Disallow `.map_err(...).ok()` pattern

What it does

I recently observed a code pattern where a Result was converted to an Option with an unnecessary map_err invocation along the way.

Lint Name

map_err_ok

Category

restriction

Advantage

Removes a call to map_err where the error value that is generated is immediately discarded.

Drawbacks

No response

Example

let x = something_that_returns_result()
    .map_err(|_e| Error::SomeOtherError)
    .ok();

Could be written as:

let x = something_that_returns_result().ok();

The requested entry point is the map_err_ok restriction lint; start by reviewing comparable restriction lints in rust-clippy and the example pattern in the issue. Add coverage for the shown Result-to-Option case and verify that the lint flags the redundant map_err while preserving the direct .ok() form.