macro_metavar_expr_concat broken when combining repeats and optionals

[bjorn3]

I tried this code:

#![feature(macro_metavar_expr_concat)]
macro_rules! define_ioctl_data {
    (struct $s:ident {
        $($field:ident: $ty:ident $([$opt:ident])?,)*
    }) => {
        pub struct $s {
            $($field: $ty,)*
        }

        impl $s {
            $($(
                fn ${concat(get_, $field)}(&self) -> $ty {
                    let _ = $opt;
                    todo!()
                }
            )?)*
        }
    };
}

define_ioctl_data! {
    struct Foo {
        a: A [aa],
        b: B [bb],
    }
}

I expected to see this happen: Expands to

pub struct Foo {
    a: A,
    b: B,
}
impl Foo {
    fn get_a(&self) -> A {
        let _ = aa;

        ::core::panicking::panic("not yet implemented")
    }
    fn get_b(&self) -> B {
        let _ = bb;
        ::core::panicking::panic("not yet implemented")
    }
}

Instead, this happened: Expands to

pub struct Foo {
    a: A,
    b: B,
}
impl Foo {
    fn get_a(&self) -> A {
        let _ = aa;

        ::core::panicking::panic("not yet implemented")
    }
    fn get_a(&self) -> B {
        let _ = bb;
        ::core::panicking::panic("not yet implemented")
    }
}

which defines get_a twice.

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rustc --version --verbose:

1.94.0-nightly (2025-12-13 430d8297c712ca7e8a48)

[Delta17920]

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