Probe when assembling upcast candidates so they don't step on eachother's toes

Author: compiler-errors

Diff for: compiler/rustc_trait_selection/src/solve/trait_goals.rs

@@ -552,16 +552,18 @@ impl<'tcx> EvalCtxt<'_, 'tcx> {
             self.walk_vtable(
                 a_principal.with_self_ty(tcx, a_ty),
                 |ecx, new_a_principal, _, vtable_vptr_slot| {
-                    if let Ok(resp) = ecx.consider_builtin_upcast_to_principal(
-                        goal,
-                        a_data,
-                        a_region,
-                        b_data,
-                        b_region,
-                        Some(new_a_principal.map_bound(|trait_ref| {
-                            ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref)
-                        })),
-                    ) {
+                    if let Ok(resp) = ecx.probe_candidate("dyn upcast").enter(|ecx| {
+                        ecx.consider_builtin_upcast_to_principal(
+                            goal,
+                            a_data,
+                            a_region,
+                            b_data,
+                            b_region,
+                            Some(new_a_principal.map_bound(|trait_ref| {
+                                ty::ExistentialTraitRef::erase_self_ty(tcx, trait_ref)
+                            })),
+                        )
+                    }) {
                         responses
                             .push((resp, BuiltinImplSource::TraitUpcasting { vtable_vptr_slot }));
                     }

Diff for: tests/ui/traits/trait-upcasting/type-checking-test-1.stderr renamed to tests/ui/traits/trait-upcasting/type-checking-test-1.current.stderr

@@ -1,5 +1,5 @@
 error[E0605]: non-primitive cast: `&dyn Foo` as `&dyn Bar<_>`
-  --> $DIR/type-checking-test-1.rs:16:13
+  --> $DIR/type-checking-test-1.rs:19:13
    |
 LL |     let _ = x as &dyn Bar<_>; // Ambiguous
    |             ^^^^^^^^^^^^^^^^ invalid cast
@@ -10,7 +10,7 @@ LL |     let _ = &x as &dyn Bar<_>; // Ambiguous
    |             +
 
 error[E0277]: the trait bound `&dyn Foo: Bar<_>` is not satisfied
-  --> $DIR/type-checking-test-1.rs:16:13
+  --> $DIR/type-checking-test-1.rs:19:13
    |
 LL |     let _ = x as &dyn Bar<_>; // Ambiguous
    |             ^ the trait `Bar<_>` is not implemented for `&dyn Foo`

Diff for: tests/ui/traits/trait-upcasting/type-checking-test-1.next.stderr

@@ -0,0 +1,9 @@
+error[E0605]: non-primitive cast: `&dyn Foo` as `&dyn Bar<_>`
+  --> $DIR/type-checking-test-1.rs:19:13
+   |
+LL |     let _ = x as &dyn Bar<_>; // Ambiguous
+   |             ^^^^^^^^^^^^^^^^ an `as` expression can only be used to convert between primitive types or to coerce to a specific trait object
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0605`.

Diff for: tests/ui/traits/trait-upcasting/type-checking-test-1.rs

@@ -1,3 +1,6 @@
+// revisions: current next
+//[next] compile-flags: -Ztrait-solver=next
+
 #![feature(trait_upcasting)]
 
 trait Foo: Bar<i32> + Bar<u32> {}
@@ -15,7 +18,7 @@ fn test_specific(x: &dyn Foo) {
 fn test_unknown_version(x: &dyn Foo) {
     let _ = x as &dyn Bar<_>; // Ambiguous
                               //~^ ERROR non-primitive cast
-                              //~^^ ERROR the trait bound `&dyn Foo: Bar<_>` is not satisfied
+                              //[current]~^^ ERROR the trait bound `&dyn Foo: Bar<_>` is not satisfied
 }
 
 fn test_infer_version(x: &dyn Foo) {