Recognize type Alias = dyn Trait in fn return types

```
error[E0277]: the size for values of type `(dyn Debug + 'static)` cannot be known at compilation time
  --> $DIR/dyn-trait-type-alias-return-type.rs:4:11
   |
LL | fn f() -> T { loop {} }
   |           ^ doesn't have a size known at compile-time
   |
   = help: the trait `Sized` is not implemented for `(dyn Debug + 'static)`
note: this type alias is unsized
  --> $DIR/dyn-trait-type-alias-return-type.rs:1:1
   |
LL | type T = dyn core::fmt::Debug;
   | ^^^^^^
   = note: the return type of a function must have a statically known size
```

Author: estebank

compiler/rustc_trait_selection/src/error_reporting/traits/suggestions.rs

@@ -1881,6 +1881,19 @@ impl<'a, 'tcx> TypeErrCtxt<'a, 'tcx> {
         let ty::Dynamic(_, _) = trait_pred.self_ty().skip_binder().kind() else {
             return false;
         };
+        if let Node::Item(hir::Item { kind: hir::ItemKind::Fn { sig: fn_sig, .. }, .. }) =
+            self.tcx.hir_node_by_def_id(obligation.cause.body_id)
+            && let hir::FnRetTy::Return(ty) = fn_sig.decl.output
+            && let hir::TyKind::Path(qpath) = ty.kind
+            && let hir::QPath::Resolved(None, path) = qpath
+            && let Res::Def(DefKind::TyAlias, def_id) = path.res
+        {
+            // Do not suggest
+            // type T = dyn Trait;
+            // fn foo() -> impl T { .. }
+            err.span_note(self.tcx.def_span(def_id), "this type alias is unsized");
+            return false;
+        }
 
         err.code(E0746);
         err.primary_message("return type cannot be a trait object without pointer indirection");

tests/ui/type-alias/dyn-trait-type-alias-return-type.rs

@@ -1,8 +1,10 @@
 type T = dyn core::fmt::Debug;
- 
+//~^ NOTE this type alias is unsized
+
 fn f() -> T { loop {} }
-//~^ ERROR return type cannot be a trait object without pointer indirection
-//~| HELP 
-//~| HELP 
+//~^ ERROR the size for values of type `(dyn Debug + 'static)` cannot be known at compilation time
+//~| HELP the trait `Sized` is not implemented for `(dyn Debug + 'static)`
+//~| NOTE doesn't have a size known at compile-time
+//~| NOTE the return type of a function must have a statically known size
 
 fn main() {}

tests/ui/type-alias/dyn-trait-type-alias-return-type.stderr

@@ -1,18 +1,17 @@
-error[E0746]: return type cannot be a trait object without pointer indirection
-  --> $DIR/dyn-trait-type-alias-return-type.rs:3:11
+error[E0277]: the size for values of type `(dyn Debug + 'static)` cannot be known at compilation time
+  --> $DIR/dyn-trait-type-alias-return-type.rs:4:11
    |
 LL | fn f() -> T { loop {} }
    |           ^ doesn't have a size known at compile-time
    |
-help: consider returning an `impl Trait` instead of a `dyn Trait`
+   = help: the trait `Sized` is not implemented for `(dyn Debug + 'static)`
+note: this type alias is unsized
+  --> $DIR/dyn-trait-type-alias-return-type.rs:1:1
    |
-LL | fn f() -> impl T { loop {} }
-   |           ++++
-help: alternatively, box the return type, and wrap all of the returned values in `Box::new`
-   |
-LL | fn f() -> Box<dyn T> { Box::new(loop {}) }
-   |           +++++++  +   +++++++++       +
+LL | type T = dyn core::fmt::Debug;
+   | ^^^^^^
+   = note: the return type of a function must have a statically known size
 
 error: aborting due to 1 previous error
 
-For more information about this error, try `rustc --explain E0746`.
+For more information about this error, try `rustc --explain E0277`.

tests/ui/unsized/issue-91801.rs

@@ -6,7 +6,7 @@ pub static ALL_VALIDATORS: &[(&'static str, &'static Validator)] =
     &[("validate that credits and debits balance", &validate_something)];
 
 fn or<'a>(first: &'static Validator<'a>, second: &'static Validator<'a>) -> Validator<'a> {
-    //~^ ERROR return type cannot be a trait object without pointer indirection
+    //~^ ERROR E0277
     return Box::new(move |something: &'_ Something| -> Result<(), ()> {
         first(something).or_else(|_| second(something))
     });

tests/ui/unsized/issue-91801.stderr

@@ -1,18 +1,17 @@
-error[E0746]: return type cannot be a trait object without pointer indirection
+error[E0277]: the size for values of type `(dyn Fn(&'a Something) -> Result<(), ()> + Send + Sync + 'a)` cannot be known at compilation time
   --> $DIR/issue-91801.rs:8:77
    |
 LL | fn or<'a>(first: &'static Validator<'a>, second: &'static Validator<'a>) -> Validator<'a> {
    |                                                                             ^^^^^^^^^^^^^ doesn't have a size known at compile-time
    |
-help: consider returning an `impl Trait` instead of a `dyn Trait`
+   = help: the trait `Sized` is not implemented for `(dyn Fn(&'a Something) -> Result<(), ()> + Send + Sync + 'a)`
+note: this type alias is unsized
+  --> $DIR/issue-91801.rs:3:1
    |
-LL | fn or<'a>(first: &'static Validator<'a>, second: &'static Validator<'a>) -> impl Validator<'a> {
-   |                                                                             ++++
-help: alternatively, box the return type, and wrap all of the returned values in `Box::new`
-   |
-LL | fn or<'a>(first: &'static Validator<'a>, second: &'static Validator<'a>) -> Box<dyn Validator<'a>> {
-   |                                                                             +++++++              +
+LL | type Validator<'a> = dyn 'a + Send + Sync + Fn(&'a Something) -> Result<(), ()>;
+   | ^^^^^^^^^^^^^^^^^^
+   = note: the return type of a function must have a statically known size
 
 error: aborting due to 1 previous error
 
-For more information about this error, try `rustc --explain E0746`.
+For more information about this error, try `rustc --explain E0277`.