Tweak Request: Add identical enum variants as associated constants

[Lytigas]

I think it would make more sense to expose duplicate variants as associated constants rather than externally. Could be disabled by default for backwards compatibility.

#include <stdint.h>
enum Opinion : int32_t {
	BAD = 0,
	BEST = 1,
	BESTEST = 1,
	WORST = -2,
	WORSTER = -2,
};

$ bindgen test.hpp --whitelist-type "Opinion" --rustified-enum ".*" -- -std=c++14 > bind.rs

yields

pub type __int32_t = ::std::os::raw::c_int;
pub const Opinion_BESTEST: Opinion = Opinion::BEST;
pub const Opinion_WORSTER: Opinion = Opinion::WORST;
#[repr(i32)]
#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
pub enum Opinion {
    BAD = 0,
    BEST = 1,
    WORST = -2,
}

Requesting something like:

impl Opinion {
    pub const BESTEST: Opinion = Opinion::BEST;
    pub const WORSTER: Opinion = Opinion::WORST;
}

I'd be happy to try myself if someone could point me the right direction.

[emilio]

This behavior looks strictly better to me so I'd be fine making it the default. Not sure since when it's available, but presumably we'd need to gate it with a rust feature gate, see features.rs for that.

Other than that, this should be somewhat straight-forward to implement.

The point where we handle the duplicated values for rust enums is:

https://github.com/rust-lang-nursery/rust-bindgen/blob/d61ab759e3512d79131eb6c455862915c6b6c4d2/src/codegen/mod.rs#L2633-L2657

We should presumably change add_constant to emit something like:

impl Opinion {
     pub const BESTEST: Opinion = Opinion::BEST;
}

if supported instead. It's probably easier to emit multiple impl blocks rather than coalescing them, even though the generated code won't be so pretty :)

[Lytigas]

Fixed by #1355