Emit rebinding only for interpolations that refer to a field

[dtolnay]

For something like:

const CODE: usize = 9;

#[derive(Error, Debug)]
#[error("{id:?} (code {CODE:?})")]
pub struct Error {
    pub id: &'static str,
}

Before:

let Self { id } = self;
match (id, CODE) {
    (__field_id, __field_CODE) => {
        __formatter.write_fmt(format_args!("{__field_id:?} (code {__field_CODE:?})"))
    }
}

After:

let Self { id } = self;
match (id,) {
    (__field_id,) => {
        __formatter.write_fmt(format_args!("{__field_id:?} (code {CODE:?})"))
    }
}