lint naive counts

[llogiq]

Lint _.iter().filter(predicate).count(), suggest _.iter().map(|x| predicate(x) as usize).sum(), which results in branchless code that is usually faster.

[mcarton]

Meh, the semantic of filter().count() is clearer, if .map(as usize).sum() really is more efficient, this is to be fixed in rustc.

[llogiq]

I agree. Leaving this open until fixed.

[llogiq]

Closing in favor of rust-lang/rust#39107