const_bitflag_or macro to enable const bitflags before const traits are available

[joshtriplett]

Until const traits become usable on stable, it'd be nice to have a macro along the lines of const_bitflag_or, which would expand to use Flags::NAME.bits() and Flags::from_bits_truncate:

const_bitflag_or!(Flags, A, B, C)
would become
Flags::from_bits_truncate(Flags::A.bits() | Flags::B.bits() | Flags::C.bits())

[Evian-Zhang]

If we write const_bitflags_or as (since the delimiter can be anything):

macro_rules! const_bitflags_or {
    ( $Flags: ty, $( $flag: ident )|+ ) => {
        <$Flags>::from_bits_truncate(0 $(| <$Flags>::$flag.bits())+)
    };
}

Then we can use it like

const_bitflags_or!(Flags, A | B | C);

which may be more graceful.

[KodrAus]

Ideally we'll be able to just mark those traits as const when #![feature(const_trait_impl)] stabilizes. In the meantime, you can do:

Flags::A.union(Flags::B).union(Flags::C)

in const evaluation.

[KodrAus]

Just coming back through some triage. I think this isn't something we're going to support in bitflags through a macro, but will absolutely jump on const_trait_impl once it stabilizes. Thanks for the input everyone!