RS.CLIPPY.REPEAT_VEC_WITH_CAPACITY

What it does

Looks for patterns such as vec![Vec::with_capacity(x); n] or iter::repeat(Vec::with_capacity(x)).

Why is this bad?

These constructs work by cloning the element, but cloning a Vec<_> does not respect the old vector's capacity and effectively discards it.

This makes iter::repeat(Vec::with_capacity(x)) especially suspicious because the user most certainly expected that the yielded Vec<_> will have the requested capacity, otherwise one can simply write iter::repeat(Vec::new()) instead and it will have the same effect.

Similarly for vec![x; n], the element x is cloned to fill the vec. Unlike iter::repeat however, the vec repeat macro does not have to clone the value n times but just n - 1 times, because it can reuse the passed value for the last slot. That means that the last Vec<_> gets the requested capacity but all other ones do not.

Example

let _: Vec<Vec<u8>> = vec![Vec::with_capacity(42); 123];
let _: Vec<Vec<u8>> = iter::repeat(Vec::with_capacity(42)).take(123).collect();

Use instead:

let _: Vec<Vec<u8>> = iter::repeat_with(|| Vec::with_capacity(42)).take(123).collect();
//                                      ^^^ this closure executes 123 times
//                                          and the vecs will have the expected capacity

Configuration

• msrv: The minimum rust version that the project supports. Defaults to the rust-version field in Cargo.toml

  (default: current version)