Lint `ignored_map_err`

[Veetaha]

Lint explanation

Detect code that uses let _ = result.map_err(); pattern instead of if-let. The reasoning is that map_err is meant for changing the error type of the result, so it does more than just inspecting the error result. if-let intentionally ignores any values that don't match the pattern and is subjectively clearer, and objectively shorter.

Example code

Bad

let _ = do_something_fallible().map_err(|err| {
	// log the error
});

Good

if let Err(err) = do_something_fallible() {
	// log the error
}