Rust didn't implement async/await for fun. The team implemented it because it was perhaps the #1 requested feature from users, including me.

You can see it here on HN. What's the #1 comment that always appears on Rust async threads? Invariably it's "why not just use goroutines? That kind of code is so much easier to write!" Now for various reasons M:N threading like in Go doesn't work in Rust. What does work, and offers most of the ergonomics of threads/goroutines, is async/await. That's why the team has prioritized it.

There are people who care less about ergonomics than optimizing every CPU cycle for socket servers. For those people, the focus on getting the ergonomics right before microoptimization of mio and tokio might be frustrating to see. But the overwhelming majority of users just want easy-to-use async I/O. Async/await is an enormous improvement in readability and maintainability over hand-written state machines. Rust would be doing a disservice to its users if it focused on microoptimization of tokio instead.

If you think that Rust doesn't care about addressing the needs of C++ users, I'm not sure what to tell you.

It's not like C++ only focuses on features with zero overhead. The zero-overhead principle means that you don't pay for what you don't use. It doesn't mean that compiler developers are forbidden from working on any language or library feature that consumes more than zero cycles at runtime.

I think the issue is that the zero-cost abstraction terminology is used in two ways often. First, to mean a feature you can use and costs nothing because the compiler will change it into a more common form for you (such as functional programming approaches where it will take multiple filters and map passes and turn it into a single loop), and second, to mean that features added that do add overhead, only do so when you use them (as in, there's no runtime to manage M:N threading unless you include a crate that does so, because developers not using that feature shouldn't have to pay for it).

I think the terminology here is looser than it should be, and these are not really the same thing, even though I've seen zero-cost abstractions used to describe them both. It causes confusion sometimes, as I believe it is doing here.

Sure, and there are a few hooks for one in the C++ standard library e.g. std::declare_reachable [1], although it doesn't include an actual implementation. Only mark and sweep is feasible, not mark and compact, for obvious reasons [2]. Here's a mention of garbage collection Stroustrop's old C++11 FAQ [3]

[1] https://en.cppreference.com/w/cpp/memory/gc/declare_reachabl...

[2] https://herbsutter.com/2011/10/25/garbage-collection-synopsi...

[3] https://web.archive.org/web/20120228143039/http://www2.resea...

Sure. It's called Rc and Arc.

Don't be intentionally obtuse. You know I mean something more like mark and sweep

Reference counting is a form of garbage collection, despite the fact that it does not detect cycles without extra work. This has been the correct terminology in the memory management literature for decades.

Is shared_ptr a zero cost abstraction?

Mostly, with the caveat that in single threaded usage, you're paying for the unnecessary overhead of atomic ops on the reference count.

Rust has `Rc`, which is not atomic but `!Send` so the type system ensures that it stays within a thread, as well as the atomic `Arc`.

> Rust kind of skipped the whole non-blocking thing and went straight to tokio, then had to backtrack a little to mio+tokio, then when mio is only half way jupmed to async/await.

Not sure exactly what you mean here; mio predates tokio by a couple years, and has been possible to use standalone since before tokio ever became a thing.

Early versions of mio had a bunch of non-essential things like a timer wheel built-in, that were later removed.

What does tokio have to do with that?

This strikes me as needlessly hostile without contributing anything.