Clippy lint · Complexity · checked with Clippy 0.1.99

clippy::reserve_after_initialization: call to `reserve` immediately after creation

What it meansInforms the user about a more concise way to create a vector with a known capacity. It is in the complexity group and warns by default.

What you see

warning: call to `reserve` immediately after creation
 --> main.rs:3:1
  |
3 | / let mut v: Vec<usize> = vec![];
4 | | v.reserve(10);
  | |______________^ help: consider using `Vec::with_capacity(/* Space hint */)`: `let mut v: Vec<usize> = Vec::with_capacity(10);`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/stable/index.html#reserve_after_initialization
  = note: `#[warn(clippy::reserve_after_initialization)]` 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::with_capacity constructor is less complex.

Code that does something simple in a complicated way. 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<usize> = vec![];
v.reserve(10);

Write this instead

let mut v: Vec<usize> = Vec::with_capacity(10);

Turn it off, or on

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

On one item

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

Whole crate (Cargo.toml)

[lints.clippy]
reserve_after_initialization = "allow"

One run

cargo clippy -- -A clippy::reserve-after-initialization

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.

Complexitygroup

warndefault level

nofixed by cargo clippy --fix

1.74.0added in Rust

Clippy's own documentation

What it does

Informs the user about a more concise way to create a vector with a known capacity.

Why is this bad?

The Vec::with_capacity constructor is less complex.

Example

let mut v: Vec<usize> = vec![];
v.reserve(10);

Use instead:

let mut v: Vec<usize> = Vec::with_capacity(10);

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

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