associated constants

[tamird]

This crate really wants to use associated constants (see rust-lang/rust#24921) but the tricks introduced in #14 are incompatible with associated constants (because associated constants can't be used to shadow locally scoped constants).

What do?

[Amanieu]

Here is an implementation which does the same thing using a dummy trait:

impl ::std::fmt::Debug for $BitFlags {
    fn fmt(&self, f: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
        // This convoluted approach is to handle #[cfg]-based flag
        // omission correctly. Some of the $Flag variants may not be
        // defined in the main struct so we create a trait which defines
        // *all* flags to the value of 0. Afterwards we define a struct
        // which overrides the definitions in the trait for all defined
        // variants, leaving only the undefined ones with the bit value
        // of 0.
        trait DummyTrait {
            // Now we define the "undefined" versions of the flags.
            // This way, all the names exist, even if some are #[cfg]ed
            // out.
            $(const $Flag: $BitFlags = $BitFlags { bits: 0 };)+
        };
        struct DummyStruct;
        impl DummyTrait for DummyStruct {
            // Then we override the "undefined" flags with flags that
            // have their proper value. Flags that are #[cfg]ed out
            // retain the default value of 0 defined by the trait.
            $($(#[$Flag_attr])* const $Flag: $BitFlags = $BitFlags { bits: $value };)+
        };
        let mut _first = true;
        $(
            // $Flag.bits == 0 means that $Flag doesn't exist
            if DummyStruct::$Flag.bits != 0 && self.contains(DummyStruct::$Flag) {
                if !_first {
                    try!(f.write_str(" | "));
                }
                _first = false;
                try!(f.write_str(stringify!($Flag)));
            }
        )+
        Ok(())
    }
}

One open question is whether Debug should print Flags::FLAG_A | Flags::FLAB_B or just FLAG_A | FLAG_B as it currently does.

[Ericson2314]

@tamird are you going to make a PR? Or does the conflict in the OP still stand?

[Amanieu]

There is already a PR (#24), however it is blocked until associated constants are stable.

[Ericson2314]

Ah thanks

[crumblingstatue]

Associated consts have been stabilized.
They will be stable in 1.20.

[Ericson2314]

Time to clean up #21 ?

[aep]

can we get a release with the new associated constants? :)

[alexcrichton]

published!