The `improper_ctypes` lint is too noisy to be useful

[alexcrichton]

Recent changes appear to generate new warnings for the improper_ctypes lint. Code such as this:

struct Internals {
    // ...
}

#[repr(C)]
pub struct internals_t {
    internals: Internals,
}

pub extern "C" fn internals_new() -> *mut internals_t {
    loop {}
}

now generates the warning:

warning: `extern` fn uses type `Internals`, which is not FFI-safe
  --> src/lib.rs:10:38
   |
10 | pub extern "C" fn internals_new() -> *mut internals_t {
   |                                      ^^^^^^^^^^^^^^^^ not FFI-safe
   |
   = note: `#[warn(improper_ctypes)]` on by default
   = help: consider adding a `#[repr(C)]` or `#[repr(transparent)]` attribute to this struct
   = note: this struct has unspecified layout
note: type defined here
  --> src/lib.rs:1:1
   |
1  | / struct Internals {
2  | |     
3  | | }
   | |_^

This pattern can be quite common if a Rust library is exposing a C interface. Many pointer types moving across the boundary are intentionally opaque to the C side of the interface, so there's no need for #[repr(C)]

The lint, however, now generates thousands of warnings on these crates and has to basically be shut off entirely, removing the usefulness of the lint in the first place.

[pnkfelix]

(Was this injected by PR #65134 ?)

[Centril]

It was, yes, but let's focus discussion in #66220.