I completely agree, it’s a pain and I was hoping for it to be higher priority too—though I sympathize with all the arguments given by other people. I only meant to point out that in the meantime, when perf matters, you can avoid boxing it; I end up doing that a lot in my code.
Re: First I also don’t like over relying on an optmizing compiler—I want my perf to be predictable.
But for the sake of the conversation for an fn pointer, if it’s in the same crate, LLVM seems to do a decent job: http://is.gd/hthlFG . Check out the LLVM IR, it gets expanded directly into 11, 12, 13, 14, inline-ing through the fn ptr.
If you make the iterator returning function inline/generic (which it probably should be), then it should work cross crate too. contain-rs relies on this a lot.