Better error message for object type with GAT

Author: compiler-errors

Diff for: compiler/rustc_trait_selection/src/traits/select/confirmation.rs

@@ -548,7 +548,7 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
                     obligation.cause.span,
                     "GATs in trait object shouldn't have been considered",
                 );
-                return Err(SelectionError::Unimplemented);
+                return Err(SelectionError::TraitNotObjectSafe(trait_predicate.trait_ref.def_id));
             }
 
             // This maybe belongs in wf, but that can't (doesn't) handle

Diff for: tests/ui/impl-trait/in-trait/object-safety.rs

@@ -19,5 +19,5 @@ fn main() {
     //~| ERROR the trait `Foo` cannot be made into an object
     let s = i.baz();
     //~^ ERROR the trait `Foo` cannot be made into an object
-    //~| ERROR the trait bound `dyn Foo: Foo`
+    //~| ERROR the trait `Foo` cannot be made into an object
 }

Diff for: tests/ui/impl-trait/in-trait/object-safety.stderr

@@ -13,13 +13,20 @@ LL |     fn baz(&self) -> impl Debug;
    |                      ^^^^^^^^^^ ...because method `baz` references an `impl Trait` type in its return type
    = help: consider moving `baz` to another trait
 
-error[E0277]: the trait bound `dyn Foo: Foo` is not satisfied
+error[E0038]: the trait `Foo` cannot be made into an object
   --> $DIR/object-safety.rs:20:15
    |
 LL |     let s = i.baz();
-   |               ^^^ the trait `Foo` is not implemented for `dyn Foo`
+   |               ^^^ `Foo` cannot be made into an object
    |
-   = help: the trait `Foo` is implemented for `u32`
+note: for a trait to be "object safe" 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>
+  --> $DIR/object-safety.rs:7:22
+   |
+LL | trait Foo {
+   |       --- this trait cannot be made into an object...
+LL |     fn baz(&self) -> impl Debug;
+   |                      ^^^^^^^^^^ ...because method `baz` references an `impl Trait` type in its return type
+   = help: consider moving `baz` to another trait
 
 error[E0038]: the trait `Foo` cannot be made into an object
   --> $DIR/object-safety.rs:20:13
@@ -54,5 +61,4 @@ LL |     fn baz(&self) -> impl Debug;
 
 error: aborting due to 4 previous errors
 
-Some errors have detailed explanations: E0038, E0277.
-For more information about an error, try `rustc --explain E0038`.
+For more information about this error, try `rustc --explain E0038`.