Constant operator overloading for bitflags

[ldm0]

Fixes #180 Fixes #210
After rust-lang/rust#68847, we now can overload the operator as const function. This PR added a feature "constant". And a user can write compile-pass code like this:

#![feature(const_fn)]
#![allow(incomplete_features)]
#![feature(const_trait_impl)]
use bitflags::bitflags;

bitflags! {
    struct Flags: u32 {
        const A = 0b00000001;
        const B = 0b00000010;
        const C = 0b00000100;
        const ABC = Self::A.bits | Self::B.bits | Self::C.bits;
    }
}

const B: Flags = Flags::A | Flags::B;

fn main() {
    println!("{:?}", B);
}

[KodrAus]

Thanks for the PR @ldm0! It looks like things are steadily moving forwards with const trait impls, so I think we'd be best waiting for that to land in some complete form first to save us too much infrastructure to manage in the bitflags library itself.

[js2xxx]

I think this feature should move forward provided that the const_fn feature has been stable since long.

[KodrAus]

Thanks for spiking this out @ldm0! We're still waiting on some kind of const overloading support in Rust itself, but I think that will be the best time for us to take advantage of it in bitflags. I'll go ahead and close this one for now since we won't be doing conditional support for unstable features in this library.

[KodrAus]

Thanks for spiking this out @ldm0! We're still waiting on some kind of const overloading support in Rust itself, but I think that will be the best time for us to take advantage of it in bitflags. I'll go ahead and close this one for now since we won't be doing conditional support for unstable features in this library.