Optimize the next-power-of-two calculation in vectors for code size

[pcwalton]

Currently it's more general than it needs to be, resulting in the next-power-of-two code appearing in lots of places due to inlining. But vec capacities are usually powers of two. I think it should be something like:

if popcount(capacity) != 1 {
    // non-power-of-two case
    capacity_slow()
} else {
    capacity << 1
}

[hirschenberger]

Do you mean in the Vec::reserve method?

[thestinger]

Ideally, the slow path would be hoisted out but it interfered with optimizations before.

[emberian]

I'm going to close this. #23670 moves cap calculation out of the inlined fast path entirely.