Use count_ones()/.count_zeros() instead of loops

What it does

It suggests to replace a loop with the usage of pop count integer methods.

Lint Name

needless_popcount_loop

Category

perf

Advantage

Shorter, less bug-prone, faster, more explicit.

Drawbacks

Slower Clippy.

Example

I'm not sure if this is doable and reasonable, but I think Clippy could recognize basic popcount loops and suggest to replace them with count_ones()/.count_zeros():

fn foo1a(x: u32) -> usize {
    (0 .. 32).filter(|i| x & (1 << i) != 0).count()
}

fn foo2a(x: u32) -> usize {
    (0 .. 32).filter(|i| x & (1 << i) == 0).count()
}

Could be written as:

fn foo1b(x: u32) -> usize {
    x.count_ones() as _
}

fn foo2b(x: u32) -> usize {
    x.count_zeros() as _
}

Start by reviewing the proposed needless_popcount_loop lint and the Rust examples in the issue, including the count_ones() and count_zeros() replacements. Done means Clippy can recognize the described popcount loops and suggest the corresponding integer methods without changing unrelated code.