Clippy lint · Performance · checked with Clippy 0.1.99

clippy::box_collection: Checks for usage of Box<T> where T is a collection such as Vec anywhere in the code. Check the Box documentation for more information

What it meansChecks for usage of Box<T> where T is a collection such as Vec anywhere in the code. Check the Box documentation for more information. It is in the performance group and warns by default.

Does it matter?

Collections already keeps their contents in a separate area on the heap. So if you Box them, you just add another level of indirection without any benefit whatsoever.

Code that could be written to run faster. Warns by default.

How to fix it

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

Flagged

struct X {
    values: Box<Vec<Foo>>,
}

Write this instead

struct X {
    values: Vec<Foo>,
}

Turn it off, or on

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

On one item

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

Whole crate (Cargo.toml)

[lints.clippy]
box_collection = "allow"

One run

cargo clippy -- -A clippy::box-collection

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.

Performancegroup

warndefault level

nofixed by cargo clippy --fix

1.57.0added in Rust

Clippy's own documentation

What it does

Checks for usage of Box<T> where T is a collection such as Vec anywhere in the code. Check the Box documentation for more information.

Why is this bad?

Collections already keeps their contents in a separate area on the heap. So if you Box them, you just add another level of indirection without any benefit whatsoever.

Example

struct X {
    values: Box<Vec<Foo>>,
}

Better:

struct X {
    values: Vec<Foo>,
}

Past names

box_vec

Configuration

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

(default: true)

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

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