Clippy lint · Pedantic · checked with Clippy 0.1.99

clippy::unnecessary_box_returns: boxed return of the sized type `std::string::String`

What it meansChecks 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. It is in the pedantic group and off by default.

What you see

warning: boxed return of the sized type `std::string::String`
 --> main.rs:3:13
  |
3 | fn foo() -> Box<String> {
  |             ^^^^^^^^^^^ help: try: `std::string::String`
  |
  = help: changing this also requires a change to the return expressions in this function
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#unnecessary_box_returns
  = note: `-W clippy::unnecessary-box-returns` 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?

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.

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

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

Write this instead

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

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]
unnecessary_box_returns = "warn"

In code

#![warn(clippy::unnecessary_box_returns)]

One run

cargo clippy -- -W clippy::unnecessary-box-returns

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

avoid-breaking-exported-api: Suppress lints whenever the suggested change would cause breakage for other crates.
unnecessary-box-size: The byte size a T in Box<T> can have, below which it triggers the clippy::unnecessary_box lint

Pedanticgroup

allowdefault level

nofixed by cargo clippy --fix

1.70.0added in Rust

Clippy's own documentation

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)

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

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