Add `unnecessary_option_map_or_else`

[sorairolake]

What it does

Checks for usage of .map_or_else() "map closure" for Option type.

Advantage

This can be written more concisely by using unwrap_or_else().

Drawbacks

No response

Example

let k = 10;
let x: Option<u32> = Some(4);
let y = x.map_or_else(|| 2 * k, |n| n);

Could be written as:

let k = 10;
let x: Option<u32> = Some(4);
let y = x.unwrap_or_else(|| 2 * k);

Summary Notes

• "best claimed by first timers" by @samueltardieu

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[sorairolake]

See also #7328 and #12169.

[samueltardieu]

@rustbot label +good-first-issue
@rustbot note "best claimed by first timers"

Please, regular contributors, let first-timers a chance to pick-up this issue, as it would help to have more people contribute to Clippy. Don't hesitate to chime it and propose your insights though!

[pnuts93]

Hi, first time contributor here, I would like to try and resolve this issue. I have read the rules on the https://github.com/rust-lang/rust-clippy/blob/master/CONTRIBUTING.md page, but please let me know if someone else was already working on it or if I missed something in the contribution process.
@rustbot claim