Unnecessary improper_ctypes warning for extern "Rust"

[josephlr]

The following code generates an improper_ctypes warning:

extern "Rust" {
    fn foo(dest: &mut [u8]) -> Result<(), Error>;
}

I would expect this sort of warning for an extern "C" declaration, but it seems incorrect for an extern "Rust" declaration. Am I misunderstanding something, or is this a warning that is safe to disable?

[dhardy]

For context, see: rust-random/getrandom#109

Error is pub struct Error(NonZeroU32); so both this and Result are not C types.

[josephlr]

Error is pub struct Error(NonZeroU32); so both this and Result are not C types.

Also the &mut [u8] is not a C type either. The main issue seems to be that the compiler checks all extern declarations as if they were extern "C", which I don't think is right (but extern "Rust" isn't documented as much as extern "C").

[cuviper]

There are instances of this in liballoc and libcore too:

rust/src/liballoc/alloc.rs

Lines 243 to 247 in 9ad1e7c

	#[allow(improper_ctypes)]
	extern "Rust" {
	#[lang = "oom"]
	fn oom_impl(layout: Layout) -> !;
	}

rust/src/libcore/panicking.rs

Lines 73 to 78 in 9ad1e7c

	// NOTE This function never crosses the FFI boundary; it's a Rust-to-Rust call
	#[allow(improper_ctypes)] // PanicInfo contains a trait object which is not FFI safe
	extern "Rust" {
	#[lang = "panic_impl"]
	fn panic_impl(pi: &PanicInfo<'_>) -> !;
	}