Empty structs should not need #[repr(C)]

[steveklabnik]

Issue by cgaebel
Thursday Nov 06, 2014 at 07:50 GMT

For earlier discussion, see rust-lang/rust#18686

This issue was labelled with: in the Rust repository

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In a lot of FFI code, I see patterns like:

struct X {}

fn some_c_function(this: *mut X);

where X is used as an opaque "black-box" object. With rust today, X needs a #[repr(C)]. That's not really necessary, and caused a bunch of noisy warnings when updating servo's rust version.

Can someone mark this bug as easy so a beginner can use it to get started?

[dgrunwald]

Empty structs do not exist in C, and it is not clear what their representation should be.
In C with GCC extensions, sizeof(empty_struct) == 0. But in C++, sizeof(empty_struct) == 1.

For the case of incomplete types in C headers, something like RFC #709 unsized types would be a better fit.

[scottmcm]

This is now covered by extern type, #1861.

[Centril]

Closing per @scottmcm's comment.