Clippy lint · Performance · checked with Clippy 0.1.99

clippy::drain_collect: draining all elements of a collection into a new collection of the same type

What it meansChecks for calls to .drain() that clear the collection, immediately followed by a call to .collect(). “Collection” in this context refers to any type with a drain method: Vec, VecDeque, BinaryHeap, HashSet,HashMap, String It is in the performance group and warns by default.

What you see

warning: draining all elements of a collection into a new collection of the same type
 --> main.rs:4:5
  |
4 |     v.drain(..).collect()
  |     ^^^^^^^^^^^^^^^^^^^^^ help: use `mem::take` to avoid creating a new allocation: `std::mem::take(v)`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#drain_collect
  = note: `#[warn(clippy::drain_collect)]` 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?

Using mem::take is faster as it avoids the allocation. When using mem::take, the old collection is replaced with an empty one and ownership of the old collection is returned.

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

fn remove_all(v: &mut Vec<i32>) -> Vec<i32> {
    v.drain(..).collect()
}

Write this instead

use std::mem;
fn remove_all(v: &mut Vec<i32>) -> Vec<i32> {
    mem::take(v)
}

Turn it off, or on

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

On one item

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

Whole crate (Cargo.toml)

[lints.clippy]
drain_collect = "allow"

One run

cargo clippy -- -A clippy::drain-collect

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

1.72.0added in Rust

Clippy's own documentation

What it does

Checks for calls to .drain() that clear the collection, immediately followed by a call to .collect().

“Collection” in this context refers to any type with a drain method: Vec, VecDeque, BinaryHeap, HashSet,HashMap, String

Why is this bad?

Using mem::take is faster as it avoids the allocation. When using mem::take, the old collection is replaced with an empty one and ownership of the old collection is returned.

Known issues

mem::take(&mut vec) is almost equivalent to vec.drain(..).collect(), except that it also moves the capacity. The user might have explicitly written it this way to keep the capacity on the original Vec.

Example

fn remove_all(v: &mut Vec<i32>) -> Vec<i32> {
    v.drain(..).collect()
}

Use instead:

use std::mem;
fn remove_all(v: &mut Vec<i32>) -> Vec<i32> {
    mem::take(v)
}

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

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