I recently published an article suggesting a different way of looking at async and await in Rust. In it, I discussed strategies for implementing state machines, and explained why I like async as a tool for building such state machines, even without threads.

In this post I’ll work through an example of why I’m so excited about this technique, by building a real driver for a notoriously tricky bus one piece at a time, using lilos.

One of the nice things about the Rust programming language is that it makes it easier to write correct concurrent (e.g. threaded) programs – to the degree that Rust’s slogan has been, at times, “fearless concurrency.”

But I’d like to tell you about the other side of Rust, which I think is under-appreciated. Rust enables you to write programs that are not concurrent. This feature is missing from most other languages, and is a source of much complexity and bugs.

“But wait,” you might be saying, “of course I can write code that isn’t concurrent in Java or Python or C!”

Can you, though? You can certainly write code that ignores concurrency, and would malfunction if (say) used from multiple threads simultaneously. But that’s not the same thing as writing code that isn’t concurrent – code that simply can’t be used concurrently, by compiler guarantee.

In Rust, you can. Let’s look at why you can do it, and why it’s awesome.

One of the common complaints I hear from systems programmers who try Rust is about mutexes, and specifically about the Rust Mutex API. The complaints usually go something like this:

• They don’t want the mutex to contain data, just a lock.
• They don’t want to have to manage a “guard” value that unlocks the mutex on drop – often, more specifically, they just want to call an unlock operation because they feel like that’s more explicit.

These changes would make the Rust mutex API equivalent to the C/Posix mutex API. In one case I’ve seen someone try to use Mutex<()> and trickery to fake it.

There’s a problem with this, though: these two aspects of Mutex’s design are inextricably linked to one another, and to Rust’s broader safety guarantees – changing either or both of them will open the door to subtle bugs and corruption due to data races.

A C-style mutex API consisting of some bundle of implicitly guarded data, plus lock and unlock functions, isn’t wise in Rust because it allows safe code to easily commit errors that break memory safety and create data races.

Perhaps controversially, I’d argue that this is also true in C. It’s just more obvious in Rust, because Rust rigorously distinguishes between the notion of “safe” code that cannot commit such errors, and “unsafe” code that can commit such errors if it wishes. C does not make this distinction, and as a result, any code using a mutex in C can trivially produce serious, potentially exploitable, bugs.

In the rest of this post I’ll walk through a typical C mutex API, compare with a typical Rust mutex API, and look at what happens if we change the Rust API to resemble C in various ways.