Clippy lint · Nursery · checked with Clippy 0.1.99

clippy::iter_on_single_items: `iter` call on a collection with only one item

What it meansChecks for calls to iter, iter_mut or into_iter on collections containing a single item It is in the nursery group and off by default.

What you see

warning: `iter` call on a collection with only one item
 --> main.rs:3:9
  |
3 | let a = [123].iter();
  |         ^^^^^^^^^^^^ help: try: `std::iter::once(&123)`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#iter_on_single_items
  = note: `-W clippy::iter-on-single-items` 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?

It is simpler to use the once function from the standard library:

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

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 a = [123].iter();
let b = Some(123).into_iter();

Write this instead

use std::iter;
let a = iter::once(&123);
let b = iter::once(123);

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]
iter_on_single_items = "warn"

In code

#![warn(clippy::iter_on_single_items)]

One run

cargo clippy -- -W clippy::iter-on-single-items

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.

Nurserygroup

allowdefault level

nofixed by cargo clippy --fix

1.65.0added in Rust

Clippy's own documentation

What it does

Checks for calls to iter, iter_mut or into_iter on collections containing a single item

Why is this bad?

It is simpler to use the once function from the standard library:

Example

let a = [123].iter();
let b = Some(123).into_iter();

Use instead:

use std::iter;
let a = iter::once(&123);
let b = iter::once(123);

Known problems

The type of the resulting iterator might become incompatible with its usage

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

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