Clippy lint · Pedantic · checked with Clippy 0.1.99

clippy::assigning_clones: assigning the result of `Clone::clone()` may be inefficient

What it meansChecks for code like foo = bar.clone(); It is in the pedantic group and off by default.

What you see

warning: assigning the result of `Clone::clone()` may be inefficient
  --> main.rs:11:5
   |
11 |     *a = b.clone();
   |     ^^^^^^^^^^^^^^ help: use `clone_from()`: `a.clone_from(&b)`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#assigning_clones
   = note: `-W clippy::assigning-clones` implied by `-W clippy::pedantic`

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?

Custom Clone::clone_from() or ToOwned::clone_into implementations allow the objects to share resources and therefore avoid allocations.

Stricter lints with some false positives. Off by default; opt in with clippy::pedantic.

How to fix it

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

Flagged

struct Thing;

impl Clone for Thing {
    fn clone(&self) -> Self { todo!() }
    fn clone_from(&mut self, other: &Self) { todo!() }
}

pub fn assign_to_ref(a: &mut Thing, b: Thing) {
    *a = b.clone();
}

Write this instead

struct Thing;

impl Clone for Thing {
    fn clone(&self) -> Self { todo!() }
    fn clone_from(&mut self, other: &Self) { todo!() }
}

pub fn assign_to_ref(a: &mut Thing, b: Thing) {
    a.clone_from(&b);
}

Turn it off, or on

It is off by default. To use it, set it to warn or deny:

Whole crate (Cargo.toml)

[lints.clippy]
assigning_clones = "warn"

In code

#![warn(clippy::assigning_clones)]

One run

cargo clippy -- -W clippy::assigning-clones

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.

Settings in clippy.toml

msrv: The minimum rust version that the project supports. Defaults to the rust-version field in Cargo.toml

Pedanticgroup

allowdefault level

nofixed by cargo clippy --fix

1.78.0added in Rust

Clippy's own documentation

What it does

Checks for code like foo = bar.clone();

Why is this bad?

Custom Clone::clone_from() or ToOwned::clone_into implementations allow the objects to share resources and therefore avoid allocations.

Example

struct Thing;

impl Clone for Thing {
    fn clone(&self) -> Self { todo!() }
    fn clone_from(&mut self, other: &Self) { todo!() }
}

pub fn assign_to_ref(a: &mut Thing, b: Thing) {
    *a = b.clone();
}

Use instead:

struct Thing;

impl Clone for Thing {
    fn clone(&self) -> Self { todo!() }
    fn clone_from(&mut self, other: &Self) { todo!() }
}

pub fn assign_to_ref(a: &mut Thing, b: Thing) {
    a.clone_from(&b);
}

Configuration

msrv: The minimum rust version that the project supports. Defaults to the rust-version field in Cargo.toml

(default: current version)

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

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