rust_enums missing

[jethrogb]

I would like my C enums to be defined as individual constants instead of a Rust enum. C enum values can hold any integer which breaks when they're interpreted as Rust enums across FFI boundaries. bitfield_enum is not sufficient since this introduces a wrapper type. Using bindgen 0.20 would introduce a public API change for my -sys crates.

[emilio]

Again, this should be easy to do, we have options to make some enum variants constant already (see #393), so it'd be a matter of expanding that codepath.

[jethrogb]

Ah, interesting. I just tried that out a little bit and noticed that returning EnumVariantCustomBehavior::Constify for a bitfield_enum is broken.

[emilio]

Yeah, I can believe so. It doesn't make a lot of sense doing that though (bitfield enums already generate constants), so probably we should just ignore that