligation::with_depth(
+                    cause,
+                    obligation.recursion_depth + 1,
+                    obligation.param_env,
+                    obligation.predicate.rebind(outlives).to_predicate(tcx),
+                ));
+            }
+            _ => bug!(),
+        };
+
+        Ok(ImplSourceBuiltinData { nested })
+    }
+
     fn confirm_builtin_unsize_candidate(
         &mut self,
         obligation: &TraitObligation<'tcx>,
@@ -701,58 +781,13 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
 
         let mut nested = vec![];
         match (source.kind(), target.kind()) {
-            // Trait+Kx+'a -> Trait+Ky+'b (upcasts).
+            // Trait+Kx+'a -> Trait+Ky+'b (auto traits and lifetime subtyping).
             (&ty::Dynamic(ref data_a, r_a), &ty::Dynamic(ref data_b, r_b)) => {
-                // Upcast coercions permit several things:
+                // See `assemble_candidates_for_unsizing` for more info.
-                //
-                // 1. Dropping auto traits, e.g., `Foo + Send` to `Foo`
-                // 2. Tightening the region bound, e.g., `Foo + 'a` to `Foo + 'b` if `'a: 'b`
-                // 3. Tightening trait to its super traits, eg. `Foo` to `Bar` if `Foo: Bar`
-                //
-                // Note that neither of the first two of these changes requires any
-                // change at runtime. The third needs to change pointer metadata at runtime.
-                //
-                // We always perform upcasting coercions when we can because of reason
-                // #2 (region bounds).
-
                 // We already checked the compatiblity of auto traits within `assemble_candidates_for_unsizing`.
-
+                let iter = data_a
-                let principal_a = data_a.principal();
+                    .principal()
-                let principal_def_id_b = data_b.principal_def_id();
+                    .map(|b| b.map_bound(ty::ExistentialPredicate::Trait))
-
-                let existential_predicate = if let Some(principal_a) = principal_a {
-                    let source_trait_ref = principal_a.with_self_ty(tcx, source);
-                    let target_trait_did = principal_def_id_b.ok_or_else(|| Unimplemented)?;
-                    let upcast_idx = util::supertraits(tcx, source_trait_ref)
-                        .position(|upcast_trait_ref| upcast_trait_ref.def_id() == target_trait_did)
-                        .ok_or_else(|| Unimplemented)?;
-                    // FIXME(crlf0710): This is less than ideal, for example,
-                    // if the trait is defined as `trait Foo: Bar<u32> + Bar<i32>`,
-                    // the coercion from Box<Foo> to Box<dyn Bar<_>> is actually ambiguous.
-                    // We currently make this coercion fail for now.
-                    //
-                    // see #65991 for more information.
-                    if util::supertraits(tcx, source_trait_ref)
-                        .skip(upcast_idx + 1)
-                        .any(|upcast_trait_ref| upcast_trait_ref.def_id() == target_trait_did)
-                    {
-                        return Err(Unimplemented);
-                    }
-                    let target_trait_ref =
-                        util::supertraits(tcx, source_trait_ref).nth(upcast_idx).unwrap();
-                    let existential_predicate = target_trait_ref.map_bound(|trait_ref| {
-                        ty::ExistentialPredicate::Trait(ty::ExistentialTraitRef::erase_self_ty(
-                            tcx, trait_ref,
-                        ))
-                    });
-                    Some(existential_predicate)
-                } else if principal_def_id_b.is_none() {
-                    None
-                } else {
-                    return Err(Unimplemented);
-                };
-
-                let iter = existential_predicate
                     .into_iter()
                     .chain(
                         data_a

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -1500,6 +1500,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 | FnPointerCandidate
                 | BuiltinObjectCandidate
                 | BuiltinUnsizeCandidate
+                | TraitUpcastingUnsizeCandidate(_)
                 | BuiltinCandidate { .. }
                 | TraitAliasCandidate(..)
                 | ObjectCandidate(_)
@@ -1517,6 +1518,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 | FnPointerCandidate
                 | BuiltinObjectCandidate
                 | BuiltinUnsizeCandidate
+                | TraitUpcastingUnsizeCandidate(_)
                 | BuiltinCandidate { has_nested: true }
                 | TraitAliasCandidate(..),
                 ParamCandidate(ref cand),
@@ -1546,6 +1548,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 | FnPointerCandidate
                 | BuiltinObjectCandidate
                 | BuiltinUnsizeCandidate
+                | TraitUpcastingUnsizeCandidate(_)
                 | BuiltinCandidate { .. }
                 | TraitAliasCandidate(..),
             ) => true,
@@ -1557,6 +1560,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 | FnPointerCandidate
                 | BuiltinObjectCandidate
                 | BuiltinUnsizeCandidate
+                | TraitUpcastingUnsizeCandidate(_)
                 | BuiltinCandidate { .. }
                 | TraitAliasCandidate(..),
                 ObjectCandidate(_) | ProjectionCandidate(_),
@@ -1630,6 +1634,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 | FnPointerCandidate
                 | BuiltinObjectCandidate
                 | BuiltinUnsizeCandidate
+                | TraitUpcastingUnsizeCandidate(_)
                 | BuiltinCandidate { has_nested: true }
                 | TraitAliasCandidate(..),
                 ImplCandidate(_)
@@ -1638,6 +1643,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                 | FnPointerCandidate
                 | BuiltinObjectCandidate
                 | BuiltinUnsizeCandidate
+                | TraitUpcastingUnsizeCandidate(_)
                 | BuiltinCandidate { has_nested: true }
                 | TraitAliasCandidate(..),
             ) => false,

src/test/ui/traits/trait-upcasting/type-checking-test-1.rs

@@ -9,12 +9,8 @@ trait Bar<T> {
 }
 
 fn test_specific(x: &dyn Foo) {
-    let _ = x as &dyn Bar<i32>; // FIXME: OK, eventually
+    let _ = x as &dyn Bar<i32>; // OK
-                                //~^ ERROR non-primitive cast
+    let _ = x as &dyn Bar<u32>; // OK
-                                //~^^ ERROR the trait bound `&dyn Foo: Bar<i32>` is not satisfied
-    let _ = x as &dyn Bar<u32>; // FIXME: OK, eventually
-                                //~^ ERROR non-primitive cast
-                                //~^^ ERROR the trait bound `&dyn Foo: Bar<u32>` is not satisfied
 }
 
 fn test_unknown_version(x: &dyn Foo) {
@@ -24,9 +20,7 @@ fn test_unknown_version(x: &dyn Foo) {
 }
 
 fn test_infer_version(x: &dyn Foo) {
-    let a = x as &dyn Bar<_>; // FIXME: OK, eventually
+    let a = x as &dyn Bar<_>; // OK
-                              //~^ ERROR non-primitive cast
-                              //~^^ ERROR the trait bound `&dyn Foo: Bar<u32>` is not satisfied
     let _: Option<u32> = a.bar();
 }
 

src/test/ui/traits/trait-upcasting/type-checking-test-1.stderr

@@ -1,43 +1,5 @@
-error[E0605]: non-primitive cast: `&dyn Foo` as `&dyn Bar<i32>`
-  --> $DIR/type-checking-test-1.rs:12:13
-   |
-LL |     let _ = x as &dyn Bar<i32>; // FIXME: OK, eventually
-   |             ^^^^^^^^^^^^^^^^^^ invalid cast
-   |
-help: consider borrowing the value
-   |
-LL |     let _ = &x as &dyn Bar<i32>; // FIXME: OK, eventually
-   |             +
-
-error[E0605]: non-primitive cast: `&dyn Foo` as `&dyn Bar<u32>`
-  --> $DIR/type-checking-test-1.rs:15:13
-   |
-LL |     let _ = x as &dyn Bar<u32>; // FIXME: OK, eventually
-   |             ^^^^^^^^^^^^^^^^^^ invalid cast
-   |
-help: consider borrowing the value
-   |
-LL |     let _ = &x as &dyn Bar<u32>; // FIXME: OK, eventually
-   |             +
-
-error[E0277]: the trait bound `&dyn Foo: Bar<i32>` is not satisfied
-  --> $DIR/type-checking-test-1.rs:12:13
-   |
-LL |     let _ = x as &dyn Bar<i32>; // FIXME: OK, eventually
-   |             ^ the trait `Bar<i32>` is not implemented for `&dyn Foo`
-   |
-   = note: required for the cast to the object type `dyn Bar<i32>`
-
-error[E0277]: the trait bound `&dyn Foo: Bar<u32>` is not satisfied
-  --> $DIR/type-checking-test-1.rs:15:13
-   |
-LL |     let _ = x as &dyn Bar<u32>; // FIXME: OK, eventually
-   |             ^ the trait `Bar<u32>` is not implemented for `&dyn Foo`
-   |
-   = note: required for the cast to the object type `dyn Bar<u32>`
-
 error[E0605]: non-primitive cast: `&dyn Foo` as `&dyn Bar<_>`
-  --> $DIR/type-checking-test-1.rs:21:13
+  --> $DIR/type-checking-test-1.rs:17:13
    |
 LL |     let _ = x as &dyn Bar<_>; // Ambiguous
    |             ^^^^^^^^^^^^^^^^ invalid cast
@@ -48,33 +10,14 @@ LL |     let _ = &x as &dyn Bar<_>; // Ambiguous
    |             +
 
 error[E0277]: the trait bound `&dyn Foo: Bar<_>` is not satisfied
-  --> $DIR/type-checking-test-1.rs:21:13
+  --> $DIR/type-checking-test-1.rs:17:13
    |
 LL |     let _ = x as &dyn Bar<_>; // Ambiguous
    |             ^ the trait `Bar<_>` is not implemented for `&dyn Foo`
    |
    = note: required for the cast to the object type `dyn Bar<_>`
 
-error[E0605]: non-primitive cast: `&dyn Foo` as `&dyn Bar<u32>`
+error: aborting due to 2 previous errors
-  --> $DIR/type-checking-test-1.rs:27:13
-   |
-LL |     let a = x as &dyn Bar<_>; // FIXME: OK, eventually
-   |             ^^^^^^^^^^^^^^^^ invalid cast
-   |
-help: consider borrowing the value
-   |
-LL |     let a = &x as &dyn Bar<_>; // FIXME: OK, eventually
-   |             +
-
-error[E0277]: the trait bound `&dyn Foo: Bar<u32>` is not satisfied
-  --> $DIR/type-checking-test-1.rs:27:13
-   |
-LL |     let a = x as &dyn Bar<_>; // FIXME: OK, eventually
-   |             ^ the trait `Bar<u32>` is not implemented for `&dyn Foo`
-   |
-   = note: required for the cast to the object type `dyn Bar<u32>`
-
-error: aborting due to 8 previous errors
 
 Some errors have detailed explanations: E0277, E0605.
 For more information about an error, try `rustc --explain E0277`.

src/test/ui/traits/trait-upcasting/type-checking-test-2.rs

@@ -13,9 +13,7 @@ fn test_specific(x: &dyn Foo<i32>) {
 }
 
 fn test_specific2(x: &dyn Foo<u32>) {
-    let _ = x as &dyn Bar<i32>; // FIXME: OK, eventually
+    let _ = x as &dyn Bar<i32>; // OK
-                                //~^ ERROR non-primitive cast
-                                //~^^ ERROR the trait bound `&dyn Foo<u32>: Bar<i32>` is not satisfied
 }
 
 fn test_specific3(x: &dyn Foo<i32>) {

src/test/ui/traits/trait-upcasting/type-checking-test-2.stderr

@@ -1,24 +1,5 @@
-error[E0605]: non-primitive cast: `&dyn Foo<u32>` as `&dyn Bar<i32>`
-  --> $DIR/type-checking-test-2.rs:16:13
-   |
-LL |     let _ = x as &dyn Bar<i32>; // FIXME: OK, eventually
-   |             ^^^^^^^^^^^^^^^^^^ invalid cast
-   |
-help: consider borrowing the value
-   |
-LL |     let _ = &x as &dyn Bar<i32>; // FIXME: OK, eventually
-   |             +
-
-error[E0277]: the trait bound `&dyn Foo<u32>: Bar<i32>` is not satisfied
-  --> $DIR/type-checking-test-2.rs:16:13
-   |
-LL |     let _ = x as &dyn Bar<i32>; // FIXME: OK, eventually
-   |             ^ the trait `Bar<i32>` is not implemented for `&dyn Foo<u32>`
-   |
-   = note: required for the cast to the object type `dyn Bar<i32>`
-
 error[E0605]: non-primitive cast: `&dyn Foo<i32>` as `&dyn Bar<u32>`
-  --> $DIR/type-checking-test-2.rs:22:13
+  --> $DIR/type-checking-test-2.rs:20:13
    |
 LL |     let _ = x as &dyn Bar<u32>; // Error
    |             ^^^^^^^^^^^^^^^^^^ invalid cast
@@ -29,7 +10,7 @@ LL |     let _ = &x as &dyn Bar<u32>; // Error
    |             +
 
 error[E0277]: the trait bound `&dyn Foo<i32>: Bar<u32>` is not satisfied
-  --> $DIR/type-checking-test-2.rs:22:13
+  --> $DIR/type-checking-test-2.rs:20:13
    |
 LL |     let _ = x as &dyn Bar<u32>; // Error
    |             ^ the trait `Bar<u32>` is not implemented for `&dyn Foo<i32>`
@@ -37,7 +18,7 @@ LL |     let _ = x as &dyn Bar<u32>; // Error
    = note: required for the cast to the object type `dyn Bar<u32>`
 
 error[E0605]: non-primitive cast: `&dyn Foo<u32>` as `&dyn Bar<_>`
-  --> $DIR/type-checking-test-2.rs:28:13
+  --> $DIR/type-checking-test-2.rs:26:13
    |
 LL |     let a = x as &dyn Bar<_>; // Ambiguous
    |             ^^^^^^^^^^^^^^^^ invalid cast
@@ -48,14 +29,14 @@ LL |     let a = &x as &dyn Bar<_>; // Ambiguous
    |             +
 
 error[E0277]: the trait bound `&dyn Foo<u32>: Bar<_>` is not satisfied
-  --> $DIR/type-checking-test-2.rs:28:13
+  --> $DIR/type-checking-test-2.rs:26:13
    |
 LL |     let a = x as &dyn Bar<_>; // Ambiguous
    |             ^ the trait `Bar<_>` is not implemented for `&dyn Foo<u32>`
    |
    = note: required for the cast to the object type `dyn Bar<_>`
 
-error: aborting due to 6 previous errors
+error: aborting due to 4 previous errors
 
 Some errors have detailed explanations: E0277, E0605.
 For more information about an error, try `rustc --explain E0277`.