`min_type_alias_impl_trait` doesn't report an error when impl Trait is used in a struct field

[nikomatsakis]

This example (playground):

#![feature(min_type_alias_impl_trait)]
type X = impl Iterator<Item = u64> + Unpin;

struct Foo(X);
impl Foo {
    fn new(z: Vec<u64>) -> Self {
        Foo(z.into_iter())
    }
}

should report an error because the use of X in that position is not permitted with min_type_alias_impl_trait. But it does not.

(Spun out from #64445)

[nikomatsakis]

Also we should test in enum fields

#![feature(type_alias_impl_trait)]

type Foo = impl Copy;

enum Wrapper {
    First(Foo),
    Second
}

[oli-obk]

I think we should just keep allowing this.

[nikomatsakis]

Decision: This works well now! 🎉