New lint: suggest the use of Result::unwrap_or_else over Result::map_or_else

[jhscheer]

What it does

Suggest the use of Result::unwrap_or_else over Result::map_or_else if map_or_else is just used to unpack a successful result while handling an error.

Categories (optional)

What is the advantage of the recommended code over the original code

Code will be shorter (and cleaner).

Drawbacks

None.

Example

.map_or_else(
    |e| handle_the_error(e),
    |n| n,
)

Could be written as:

.unwrap_or_else(|e| handle_the_error(e))

[camsteffen]

I think an enhancement to map_identity would do.

[Valentine-Mario]

@rustbot claim

[KisaragiEffective]

I've found another case.

let res: Result<usize, usize> = foo();
let v: usize = foo.map_or_else(|x| x, |x| x);

can be following code:

let v: usize = foo.unwrap_or_else(|x| x)

[GuillaumeGomez]

Adding a lint for it.