Enable $($pattern)|+ or equivalent in macro_rules!

[SimonSapin]

I use this macro a lot:

macro_rules! is_match {
    ($value:expr, $($pattern:pat)|+) => {
        match $value {
            $($pattern)|+ => true,
            _ => false
        }
    }
}

But today’s Nightly broke it (#20563 (comment)) with apparently no fix that don’t involve changing the usage syntax.

error: `$pattern:pat` is followed by `|`, which is not allowed for `pat` 

A work around (that I’m going to use) is to use tt:

#[macro_export]
macro_rules! matches {
    ($expression: expr, $($pattern:tt)+) => {
        _tt_as_expr_hack! {
            match $expression {
                $($pattern)+ => true,
                _ => false
            }
        }
    }
}

/// Work around "error: unexpected token: `an interpolated tt`", whatever that means.
#[macro_export]
macro_rules! _tt_as_expr_hack {
    ($value:expr) => ($value)
}

But it’s not great since it allows sneaking another match arm into the macro expansion:

fn foo(arg: &str, flux: i32) {
    matches!(arg, "bar" | "baz" => flux> 9000, "fuzz");
}

Therefore, I think think that something like the original macro should be possible to write.

[steveklabnik]

Triage: I am not aware of any change here. @SimonSapin do you still feel this pain? (and is this RFC-worthy?)

[SimonSapin]

I’m not very happy with the work-around described above, but it’s hidden away in https://crates.io/crates/matches/ so I don’t think need to about it.

[pnkfelix]

| has been added to FOLLOW(pat) in RFC amendment rust-lang/rfcs#1384 , so once the associated implementation lands ( #30450 ) this can probably be closed ... I will try to remember to add a specific unit test for it.

[pnkfelix]

Okay it landed and here's the test: https://github.com/rust-lang/rust/pull/30694/files#diff-b3d1417aaa1fb8f3fa7c449b1f54cbfc

So I'm closing this ticket.

[SimonSapin]

Confirmed that the original macro in this issue (with $($pattern:pat)|+) now works without the "tt as expr" hack on Nightly. Thanks! (I’ll keep the hack in the crate at least until the fix hits stable.)