clippy::single_option_map: `fn` that only maps over argument

What you see

warning: `fn` that only maps over argument
 --> main.rs:3:1
  |
3 | / fn double(param: Option<u32>) -> Option<u32> {
4 | |     param.map(|x| x * 2)
5 | | }
  | |_^
  |
  = help: move the `.map` to the caller or to an `_opt` function
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#single_option_map
  = note: `-W clippy::single-option-map` implied by `-W clippy::nursery`

Real output of Clippy 0.1.99 (Rust 1.99) on the example below. Names and line numbers in yours will differ.

Does it matter?

Taking and returning an Option<T> may require additional Some(_) and unwrap if all you have is a T.

New lints still being finished. Off by default and may have false positives.

How to fix it

Clippy does not fix this one automatically. Change the code by hand:

Flagged

fn double(param: Option<u32>) -> Option<u32> {
    param.map(|x| x * 2)
}

Write this instead

fn double(param: u32) -> u32 {
    param * 2
}

Turn it off, or on

It is off by default. To use it, set it to warn or deny:

[lints.clippy]
single_option_map = "warn"

#![warn(clippy::single_option_map)]

cargo clippy -- -W clippy::single-option-map

The [lints] table needs Rust 1.74 or newer. In a workspace, put it under [workspace.lints.clippy] and add [lints] workspace = true to each member.