Clippy lint · Complexity · checked with Clippy 0.1.99

clippy::swap_with_temporary: swapping with a temporary value is inefficient

What it meansChecks for usage of std::mem::swap with temporary values. It is in the complexity group and warns by default.

What you see

warning: swapping with a temporary value is inefficient
 --> main.rs:4:5
  |
4 |     std::mem::swap(s, &mut String::from("replaced"));
  |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use assignment instead: `*s = String::from("replaced")`
  |
note: this expression returns a temporary value
 --> main.rs:4:28
  |
4 |     std::mem::swap(s, &mut String::from("replaced"));
  |                            ^^^^^^^^^^^^^^^^^^^^^^^^
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#swap_with_temporary
  = note: `#[warn(clippy::swap_with_temporary)]` 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?

Storing a new value in place of a temporary value which will be dropped right after the swap is an inefficient way of performing an assignment. The same result can be achieved by using a regular assignment.

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

fn replace_string(s: &mut String) {
    std::mem::swap(s, &mut String::from("replaced"));
}

Write this instead

fn replace_string(s: &mut String) {
    *s = String::from("replaced");
}

Turn it off, or on

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

On one item

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

Whole crate (Cargo.toml)

[lints.clippy]
swap_with_temporary = "allow"

One run

cargo clippy -- -A clippy::swap-with-temporary

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

1.88.0added in Rust

Clippy's own documentation

What it does

Checks for usage of std::mem::swap with temporary values.

Why is this bad?

Storing a new value in place of a temporary value which will be dropped right after the swap is an inefficient way of performing an assignment. The same result can be achieved by using a regular assignment.

Examples

fn replace_string(s: &mut String) {
    std::mem::swap(s, &mut String::from("replaced"));
}

Use instead:

fn replace_string(s: &mut String) {
    *s = String::from("replaced");
}

Also, swapping two temporary values has no effect, as they will both be dropped right after swapping them. This is likely an indication of a bug. For example, the following code swaps the references to the last element of the vectors, instead of swapping the elements themselves:

fn bug(v1: &mut [i32], v2: &mut [i32]) {
    // Incorrect: swapping temporary references (`&mut &mut` passed to swap)
    std::mem::swap(&mut v1.last_mut().unwrap(), &mut v2.last_mut().unwrap());
}