The author actually touches on why it couldn't optimise this away: .* doesn't match any character sequence, it is equivalent to [^\n]* by default

Also, assuming RE2 works like other engines, it also couldn't convert to a partial match without the .* prefix+suffix even if .* matched any char, because you might then ask for capture group 0

Do what? PartialMatch inserts them for you. FullMatch doesn't. The compiler can't guess your intent.