Suggest Iterator::count instead of collecting + len

[Luro02]

What it does

Suggests the use of Iterator::count instead of collecting into a collection and then calling its len function.

Categories (optional)

• Kind: clippy::complexity

What is the advantage of the recommended code over the original code

• Simpler to read
• No Heap Allocations

Drawbacks

None.

Example

fn count(string: &str) -> usize {
    let buffer: Vec<&str> = string.split('/').collect();
    buffer.len()
}

Could be written as:

fn count_rewritten(string: &str) -> usize {
    string.split('/').count()
}

playground

[llogiq]

That should be caught by needless_collect.

[mgacek8]

@rustbot claim

[llogiq]

Note that this should only lint for collections where the len() actually equals the count(). Vec and VecDeque as well as BinaryHeap. Notably, neither the *Sets nor Strings uphold this equality.

[mgacek8]

Note that this should only lint for collections where the len() actually equals the count().

Yep, that's right. Currently this lint (in case of indirect usage, as in the above example) respects Vec, VecDeque and LinkedList.
The problem is that it doesn't work when a type annotation is used.

[mgacek8]

as well as BinaryHeap

BinaryHeap is missing, so I'll add that