Support building without core

[dtolnay]

As described in rust-lang/rust#45079 (comment) bitflags currently depends on core here, but as a macro-only crate it shouldn't need to. This makes it impossible to use bitflags within the implementation of libcore.

[dtolnay]

@tamird may be interested in working on this.

[tamird]

Yep, was planning to work on that after #127 merged. Could you give that a review?

[tamird]

OK, I pulled on this thread a bit, but it seems that path variables can't be chained. In other words, we'd need a separate path variable for every one of the identifiers being brought in, not just one for core. Am I missing something?

[dtolnay]

Is this what you mean by can't be chained?

macro_rules! m {
    ($p:path) => {
        use $p::HashMap;
        //    ^^ expected one of `;` or `as` here
    };
}

m!(std::collections);

If so, something like this might work as a workaround.

macro_rules! m {
    ($($p:ident)::+) => {
        use $($p)::+::HashMap;
    };
}

[tamird]

That's exactly what I meant, thank you! I'll try that.

[dtolnay]

We are not planning to pursue this. #134 (comment)