Clippy lint · Complexity · checked with Clippy 0.1.99

clippy::unnecessary_first_then_check: unnecessary use of `first().is_none()` to check if slice is empty

What it meansChecks the usage of .first().is_some() or .first().is_none() to check if a slice is empty. It is in the complexity group and warns by default.

What you see

warning: unnecessary use of `first().is_none()` to check if slice is empty
 --> main.rs:4:6
  |
4 | if v.first().is_none() {
  |      ^^^^^^^^^^^^^^^^^ help: replace this with: `is_empty()`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#unnecessary_first_then_check
  = note: `#[warn(clippy::unnecessary_first_then_check)]` 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?

Using .is_empty() is shorter and better communicates the intention.

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 v = vec![1, 2, 3];
if v.first().is_none() {
    // The vector is empty...
}

Write this instead

let v = vec![1, 2, 3];
if v.is_empty() {
    // The vector is empty...
}

Turn it off, or on

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

On one item

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

Whole crate (Cargo.toml)

[lints.clippy]
unnecessary_first_then_check = "allow"

One run

cargo clippy -- -A clippy::unnecessary-first-then-check

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

1.83.0added in Rust

Clippy's own documentation

What it does

Checks the usage of .first().is_some() or .first().is_none() to check if a slice is empty.

Why is this bad?

Using .is_empty() is shorter and better communicates the intention.

Example

let v = vec![1, 2, 3];
if v.first().is_none() {
    // The vector is empty...
}

Use instead:

let v = vec![1, 2, 3];
if v.is_empty() {
    // The vector is empty...
}

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

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