Declarative macro transcription does not take into account precedence

[scrabsha]

Let's consider the following code:

macro_rules! two_times {
    ($e:expr) => {
        2 * $e
    };
}

fn main() {
    let a = two_times!(1 + 1);
    assert_eq!(a, 4);
}

Let's break down its expansion according to the current Ferrocene specs:

• $e matches the expression 1 + 1 (20.4.1.2:6),
• When transcribing, $e is replaced with 1 + 1 (20.4.2:3),
• The final expansion is 2 * 1 + 1.

Well, that's not what currently happens (playground link). rustc -Zunpretty=expanded even adds parenthesis around the 1 + 1.

My understanding as that expr metavariables that match an arithmetic expression should have a higher precedence than the rest of the macro substitution. As far as I can tell, this is not mentioned in Ferrocene.

Side note: I don't have much experience in how Ferrocene specs are structured, so I may have used the wrong word/missed something important. Sorry for that.

[Veykril]

Good catch we are not specifying none groups (aka invisible delimiters) at all currently which is in general a somewhat undesigned space in rust still rust-lang/rust#67062. We should be specifying the grouping behavior for expressions at least though.