[lasagnaphil]

This isn’t really talking about Rust, it’s actually talking about the optimization capabilities of LLVM (the compiler backend, which quite a lot of languages use, such as Clang for C++, Rust, Swift, Julia, Zig, ...) These languages all have similar chances of performing those same optimizations, at least in those simple cases.

What I’m interested is how the IL code for the compiler frontends for each of those languages are more well-optimizable for LLVM (in practical situations, not just a few lines of simple numerical code.) I’ve heard that you need to be careful about encoding IL code in the right way such that LLVM does not generate needless memcpy’s or something but I’m not that much of a compiler expert...

[kibwen]

rustc does indeed deliberately seek to emit LLVM IR that resembles what Clang would emit, in order to benefit from the same sorts of optimizations that default LLVM is tuned for.

[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.