Clippy lint · Complexity · checked with Clippy 0.1.99

clippy::map_identity: unnecessary map of the identity function

What it meansChecks for instances of map(f) where f is the identity function. It is in the complexity group and warns by default.

What you see

warning: unnecessary map of the identity function
 --> main.rs:4:25
  |
4 | let y: Vec<_> = x.iter().map(|x| x).map(|x| 2*x).collect();
  |                         ^^^^^^^^^^^ help: remove the call to `map`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#map_identity
  = note: `#[warn(clippy::map_identity)]` on by default

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?

It can be written more concisely without the call to map.

Code that does something simple in a complicated way. 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 x = [1, 2, 3];
let y: Vec<_> = x.iter().map(|x| x).map(|x| 2*x).collect();

Write this instead

let x = [1, 2, 3];
let y: Vec<_> = x.iter().map(|x| 2*x).collect();

Turn it off, or on

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

On one item

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

Whole crate (Cargo.toml)

[lints.clippy]
map_identity = "allow"

One run

cargo clippy -- -A clippy::map-identity

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.

Complexitygroup

warndefault level

yesfixed by cargo clippy --fix

1.47.0added in Rust

Clippy's own documentation

What it does

Checks for instances of map(f) where f is the identity function.

Why is this bad?

It can be written more concisely without the call to map.

Example

let x = [1, 2, 3];
let y: Vec<_> = x.iter().map(|x| x).map(|x| 2*x).collect();

Use instead:

let x = [1, 2, 3];
let y: Vec<_> = x.iter().map(|x| 2*x).collect();

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

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