Clippy lint · Complexity · checked with Clippy 0.1.99

clippy::manual_swap: this looks like you are swapping `b` and `a` manually

What it meansChecks for manual swapping. Note that the lint will not be emitted in const blocks, as the suggestion would not be applicable. It is in the complexity group and warns by default.

What you see

warning: this looks like you are swapping `b` and `a` manually
 --> main.rs:6:1
  |
6 | / let t = b;
7 | | b = a;
8 | | a = t;
  | |______^ help: try: `std::mem::swap(&mut b, &mut a);`
  |
  = note: or maybe you should use `std::mem::replace`?
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#manual_swap
  = note: `#[warn(clippy::manual_swap)]` 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 std::mem::swap function exposes the intent better without deinitializing or copying either variable.

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 mut a = 42;
let mut b = 1337;

let t = b;
b = a;
a = t;

Write this instead

let mut a = 1;
let mut b = 2;
std::mem::swap(&mut a, &mut b);

Turn it off, or on

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

On one item

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

Whole crate (Cargo.toml)

[lints.clippy]
manual_swap = "allow"

One run

cargo clippy -- -A clippy::manual-swap

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 manual swapping.

Note that the lint will not be emitted in const blocks, as the suggestion would not be applicable.

Why is this bad?

The std::mem::swap function exposes the intent better without deinitializing or copying either variable.

Example

let mut a = 42;
let mut b = 1337;

let t = b;
b = a;
a = t;

Use std::mem::swap():

let mut a = 1;
let mut b = 2;
std::mem::swap(&mut a, &mut b);

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

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