False positive improper_ctypes warning for () used through trait projection

[alexcrichton]

This code:

pub trait Foo {
    type Ret;
}

impl Foo for u32 {
    type Ret = ();
}

pub extern "C" fn foo() -> <u32 as Foo>::Ret {
}

generates this warning on nightly:

warning: `extern` fn uses type `()`, which is not FFI-safe
 --> src/lib.rs:9:28
  |
9 | pub extern "C" fn foo() -> <u32 as Foo>::Ret {
  |                            ^^^^^^^^^^^^^^^^^ not FFI-safe
  |
  = note: `#[warn(improper_ctypes)]` on by default
  = help: consider using a struct instead
  = note: tuples have unspecified layout

but I believe that this is an false positive because () in a return position is indeed FFI safe

[Centril]

Duplicate of #66202?

[Centril]

cc @davidtwco @rkruppe @eddyb

[davidtwco]

Duplicate of #66202?

I think so.

[hanna-kruppe]

Agreed, closing as such.