`CxxVector` explicit shim impl fails: `patterns aren't allowed in foreign function declarations`

#[cxx::bridge]
mod ffi1 {
    unsafe extern "C++" {
        type A;
    }

    impl CxxVector<A> {}
}

This code will error with the following:

patterns aren't allowed in foreign function declarations

This is because the __vector_size and __vector_capacity extern functions generate with _ as an argument rather than a single-letter variable. For whatever reason, rust is considering this a pattern match and erroring. The solution, as per the other vector methods, is to just make the argument a single letter or something like _v.