Understanding matches! macro

I’m trying to understand how the matches! macro is written.

macro_rules! matches {
    ($expression:expr, $(|)? $( $pattern:pat_param )|+ $( if $guard: expr )? $(,)?) => {
        match $expression {
            $( $pattern )|+ $( if $guard )? => true,
            _ => false
        }
    };
}

Two metavariables are not clear to me;

1. $(|)?
2. $(,)?

I understand the functionality of ?:

indicates an optional fragment with zero or one occurrences.

But I’m not understanding where these metavariables are used for.

>Solution :

They are not used for anything, they are syntactic allowances.

$(|)? makes it so you can start the pattern with a leading |, for uniformity with e.g. match patterns which also allow that:

matches!(a, |1|2);

If you create a copy of the macro and remove that group, you’ll see that that syntax does not work anymore.

This is because while the leading | is part of the Pattern production pat_param matches the PatternNoTopAlt production, and so does not include or allow for a leading |.

Similarly, the trailing $(,)? allows for a trailing comma on the pattern: rust supports trailing commas in function calls, so most expression-like and function-like macros will also support it as they make for convenient uniformity:

matches!(
    a,
    1|2,
);

Here again, if you create a copy of the macro and remove that group, trailing commas will fail to parse.