[marcusb]

Rust has debug asserts for that. Using expect with a comment about why the condition should not/can't ever happen is idiomatic for cases where you never expect an Err.

This reads to me more like the error type returned by append with names is not (ErrorFlags, i32) and wasn't trivially convertible into that type so someone left an unwrap in place on an "I'll fix it later" basis, but who knows.