Convert to associated constants

[tamird]

Two problems with this:

• breaking API change
• requires nightly

I suppose this will need to wait until associated constants are stable.

[alexcrichton]

Yeah for now this crate builds on stable, so we'll have to wait for associated constants to be stable to merge this unfortunately.

[phil-opp]

@tamird It seems like the associated_const feature will remain unstable for the near future. So it might make sense to pre-release this on crates.io as a separate crate (e.g. bitflags_associated_consts). This would allow nightly users to take advantage of the nicer API.

[tamird]

This needs a rebase to pick up various changes made on master; I'll try to get to it this week.

[tamird]

Rebased; probably won't pass until 1.20 is stable.

[tamird]

Rebased again. Beyond the use of associated constants, this also uses compiletest, which requires nightly. Do we want to just remove the compile-fail test?

[alexcrichton]

The stable/beta builds seem to still be failing due to experimental features?

[tamird]

Yes, I believe that's because this PR adds a dependency on compiletest, which requires nightly (see my last comment). Should I just remove the compile-fail tests?

[tamird]

Removed compiletest; lmk if you'd like me to squash the commits, but this should be green now.

[sfackler]

r? @alexcrichton

[alexcrichton]

👍

Thanks @tamird!