Implement trait upcasting coercion type-checking.

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

@@ -693,22 +693,12 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         let may_apply = match (source.kind(), target.kind()) {
             // Trait+Kx+'a -> Trait+Ky+'b (upcasts).
             (&ty::Dynamic(ref data_a, ..), &ty::Dynamic(ref data_b, ..)) => {
-                // Upcasts permit two 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`
-                //
-                // Note that neither of these changes requires any
-                // change at runtime. Eventually this will be
-                // generalized.
-                //
-                // We always upcast when we can because of reason
-                // #2 (region bounds).
-                data_a.principal_def_id() == data_b.principal_def_id()
-                    && 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))
+                // See `confirm_builtin_unsize_candidate` for more info.
+                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
             }
 
             // `T` -> `Trait`

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

@@ -703,10 +703,56 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         match (source.kind(), target.kind()) {
             // Trait+Kx+'a -> Trait+Ky+'b (upcasts).
             (&ty::Dynamic(ref data_a, r_a), &ty::Dynamic(ref data_b, r_b)) => {
-                // See `assemble_candidates_for_unsizing` for more info.
-                let iter = data_a
-                    .principal()
-                    .map(|b| b.map_bound(ty::ExistentialPredicate::Trait))
+                // 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).
+
+                // We already checked the compatiblity of auto traits within `assemble_candidates_for_unsizing`.
+
+                let principal_a = data_a.principal();
+                let principal_def_id_b = data_b.principal_def_id();
+
+                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_typeck/src/check/coercion.rs

@@ -576,6 +576,7 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
         )];
 
         let mut has_unsized_tuple_coercion = false;
+        let mut has_trait_upcasting_coercion = false;
 
         // Keep resolving `CoerceUnsized` and `Unsize` predicates to avoid
         // emitting a coercion in cases like `Foo<$1>` -> `Foo<$2>`, where
@@ -590,7 +591,16 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
                     if traits.contains(&trait_pred.def_id()) =>
                 {
                     if unsize_did == trait_pred.def_id() {
+                        let self_ty = trait_pred.self_ty();
                         let unsize_ty = trait_pred.trait_ref.substs[1].expect_ty();
+                        if let (ty::Dynamic(ref data_a, ..), ty::Dynamic(ref data_b, ..)) =
+                            (self_ty.kind(), unsize_ty.kind())
+                        {
+                            if data_a.principal_def_id() != data_b.principal_def_id() {
+                                debug!("coerce_unsized: found trait upcasting coercion");
+                                has_trait_upcasting_coercion = true;
+                            }
+                        }
                         if let ty::Tuple(..) = unsize_ty.kind() {
                             debug!("coerce_unsized: found unsized tuple coercion");
                             has_unsized_tuple_coercion = true;
@@ -666,6 +676,16 @@ impl<'f, 'tcx> Coerce<'f, 'tcx> {
             .emit();
         }
 
+        if has_trait_upcasting_coercion && !self.tcx().features().trait_upcasting {
+            feature_err(
+                &self.tcx.sess.parse_sess,
+                sym::trait_upcasting,
+                self.cause.span,
+                "trait upcasting coercion is experimental",
+            )
+            .emit();
+        }
+
         Ok(coercion)
     }
 

src/test/ui/feature-gates/feature-gate-trait_upcasting.rs

@@ -9,5 +9,5 @@ impl Bar for () {}
 fn main() {
     let bar: &dyn Bar = &();
     let foo: &dyn Foo = bar;
-    //~^ ERROR trait upcasting is experimental [E0658]
+    //~^ ERROR trait upcasting coercion is experimental [E0658]
 }

src/test/ui/feature-gates/feature-gate-trait_upcasting.stderr

@@ -1,4 +1,4 @@
-error[E0658]: trait upcasting is experimental
+error[E0658]: trait upcasting coercion is experimental
   --> $DIR/feature-gate-trait_upcasting.rs:11:25
    |
 LL |     let foo: &dyn Foo = bar;

src/test/ui/issues/issue-11515.rs

@@ -1,10 +1,10 @@
 #![feature(box_syntax)]
 
 struct Test {
-    func: Box<dyn FnMut() + 'static>
+    func: Box<dyn FnMut() + 'static>,
 }
 
 fn main() {
     let closure: Box<dyn Fn() + 'static> = Box::new(|| ());
-    let test = box Test { func: closure }; //~ ERROR trait upcasting is experimental [E0658]
+    let test = box Test { func: closure }; //~ ERROR trait upcasting coercion is experimental [E0658]
 }

src/test/ui/issues/issue-11515.stderr

@@ -1,4 +1,4 @@
-error[E0658]: trait upcasting is experimental
+error[E0658]: trait upcasting coercion is experimental
   --> $DIR/issue-11515.rs:9:33
    |
 LL |     let test = box Test { func: closure };

src/test/ui/traits/trait-upcasting/issue-11515-upcast-fn_mut-fn.rs

@@ -0,0 +1,13 @@
+// run-pass
+#![feature(box_syntax, trait_upcasting)]
+#![allow(incomplete_features)]
+
+struct Test {
+    func: Box<dyn FnMut() + 'static>,
+}
+
+fn main() {
+    let closure: Box<dyn Fn() + 'static> = Box::new(|| ());
+    let mut test = box Test { func: closure };
+    (test.func)();
+}

src/test/ui/traits/trait-upcasting/multiple-occurence-ambiguousity.rs

@@ -0,0 +1,22 @@
+// check-fail
+#![feature(trait_upcasting)]
+#![allow(incomplete_features)]
+
+trait Bar<T> {
+    fn bar(&self, _: T) {}
+}
+
+trait Foo : Bar<i32> + Bar<u32> {
+    fn foo(&self, _: ()) {}
+}
+
+struct S;
+
+impl Bar<i32> for S {}
+impl Bar<u32> for S {}
+impl Foo for S {}
+
+fn main() {
+    let s: &dyn Foo = &S;
+    let t: &dyn Bar<_> = s; //~ ERROR mismatched types
+}

src/test/ui/traits/trait-upcasting/multiple-occurence-ambiguousity.stderr

@@ -0,0 +1,14 @@
+error[E0308]: mismatched types
+  --> $DIR/multiple-occurence-ambiguousity.rs:21:26
+   |
+LL |     let t: &dyn Bar<_> = s;
+   |            -----------   ^ expected trait `Bar`, found trait `Foo`
+   |            |
+   |            expected due to this
+   |
+   = note: expected reference `&dyn Bar<_>`
+              found reference `&dyn Foo`
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0308`.