Replace zero-size structs in FFI with empty enums

[nukep]

In Rust nightly, a bunch of warnings are being emitted for zero-size structs in sdl2-sys:
https://travis-ci.org/AngryLawyer/rust-sdl2/jobs/72803339

These are opaque types, and it isn't appropriate to add any fields to them. A mentioned alternative in rust-lang/rust#27303 is to use enum MyStruct {} instead.

[bfops]

For me, zero-size enums cause the error

sdl2-sys/src/video.rs:14:1: 14:23 error: unsupported representation for zero-variant enum [E0084]
sdl2-sys/src/video.rs:14 pub enum SDL_Window {}

[bfops]

I think c_void might make sense in these contexts, since these types are only ever used as pointers, as far as I know.

[nukep]

You might need to get rid of the #[repr(C)] before it, because it will cause an error.

[bfops]

Removing the #[repr(C)] just causes different warnings, since these types are part of the FFI.

[bfops]

Specifically warning: found struct without foreign-function-safe representation annotation in foreign module, consider adding a #[repr(C)] attribute to the type, #[warn(improper_ctypes)] on by default

[nukep]

I wasn't able to reproduce the issue. http://is.gd/8SpNgV

It won't hurt to use c_void realistically, but I like the empty enum version because that way you can only use the type via a pointer, i.e. *mut SDL_Renderer.

[bfops]

It still happens for me on the latest nightly. Did you try with actual sdl2-sys, or just in the sandbox?

As far as I know, by design, c_void can't be instantiated either (it wouldn't make much sense if it could), so it can also only be used via pointer. In my opinion, c_void makes it more explicit that the intent is for a type not to be instantiated, where an empty enum could imply that variants may be added later. I agree that it doesn't really make a difference here though.

[nukep]

That's weird. I only tried in the sandbox with nightly, though. shrugs

c_void works perfectly, so I won't worry about it. The only downside I can think of for using c_void is that std::mem::size_of::<c_void>() > 0, but that's not a realistic downside if c_void can't be instantiated.