Function rustc_expand::mbe::macro_parser::or_pat_mode

fn or_pat_mode(edition: Edition) -> OrPatNonterminalMode

In edition 2015/18, :pat can only match pat<no_top_alt> because otherwise, we have breakage. As of edition 2021, :pat matches top_pat.

See https://github.com/rust-lang/rust/issues/54883 for more info.