RS.CLIPPY.UNNECESSARY_BOX_RETURNS

Needlessly returning a Box

This checker is a Clippy lint created by The Rust Project Contributors. The documentation shown here is a copy of the original documentation for: unnecessary_box_returns. Copyright ©2025 The Rust Team. All rights reserved.

What it does

Checks for a return type containing a Box<T> where T implements Sized

The lint ignores Box<T> where T is larger than unnecessary_box_size, as returning a large T directly may be detrimental to performance.

Why is this bad?

It's better to just return T in these cases. The caller may not need the value to be boxed, and it's expensive to free the memory once the Box<T> been dropped.

Example

fn foo() -> Box<String> {
    Box::new(String::from("Hello, world!"))
}

Use instead:

fn foo() -> String {
    String::from("Hello, world!")
}

Configuration

avoid-breaking-exported-api: Suppress lints whenever the suggested change would cause breakage for other crates.

(default: true)
unnecessary-box-size: The byte size a T in Box<T> can have, below which it triggers the clippy::unnecessary_box lint

(default: 128)