Clippy lint · Performance · checked with Clippy 0.1.99

clippy::vec_init_then_push: calls to `push` immediately after creation

What it meansChecks for calls to push immediately after creating a new Vec. If the Vec is created using with_capacity this will only lint if the capacity is a constant and the number of pushes is greater than or equal to the initial capacity. If the Vec is extended after the initial sequence of pushes and it was default initialized then this will only lint after there were at least four pushes. This number may change in the future. It is in the performance group and warns by default.

What you see

warning: calls to `push` immediately after creation
 --> main.rs:3:1
  |
3 | / let mut v = Vec::new();
4 | | v.push(0);
5 | | v.push(1);
6 | | v.push(2);
  | |__________^ help: consider using the `vec![]` macro: `let v = vec![..];`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#vec_init_then_push
  = note: `#[warn(clippy::vec_init_then_push)]` 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?

The vec![] macro is both more performant and easier to read than multiple push calls.

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

How to fix it

Clippy does not fix this one automatically (its suggestion may change behaviour, so it only shows it). Change the code by hand:

Flagged

let mut v = Vec::new();
v.push(0);
v.push(1);
v.push(2);

Write this instead

let v = vec![0, 1, 2];

Turn it off, or on

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

On one item

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

Whole crate (Cargo.toml)

[lints.clippy]
vec_init_then_push = "allow"

One run

cargo clippy -- -A clippy::vec-init-then-push

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

nofixed by cargo clippy --fix

1.51.0added in Rust

Clippy's own documentation

What it does

Checks for calls to push immediately after creating a new Vec.

If the Vec is created using with_capacity this will only lint if the capacity is a constant and the number of pushes is greater than or equal to the initial capacity.

If the Vec is extended after the initial sequence of pushes and it was default initialized then this will only lint after there were at least four pushes. This number may change in the future.

Why is this bad?

The vec![] macro is both more performant and easier to read than multiple push calls.

Example

let mut v = Vec::new();
v.push(0);
v.push(1);
v.push(2);

Use instead:

let v = vec![0, 1, 2];

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

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