Lets say you've thrown away the ast, don't have a token stream, etc, but do still have access to the source of the function. Getting the function length is O(1)

Lets say you've thrown away the ast, don't have a token stream, etc, but do still have access to the source of the function. Getting the function length is O(1). At worst, its a subtraction. Lexing it is considerably more expensive, and doing so just to improve the heuristic may not be worth the time.