Yeah, there's no particularly good way to optimize those calls in today's compiler. I mumbled something about how is_heap should be annotated to predict true (Does Rust even have a way to do this? Clang has some kind of non-standard attr I think.) or whatever, but I have nothing concrete to say as to the performance hit.
Yeah, that's what I've been saying. Absent that, I don't think making std::string::String an SSO string is worth it.
I'm glad to see the discussion this has generated. I think the issue is cleared up for me!