`meta_variable_misuse` trips on `${count($foo)}` outside of a repetition

[PatchMixolydic]

Code

(playground, updated 2024-01-25 to account for a change to the syntax of macro metavariable expressions)

#![feature(macro_metavar_expr)]
#![warn(meta_variable_misuse)]

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

fn main() {
    assert_eq!(foo!(a, b, c), 3);
}

Current output

warning: variable 'id' is still repeating at this depth
 --> src/main.rs:6:10
  |
5 |     ($($id:ident),* $(,)?) => {
  |                   - expected repetition
6 |         ${count($id)}
  |          ^^^^^^^^^^^^
  |
note: the lint level is defined here
 --> src/main.rs:2:9
  |
2 | #![warn(meta_variable_misuse)]
  |         ^^^^^^^^^^^^^^^^^^^^

Desired output

No diagnostics should be emitted.

Rationale and extra context

This code builds and runs just fine; ${count($foo)} doesn't have to be in a repetition of $foo.

Anything else?

@rustbot modify labels +A-macros +D-papercut +F-macro-metavar-expr