Can't use generated bitflags to initialise a const

[PhilipDaniels]

This works in plain Rust

const MAMMAL: u8 = 1;
const AQUATIC: u8 = 2;
const SEAL: u8 = MAMMAL | AQUATIC;

but the equivalent when using bitflags does not

use bitflags::bitflags;

bitflags! {
    pub struct FLAGS: u8 {
        const MAMMAL  = 1;
        const AQUATIC = 2;
    }
}

const SECOND_SEAL: FLAGS = FLAGS::MAMMAL | FLAGS::AQUATIC;

You get an error error[E0015]: calls in constants are limited to constant functions, tuple structs and tuple variants when trying to BITOR the two flags.

It would be nice if it did, because in my program I have a large initial array of flags to create and there are many different combinations making it impractical to spell them all out inside the bitflags! call.

Edit: Simplified the repro.

[KodrAus]

So this is a little unfortunate. It seems like a limitation in Rust's const evaluation where operators on primitive types are specially understood as being available in a constant context. We can't mark our BitOr implementation as being constant, even though it is.

One workaround is to do something like this:

const SECOND_SEAL: FLAGS = FLAGS::from_bits_truncate(FLAGS::MAMMAL.bits() | FLAGS::AQUATIC.bits());

[PhilipDaniels]

Thanks.

I wondered whether it was possible to mark BitOr as const fn or something...but I don't follow the rustc improvements that closely. I worked around my issue my not using bitflags, instead I declared an empty struct and gave it some associated constants.

[ArekPiekarz]

If it is a Rust limitation, should an issue about it be reported on their github?

[KodrAus]

There's some experimental work to support const trait impls that should enable using bitops here: rust-lang/rust#67792 but it sounds like there's still a lot of work to be done before that will reach us.

In the meantime, we just merged in #244 which includes some nicer const set operators that avoids the need to wrap integers. So your second example could become:

const SECOND_SEAL: FLAGS = FLAGS::MAMMAL.union(FLAGS::AQUATIC);