Clippy lint · Complexity · checked with Clippy 0.1.99

clippy::extra_unused_lifetimes: this lifetime isn't used in the function definition

What it meansChecks for lifetimes in generics that are never used anywhere else. It is in the complexity group and warns by default.

What you see

warning: this lifetime isn't used in the function definition
 --> main.rs:4:20
  |
4 | fn unused_lifetime<'a>(x: u8) {
  |                    ^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#extra_unused_lifetimes
  = note: `#[warn(clippy::extra_unused_lifetimes)]` 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?

The additional lifetimes make the code look more complicated, while there is nothing out of the ordinary going on. Removing them leads to more readable code.

Code that does something simple in a complicated way. Warns by default.

How to fix it

Clippy does not fix this one automatically. Change the code by hand:

Flagged

// unnecessary lifetimes
fn unused_lifetime<'a>(x: u8) {
    // ..
}

Write this instead

fn no_lifetime(x: u8) {
    // ...
}

Turn it off, or on

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

On one item

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

Whole crate (Cargo.toml)

[lints.clippy]
extra_unused_lifetimes = "allow"

One run

cargo clippy -- -A clippy::extra-unused-lifetimes

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

nofixed by cargo clippy --fix

pre 1.29.0added in Rust

Clippy's own documentation

What it does

Checks for lifetimes in generics that are never used anywhere else.

Why is this bad?

The additional lifetimes make the code look more complicated, while there is nothing out of the ordinary going on. Removing them leads to more readable code.

Example

// unnecessary lifetimes
fn unused_lifetime<'a>(x: u8) {
    // ..
}

Use instead:

fn no_lifetime(x: u8) {
    // ...
}

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

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