Warning at use site for #[repr(C)] type that contains non-#[repr(C)] types

[huonw]

#![allow(dead_code)]

struct Foo;
#[repr(C)]
struct Bar { x: Foo }

extern { fn whatever(x: Bar); }

fn main() {}

<anon>:7:25: 7:28 warning: found type without foreign-function-safe
                                             representation annotation in foreign module, consider adding a #[repr(...)] attribute to the type, #[warn(ctypes)] on by default
<anon>:7 extern { fn whatever(x: Bar); }
                                 ^~~

This could either be a warning on Bar, that Foo isn't FFI safe; or not a warning at all. (Or, at the very least, a note pointing to the x field, to help diagnose the problem.)

[o11c]

Significantly, this occurs even if the struct contains only a pointer-to-non-repr-C-struct

[frewsxcv]

Triage comment: the warning has not changed

playpen link

[steveklabnik]

Since new lints have a big impact on users of rustc, the policy is that they should go through the RFC process like other user-facing changes. As such, I'm going to give this one a close, but if anyone comes across this ticket and wants this lint, consider adding it to clippy and/or writing up an RFC. Thanks!