Macro `:meta` matcher doesn't match `true` or `false`

[tgross35]

true and false are now accepted config options. However, they aren't accepted by the meta fragment specifier:

macro_rules! demo {
    ($m:meta) => {
        #[cfg($m)]
        println!("hi {}", stringify!($m));
    }
}

fn main() {
    demo!(any());
    demo!(all());
    demo!(true);
    demo!(false);
}

error: expected identifier, found keyword `true`
  --> src/main.rs:11:11
   |
11 |     demo!(true);
   |           ^^^^ expected identifier, found keyword

error: expected identifier, found keyword `false`
  --> src/main.rs:12:11
   |
12 |     demo!(false);
   |           ^^^^^ expected identifier, found keyword

playground

We should consider whether we want to accept this in the next edition, keeping the current :meta matcher behavior as :meta_2024.

[jhpratt]

Is it actually a breaking change to accept this in current editions? That was the justification for having :expr_2024; if there's no possible breakage I don't see why we shouldn't allow this outright.

[tgross35]

I was assuming this would affect whether or not you go to a fallback if you try to match a :meta and then something else that would match true/false. But maybe this isn't the case? Changing the macro in the demo to this:

macro_rules! demo {
    ($m:meta) => {
        println!("match meta {}", stringify!($m));
    };
    ($tt:tt) => {
        println!("match expr {}", stringify!($tt));
    };
}

gets the same errors as in the top post. It seems like true/false do get caught by meta and then rejected, as opposed to falling through meta.

https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=7827c56d0abc3b99092f508684328413

So you're right, I guess we could just do this now.

[tgross35]

Nominating for lang: should :meta match true/false?

@rustbot label +I-lang-nominated +I-lang-easy-decision

[petrochenkov]

meta is not supposed to match syntax of cfg predicates, or other syntaxes that other attributes may define, it matches the stuff inside the attribute brackets #[...].

• #[attr] - ok, attr is meta
• #[attr { a b c }] - ok, attr { a b c } is meta
• #[true] - not ok, true is not meta