Clippy lint · Performance · checked with Clippy 0.1.99

clippy::replace_box: creating a new box with default content

What it meansDetects assignments of Default::default() or Box::new(value) to a place of type Box<T>. It is in the performance group and warns by default.

What you see

warning: creating a new box with default content
 --> main.rs:4:1
  |
4 | b = Default::default();
  | ^^^^^^^^^^^^^^^^^^^^^^ help: replace existing content with default instead: `*b = Default::default()`
  |
  = note: this creates a needless allocation
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#replace_box
  = note: `#[warn(clippy::replace_box)]` 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?

This incurs an extra heap allocation compared to assigning the boxed storage.

Code that could be written to run faster. 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 b = Box::new(1u32);
b = Default::default();

Write this instead

let mut b = Box::new(1u32);
*b = Default::default();

Turn it off, or on

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

On one item

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

Whole crate (Cargo.toml)

[lints.clippy]
replace_box = "allow"

One run

cargo clippy -- -A clippy::replace-box

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.

Performancegroup

warndefault level

yesfixed by cargo clippy --fix

1.92.0added in Rust

Clippy's own documentation

What it does

Detects assignments of Default::default() or Box::new(value) to a place of type Box<T>.

Why is this bad?

This incurs an extra heap allocation compared to assigning the boxed storage.

Example

let mut b = Box::new(1u32);
b = Default::default();

Use instead:

let mut b = Box::new(1u32);
*b = Default::default();

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

Up next

Also worth your time

Did this answer your question?