This is one of those lints where there's probably an entire backstory to it.
Probably not much of a backstory, it's common to make copy-and-paste mistakes when implementing multiple similar things. For instance, when implementing both the Add and Sub traits, it would be normal to start with one of them, and duplicate it to make the other, making the relevant changes in the copy (Add -> Sub, add -> sub, + -> -); it's easy to forget to make one of these changes.
This sounds horrid.
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.
Knowing clippy I wouldn’t be surprised if that compiled, but resulted in a warning that you can simplify the double negative.
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.
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