Let's say you have a Result<(&str, &str), &str>, as coming from a lexer. It makes sense to fail if a token matches, so then you want to (!result.map(HelpfulError))? or (!result).map_err(HelpfulError)?. Alternatively you need to make your lexer take an &mut &str (not to be confused with &mut str) and output bools and use bools everywhere, which is, in fact, a lot easier than trying to work with the existing Result.
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