Use .extend(some_iter) over .extend(some_iter.collect())

[jovenlin0527]

What it does

Extend::extend takes an IntoIterator. Since every Iterator is also an IntoIterator, you can extend from an iterator; collect() an iterator before extend() from it leads to extra memory allocations and extra copying, and is probably a mistake.

Lint Name

extend_collect

Category

suspicious, perf

Advantage

Fewer memory allocations and fewer copying.

Drawbacks

Maybe collect() actually leads to a performance boost because of, say, better cache behavior, and is actually intentional.

Example

let mut v = vec![0i32];
v.extend((1..10).collect::<Vec<_>>());

Could be written as:

let mut v = vec![0i32];
v.extend(1..10);

[jovenlin0527]

I saw this in the wild: https://github.com/huhu/rust-search-extension/blob/8942eadf1e7c721294e87c7e9beac0356e0fdd2d/rust/src/tasks/crates.rs#L66

[samueltardieu]

This is not even specific to extend. It could be used everywhere a IntoIterator is expected.