Clippy lint · Suspicious · checked with Clippy 0.1.99

clippy::unnecessary_option_map_or_else: unused "map closure" when calling `Option::map_or_else` value

What it meansChecks for usage of .map_or_else() “map closure” for Option type. It is in the suspicious group and warns by default.

What you see

warning: unused "map closure" when calling `Option::map_or_else` value
 --> main.rs:5:9
  |
5 | let y = x.map_or_else(|| 2 * k, |n| n);
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#unnecessary_option_map_or_else
  = note: `#[warn(clippy::unnecessary_option_map_or_else)]` on by default
help: consider using `unwrap_or_else`
  |
5 - let y = x.map_or_else(|| 2 * k, |n| n);
5 + let y = x.unwrap_or_else(|| 2 * k);
  |

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?

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

Code that is most likely wrong or useless. Warns by default.

How to fix it

Clippy can fix this one for you: run cargo clippy --fix (add --allow-dirty if you have uncommitted changes). Review the diff before you commit.

Flagged

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

Write this instead

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

Turn it off, or on

Allow it where it does not fit your code, as narrowly as you can:

On one item

#[allow(clippy::unnecessary_option_map_or_else)]
fn my_function() { /* ... */ }

Whole crate (Cargo.toml)

[lints.clippy]
unnecessary_option_map_or_else = "allow"

One run

cargo clippy -- -A clippy::unnecessary-option-map-or-else

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.

Suspiciousgroup

warndefault level

yesfixed by cargo clippy --fix

1.92.0added in Rust

Clippy's own documentation

What it does

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

Why is this bad?

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

Example

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

Use instead:

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

From the Clippy lint list (0.1.99), part of the Rust project, MIT or Apache-2.0. Official entry.

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