It would require making "expression" and "pattern" into the same nonterminal in the grammar, and then deciding afterwards whether something that was parsed was an expression or a pattern. This is sometimes called a "cover grammar" because it "covers" two distinct nonterminals. It is possible--ECMAScript 6 does it--but it's definitely a complex way to parse. It doesn't seem compatible with Rust's design goal of being easy to parse and regular, e.g. for tooling.