Illegally adds comma to macro invocation

[Person-93]

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Rustfmt adds a comma to the following and causes a compiler error.

macro_rules! idx {
    ($(#[$attr:meta])* $vis:vis struct $name:ident; $metadata:ident) => {
        $(#[$attr])*
        $vis struct $name(usize);
    };
}

idx!(
    #[derive(Debug)]
    pub struct FooIdx; // rustfmt adds a comma here
    Meta
);