clippy::double_ended_iterator_last: called `Iterator::last` on a `DoubleEndedIterator`; this will needlessly iterate the entire iterator

What you see

warning: called `Iterator::last` on a `DoubleEndedIterator`; this will needlessly iterate the entire iterator
 --> main.rs:3:16
  |
3 | let last_arg = "echo hello world".split(' ').last();
  |                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#double_ended_iterator_last
  = note: `#[warn(clippy::double_ended_iterator_last)]` on by default
help: try
  |
3 - let last_arg = "echo hello world".split(' ').last();
3 + let last_arg = "echo hello world".split(' ').next_back();
  |

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?

Iterator::last is implemented by consuming the iterator, which is unnecessary if the iterator is a DoubleEndedIterator. Since Rust traits do not allow specialization, Iterator::last cannot be optimized for DoubleEndedIterator.

Code that could be written to run faster. 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 last_arg = "echo hello world".split(' ').last();

Write this instead

let last_arg = "echo hello world".split(' ').next_back();

Turn it off, or on

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

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

[lints.clippy]
double_ended_iterator_last = "allow"

cargo clippy -- -A clippy::double-ended-iterator-last

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.