clippy::extend_with_drain: use of `extend` instead of `append` for adding the full range of a second vector

What you see

warning: use of `extend` instead of `append` for adding the full range of a second vector
 --> main.rs:6:1
  |
6 | a.extend(b.drain(..));
  | ^^^^^^^^^^^^^^^^^^^^^ help: try: `a.append(&mut b)`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#extend_with_drain
  = note: `#[warn(clippy::extend_with_drain)]` 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 append instead of extend is more concise and faster

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 a = vec![1, 2, 3];
let mut b = vec![4, 5, 6];

a.extend(b.drain(..));

Write this instead

let mut a = vec![1, 2, 3];
let mut b = vec![4, 5, 6];

a.append(&mut b);

Turn it off, or on

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

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

[lints.clippy]
extend_with_drain = "allow"

cargo clippy -- -A clippy::extend-with-drain

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.