`${count(t,)}` is interpreted as `${count(t, 0)}`

[lowr]

I tried this code:

#![feature(macro_metavar_expr)]

macro_rules! foo {
    ( $( $($t:ident),* );* ) => { ${count(t,)} }
}

fn test() {
    foo!(a, a; b, b);
}

I expected to see this happen:

Either:

• The macro definition is rejected, leading to a compile error.
• ${count(t,)} is interpreted as ${count(t)}, the macro call is expanded to 4.

Instead, this happened:
Compiled successfully, ${count(t,)} is interpreted as ${count(t, 0)}, the macro call is expanded to 2.

This form of ${count()} metavar expression is not specified in RFC 3086, so this is more of an ask for clarification than a bug report.

Meta

rustc --version --verbose:

1.71.0-nightly (2023-05-23 5ea3f0ae08c07472239a)

cc #83527
@rustbot label requires-nightly F-macro-metavar-expr