[kbenson]

It's both, it's just complicated by the fact that LLVM is optimizing it to a known formula.

The zero cost abstractions are the fact that you get the same output, special optimization and all, when you do the original loop, or when you use the "(1..n).fold(0, |x,y| x + y)" variant, or "(1..n).sum()" variant. Rust is converting those to intermediate code that is the same, or close enough, that LLVM is able to apply the same optimization.

Would it be better is some sample code was chosen that wasn't special cased to the degree that the entire algorithm was replaced wholesale? Probably. It doesn't invalidate the premise though, just slightly obscures it.