I think Haskell's warning-in-the-doc approach is not strong enough. I'd be in favor of distinguishing small and huge values using the type system. Have a Rust enum that contains either a small-ish number (the absolute value being 10^100 or less, but the threshold should be configurable preferably as a type parameter) or a huge number. Then the user will be required to handle it. Most of the time the user does not want huge numbers, so they will fail the parse explicitly when they do a match and find it.