[format_macro_matchers] Closure syntax

[whatisaphone]

I occasionally write macros that include faux-closure syntax, for example:

macro_rules! map_and_then_print {
    ($value:expr, |$pat:pat| $map:expr) => {{
        let $pat = $value;
        let s = $map;
        println!("{}", s);
    }};
}

map_and_then_print!(1, |x| x + 3);
// Prints "4"

rustfmt formats the usage as I'd expect:

map_and_then_print!(1, |x| x + 3);

But the declaration gets formatted a bit strangely:

    ($value:expr, | $pat:pat | $map:expr) => {{

rustfmt doesn't understand that it's supposed to resemble a closure, so it puts a few extra spaces in. I would have expected it to look like this:

    ($value:expr, |$pat:pat| $map:expr) => {{

Here is rustfmt.toml:

edition = "2018"
format_macro_matchers = true

Is this kind of thing in scope for the project?

[topecongiro]

Is this kind of thing in scope for the project?

It is, though it is not the highest priority and very hard to get it done right.

[ytmimi]

Confirming this is reproducible with rustfmt 1.5.1-nightly (a451a39d 2022-07-25).

Linking tracking issue for format_macro_matchers #3354