[repr(C)] enum only accepts int constants

[mahkoh]

static X: i32 = 1;
static Y: i32 = 2;

#[repr(C)]
enum Foo {
    Bar = X,
    Baz = Y,
}

This worked in 1e6b699 but not in a490871.

cc @huonw

[mahkoh]

Looks like a5bb0a3 .

cc @pcwalton

[chris-morgan]

The solution is #[repr(i32)].

It is bad that this was not marked as a breaking change. #15234 does list a breaking change, but not this one.

[huonw]

@chris-morgan that is not the real solution, since repr(i32) is not guaranteed to match repr(C) everywhere. (On that note, repr(C) defaulting to int isn't great either, since Rust's int doesn't match C's int.)

[mahkoh]

@chris-morgan: bindgen creates c_uint (= u32 on my system) statics for enums with no negative entries and c_int (= i32 on my system) for enums with negative entries. So you can't just use i32 always.

[mahkoh]

Some more problems with int:

You can have enums with u64::MAX variants in C which will have a size of 8.

This is complicated to get right in Rust because u64::MAX will overflow and give you a -1 variant on x86_64. On x86 it's outright impossible to get a repr(C) enum of size 8.

[mahkoh]

The constants should simply be untyped like the const RFC suggested.