Clippy lint · Suspicious · checked with Clippy 0.1.99

clippy::by_ref_peekable_peek: calling `.by_ref().peekable().peek()` will advance the underlying iterator and consume its first output

What it meansChecks for usages of Iterator::by_ref().peekable().peek(). It is in the suspicious group and warns by default.

What you see

warning: calling `.by_ref().peekable().peek()` will advance the underlying iterator and consume its first output
 --> main.rs:4:9
  |
4 | let x = iter.by_ref().peekable().peek();  // 1
  |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#by_ref_peekable_peek
  = note: `#[warn(clippy::by_ref_peekable_peek)]` on by default
help: to peek the first item without advancing the underlying iterator, use
  |
4 - let x = iter.by_ref().peekable().peek();  // 1
4 + let x = iter.clone().next().as_ref();  // 1
  |
help: to advance the underlying iterator, use
  |
4 - let x = iter.by_ref().peekable().peek();  // 1
4 + let x = iter.next().as_ref();  // 1
  |
help: to make the iterator peekable, use
  |
3 ~ let mut iter = [1, 2, 3].into_iter().peekable();
4 ~ let x = iter.peek();  // 1
  |

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?

While it might look like this will allow peeking on the first element of an iterator without consuming it and without consuming the iterator itself, it will in practice consume the first element. The implementation of Peekable::peek() produces the first element of the underlying iterator, and stores it internally so that it can be later produced. As a consequence, it advances the underlying iterator, whose .next() method will now produce its second element.

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

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 mut iter = [1, 2, 3].into_iter();
let x = iter.by_ref().peekable().peek();  // 1
let y = iter.by_ref().peekable().peek();  // 2

Write this instead

let mut iter = [1, 2, 3].into_iter();
let x = iter.next().as_ref();  // 1
let y = iter.next().as_ref();  // 2

Turn it off, or on

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

On one item

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

Whole crate (Cargo.toml)

[lints.clippy]
by_ref_peekable_peek = "allow"

One run

cargo clippy -- -A clippy::by-ref-peekable-peek

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

nofixed by cargo clippy --fix

1.98.0added in Rust

Clippy's own documentation

What it does

Checks for usages of Iterator::by_ref().peekable().peek().

Why is this bad?

While it might look like this will allow peeking on the first element of an iterator without consuming it and without consuming the iterator itself, it will in practice consume the first element.

The implementation of Peekable::peek() produces the first element of the underlying iterator, and stores it internally so that it can be later produced. As a consequence, it advances the underlying iterator, whose .next() method will now produce its second element.

Example

let mut iter = [1, 2, 3].into_iter();
let x = iter.by_ref().peekable().peek();  // 1
let y = iter.by_ref().peekable().peek();  // 2

If this does what you intended, use the following instead, which is shorter and clearer:

let mut iter = [1, 2, 3].into_iter();
let x = iter.next().as_ref();  // 1
let y = iter.next().as_ref();  // 2

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

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