Impl trait in type aliases: defining use via struct

[dhardy]

Sub-issue of #63063

It should be possible for the defining implementation of Type-Alias-Impl-Trait (aka TAIT) to be a struct usage:

#![feature(type_alias_impl_trait)]
type A = impl Drop;

struct S {
    a: A,
}

fn foo() -> S {
    S { a: Box::new(1) }
}

First issue: currently, non-defining uses of TAIT in the defining scope are not allowed; see this comment.

Second issue: defining usage via a struct field.

Related cases:

• compound type aliases are already supported: type B = (A, ()); where A is TAIT (also applies to array types)
• enum usages usually (when N>1) do not involve all variants and often do not involve all type-components, so extension to enum would only be possible in very limited circumstances: already supported for Option but not other enums

[jackh726]

Current errors on master:

error[E0308]: mismatched types
 --> src/main.rs:9:12
  |
2 | type A = impl Drop;
  |          --------- the expected opaque type
...
9 |     S { a: Box::new(1) }
  |            ^^^^^^^^^^^ expected opaque type, found struct `Box`
  |
  = note: expected opaque type `impl Drop`
                  found struct `Box<{integer}>`

error: could not find defining uses
 --> src/main.rs:2:10
  |
2 | type A = impl Drop;
  |          ^^^^^^^^^

error: aborting due to 2 previous errors

[oli-obk]

This is a duplicate of #64445