clippy is an optional linting tool, don't have to use it, and if you do use it you can configure it to not care about this
Why?
Not OP, but what happens in the case you want to add by subtracting a negative value?
You disable the warning for the function?
Yes, which also serves as a way to explicitly point out that the "surprising operation" is intended. Hopefully this even reminds you of putting an explanatory comment there.
Clippy is a linter. You don't use linters by mechanically following its warnings as gospel, you use it by double-checking the code wherever it warns and removing the warning from that function if it's a false positive.
Knowing clippy I wouldn’t be surprised if that compiled, but resulted in a warning that you can simplify the double negative.