Clippy lint · Restriction · checked with Clippy 0.1.99

clippy::map_with_unused_argument_over_ranges: map of a closure that does not depend on its parameter over a range

What it meansChecks for Iterator::map over ranges without using the parameter which could be more clearly expressed using std::iter::repeat(...).take(...) or std::iter::repeat_n. It is in the restriction group and off by default.

What you see

warning: map of a closure that does not depend on its parameter over a range
 --> main.rs:3:31
  |
3 | let random_numbers : Vec<_> = (0..10).map(|_| { 3 + 1 }).collect();
  |                               ^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#map_with_unused_argument_over_ranges
  = note: requested on the command line with `-W clippy::map-with-unused-argument-over-ranges`
help: remove the explicit range and use `repeat_n`
  |
3 - let random_numbers : Vec<_> = (0..10).map(|_| { 3 + 1 }).collect();
3 + let random_numbers : Vec<_> = std::iter::repeat_n({ 3 + 1 }, 10).collect();
  |

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 expresses the intent more clearly to take the correct number of times from a generating function than to apply a closure to each number in a range only to discard them.

Lints that forbid language features for specific codebases (no unwrap, no indexing...). Off by default; pick them one by one, never the whole group.

How to fix it

Clippy does not fix this one automatically (its suggestion may change behaviour, so it only shows it). Change the code by hand:

Flagged

let random_numbers : Vec<_> = (0..10).map(|_| { 3 + 1 }).collect();

Write this instead

let f : Vec<_> = std::iter::repeat( 3 + 1 ).take(10).collect();

Turn it off, or on

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

Whole crate (Cargo.toml)

[lints.clippy]
map_with_unused_argument_over_ranges = "warn"

In code

#![warn(clippy::map_with_unused_argument_over_ranges)]

One run

cargo clippy -- -W clippy::map-with-unused-argument-over-ranges

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.

Settings in clippy.toml

msrv: The minimum rust version that the project supports. Defaults to the rust-version field in Cargo.toml

Restrictiongroup

allowdefault level

nofixed by cargo clippy --fix

1.84.0added in Rust

Clippy's own documentation

What it does

Checks for Iterator::map over ranges without using the parameter which could be more clearly expressed using std::iter::repeat(...).take(...) or std::iter::repeat_n.

Why is this bad?

It expresses the intent more clearly to take the correct number of times from a generating function than to apply a closure to each number in a range only to discard them.

Example

let random_numbers : Vec<_> = (0..10).map(|_| { 3 + 1 }).collect();

Use instead:

let f : Vec<_> = std::iter::repeat( 3 + 1 ).take(10).collect();

Known Issues

This lint may suggest replacing a Map<Range> with a Take<RepeatWith>. The former implements some traits that the latter does not, such as DoubleEndedIterator.

Configuration

msrv: The minimum rust version that the project supports. Defaults to the rust-version field in Cargo.toml

(default: current version)

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

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