Incorrect warning about pointer to non_exhaustive ffi struct not being ffi safe

[maia-s]

Rust warns that pointers to structs tagged with non_exhaustive aren't FFI safe in downstream crates. We ran into this in the sdl3-sys crate, where I've marked some FFI types, e.g. SDL_Surface, as non_exhaustive because the internal definition of the type used by the SDL library is larger than the public definition, and the non-public part isn't stable. Marking it with non_exhaustive prevents code from manually constructing their own value that would cause immediate UB if passed to SDL. The non_exhaustive structs are only used through pointers.

Reproduction

Crate a, lib.rs:

#[repr(C)]
#[non_exhaustive]
pub struct Struct {
    pub field: u8
}

extern "C" {
    pub fn create_struct() -> *mut Struct;
    pub fn destroy_struct(s: *mut Struct);
}

Crate b, lib.rs:

use a::Struct;

extern "C" {
    pub fn use_struct(s: *mut Struct);
}

I expected to see this happen:

Either neither crate should warn about this, or both should. non_exhaustive is as useful for FFI types as it is for native Rust types, so I'd prefer that neither crate warned.

If the warning is intentional, I'd like a way to disable it on the type itself (assuming it's safe, but I don't see why it wouldn't be)

Instead, this happened:

Crate b warns that Struct isn't FFI safe. There's no warning for crate a.

warning: `extern` block uses type `Struct`, which is not FFI-safe
 --> b/src/lib.rs:4:26
  |
4 |     pub fn use_struct(s: *mut Struct);
  |                          ^^^^^^^^^^^ not FFI-safe
  |
  = note: this struct is non-exhaustive
  = note: `#[warn(improper_ctypes)]` on by default

warning: `b` (lib) generated 1 warning

Meta

rustc --version --verbose:

% rustc --version --verbose
rustc 1.82.0 (f6e511eec 2024-10-15)
binary: rustc
commit-hash: f6e511eec7342f59a25f7c0534f1dbea00d01b14
commit-date: 2024-10-15
host: aarch64-apple-darwin
release: 1.82.0
LLVM version: 19.1.1

% rustc +nightly --version --verbose
rustc 1.84.0-nightly (798fb83f7 2024-10-16)
binary: rustc
commit-hash: 798fb83f7d24e31b16acca113496f39ff168c143
commit-date: 2024-10-16
host: aarch64-apple-darwin
release: 1.84.0-nightly
LLVM version: 19.1.1