The previous two posts in this series tried to discuss the design of Rust through the lens of some higher level language concepts:

• First, the notion that programming languages have different registers
• Second, the idea that not all patterns should be made into abstractions

This post does not introduce any such high-minded concept. It is entirely down in the weeds. That’s partly because it is based on content I removed from the previous post so that post could focus on the higher point.

This is the second post in an informal series commenting on the design of async Rust in 2023. In my previous post, after a discussion of the “registers” in which control-flow effects could be handled in Rust, I promised to turn my attention to recent proposals around a concept called “keyword generics.” For reference, there are two posts by the current design team of Rust that are my reference point for this commentary:

• Keyword Generics Progress Report: February 2023, a status update from the group working on “keyword generics”
• Trait transformers (send bounds, part 3), a blog post about a related idea by Niko Matsakis

I’m not going to reiterate these blog posts at length, but in brief “keyword generics” is a proposal to introduce a new kind of abstraction to Rust, to allow types to be abstracted over certain effects. The examples have focused on async and const as the effects, but there is sometimes discussion of try as well. Astute readers will notice this is an overlapping but not identical set of effects to the effects I identified in my last post; I did not mention const as an effect, and as far as I know the keyword generics working group has not devoted much or any time to considering iteration as an effect.

It’s been nearly two and half years since I was an active contributor to the Rust project. There are some releases that I’ve been very excited about since then, and I’m heartened by Niko’s recent blog post emphasizing stability and polish over grand new projects. But I’ve also felt a certain apprehension at a lot of the directions the project has taken, which has often occupied my thoughts. From that preoccupation this blog post has emerged, hopefully the first in a series over the next few weeks outlining my thoughts on the design of Rust in 2023, especially in connection to async, and I hope its impact will be chiefly positive.

In the previous post in this series, I wrote about how the core state machine of ringbahn is implemented. In this post I want to talk about another central concept in ringbahn: “drivers”, external libraries which determine how ringbahn schedules IO operations over an io-uring instance.

A bit over a year ago, I wrote some notes on a “smaller Rust” - a higher level language that would take inspiration from some of Rust’s type system innovations, but would be simpler by virtue of targeting a domain with less stringent requirements for user control and performance. During my time of unemployment this year, I worked on sketching out what a language like that would look like in a bit more detail. I wanted to write a bit about what new conclusions I’ve come to during that time.

Today I made a new release of the iou library, which contains idiomatic Rust bindings to the liburing library. This library allows users to manipulate the new io-uring interface for asynchronous IO on Linux. For more context, you can read my previous post on the first release of iou last year.

This new release greatly expands the API of iou, introduces some valuable improvements, and contains some breakages. I figured I would let this blog post serve as some basic release notes.

I’ve just released a new crate called propane, which is a library for writing generator functions. It can only run on nightly:

#![feature(generators, generator_trait, try_trait)]

#[propane::generator]
fn fizz_buzz() -> String {
    for x in 1..101 {
        match (x % 3 == 0, x % 5 == 0) {
            (true, true)  => yield String::from("FizzBuzz"),
            (true, false) => yield String::from("Fizz"),
            (false, true) => yield String::from("Buzz"),
            (..)          => yield x.to_string(),
        }
    }
}

It’s hard to believe that its been more than 3 years since I opened RFC 2000, which defined the const generics for Rust. At the same time, reading the RFC thread, there’s also been a huge amount of change in this area: for one thing, at the time the RFC was written, const fns weren’t stable, and consts weren’t even being evaluated using miri yet. There’s been a lot of work over the years on the const generics feature, but still nothing has shipped. However, I think we have defined a very useful subset of const generics which is stable enough to ship in the near term.

Last time I wrote about ringbahn, a safe API for using io-uring from Rust. I wrote that I would soon write a series of posts about the mechanism that makes ringbahn work. In the first post in that series, I want to look at the core state machine of ringbahn which makes it memory safe. The key types involved are the Ring and Completion types.

While implementing ringbahn, I introduced at least two bugs that caused memory safety errors, resulting in segfaults, allocator aborts, and bizarre undefined behavior. I’ve fixed both bugs that I could find, and now I have no evidence that there are more memory safety issues in the current codebase (though that doesn’t mean there aren’t, of course). I wanted to write about both of these bugs, because they had an interesting thing in common: they were both caused by destructors.