Support extern opaque types.

[crlf0710]

#![feature(extern_types)]

extern "C" {
    type A;
    fn foo() -> Box<A>;
    fn bar();
}

impl A {
    fn bar(&self) {
        unsafe {bar()};
    }
}

fn main() {
    let a = unsafe { foo() } ;
    a.bar();
}

This code compiles fine in rustc, but rust-analyzer gives Box<{unknown}> inlay hint to a.

[jackh726]

While not technically "built-in", I think this falls under #368, so I'm gonna link this there.

Overall, I think this should be pretty easy. I think we really only need to add an extra variant to TypeName. Other than that, we won't add any clauses for Sized/Copy/Clone traits.

[memoryleak47]

I'll try this one.

[jackh726]

@rustbot assign @memoryleak47

[jackh726]

Ah right, I can't do that.

[crlf0710]

Thanks! Hopefully this will help solve rust-lang/rust-analyzer#4150 .