syntax: decrease binary size
syntax: decrease binary size

Avoid depending on expression Debug stringifier in release builds.

The code weighs over a kilobyte, which doesn't seem worth it for an unreachable.
rust-lang/regex,rust-lang/regex,rust-lang/regex,rust-lang-nursery/regex,rust-lang/regex,rust-lang-nursery/regex,rust-lang-nursery/regex,rust-lang-nursery/regex,rust-lang/regex,rust-lang-nursery/regex