Clippy lint · Complexity · checked with Clippy 0.1.99

clippy::needless_lifetimes: the following explicit lifetimes could be elided: 'a

What it meansChecks for lifetime annotations which can be removed by relying on lifetime elision. It is in the complexity group and warns by default.

What you see

warning: the following explicit lifetimes could be elided: 'a
 --> main.rs:4:15
  |
4 | fn in_and_out<'a>(x: &'a u8, y: u8) -> &'a u8 {
  |               ^^      ^^                ^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#needless_lifetimes
  = note: `#[warn(clippy::needless_lifetimes)]` on by default
help: elide the lifetimes
  |
4 - fn in_and_out<'a>(x: &'a u8, y: u8) -> &'a u8 {
4 + fn in_and_out(x: &u8, y: u8) -> &u8 {
  |

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 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

// Unnecessary lifetime annotations
fn in_and_out<'a>(x: &'a u8, y: u8) -> &'a u8 {
    x
}

Write this instead

fn elided(x: &u8, y: u8) -> &u8 {
    x
}

Turn it off, or on

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

On one item

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

Whole crate (Cargo.toml)

[lints.clippy]
needless_lifetimes = "allow"

One run

cargo clippy -- -A clippy::needless-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

yesfixed by cargo clippy --fix

pre 1.29.0added in Rust

Clippy's own documentation

What it does

Checks for lifetime annotations which can be removed by relying on lifetime elision.

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.

Known problems

This lint ignores functions with where clauses that reference lifetimes to prevent false positives.

Example

// Unnecessary lifetime annotations
fn in_and_out<'a>(x: &'a u8, y: u8) -> &'a u8 {
    x
}

Use instead:

fn elided(x: &u8, y: u8) -> &u8 {
    x
}

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

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