Lessons from the impl period

5 January 2018

So, as you likely know, we tried something new at the end of 2017. For roughly the final quarter of the year, we essentially stopped doing design work, and instead decided to focus on implementation – what we called the “impl period”. We had two goals for the impl period: (a) get a lot of high-value implementation work done and (b) to do that by expanding the size of our community, and making it easy for new people to get involved. To that end, we spun up about 40 working groups, which is really a tremendous figure when you think about it, each of which was devoted to a particular task.

For me personally, this was a very exciting three months. I really enjoyed the enthusiasm and excitement that was in the air. I also enjoyed the opportunity to work in a group of people collectively trying to get our goals done – one thing I’ve found working on an open-source project is that it is often a much more “isolated” experience than working in a more traditional company. The impl period really changed that feeling.

I wanted to write a brief post kind of laying out my experience and trying to dive a bit into what I felt worked well and what did not. I’d very much like to hear back from others who participated (or didn’t). I’ve opened up a dedicated thread on internals for discussion, please leave comments there!


If you don’t want to read the details, here are the major points:

  • Overall, the impl period worked great. Having structure to the year felt liberating and I think we should do more of it.
  • We need to grow and restructure the compiler team around the idea of mentoring and inclusion. I think having more focused working groups will be a key part of that.
  • We have work to do on making the compiler code base accessible, beginning with top-down documentation but also rustdoc.
  • We need to develop skills and strategies for how to split tasks up.
  • IRC isn’t great, but Gitter wasn’t either. The search for a better chat solution continues. =)

Worked well: establishing focus and structure to the year

Working on Rust often has this kind of firehose quality: so much is going on at once. At any one time, we are:

  • fixing bugs in existing code,
  • developing code for new features that have been designed,
  • discussing the minutae and experience of some existing feature we may consider stabilizing,
  • designing new features and APIs via RFCs.

It can get pretty exhausting to keep all that in your head at once. I really enjoyed having a quarter to just focus on one thing – implementing. I would like us to introduce more structure into future years, so that we can have a time when we are just focused on design, and so forth.

I also appreciated that the impl period imposed a kind of “soft deadline”. I found that helpful for defining our scope. I felt like it ensured that difficult discussions did reach an end point.

That said, I don’t think we managed this deadline especially well this year. The final discussions were pretty frantic and it was hard – no, impossible – to keep up with all of them (I know I certainly couldn’t, and I work on Rust full time). Clearly in the future we need to manage the schedule better, and make sure that design work is happening at a more measured pace. I think that having more structure to the year can help with that, by ensuring that we do the design work at the time it needs to get done.

Worked well: newcomers developing key, important features

Earlier, I said that the goals of impl period were to (a) get a lot of high-value implementation work done and (b) to do that by expanding the size of our community. There is a bit of a tension there: if you have some high-value new feature, there is a tendency to think that we should have an established developer do it. After all, they know the codebase, and they will get it done the fastest. That is (often) true, but it is not the complete story.

What we wanted to do in the impl period was to focus on bringing new people into the project. Hopefully, many of those people will stick around, working on new projects, and eventually becoming experienced Rust compiler developers themselves. This increases our overall bandwidth and grows our community, making us stronger.

And even when people don’t have time to keep hacking on the Rust compiler, there are still advantages to developing through mentoring. The fact is that coding takes a lot of time. A single experienced developer can only really effectively code up a single feature at a time, but they can be mentoring many people at once.

Still, it must be said, there are plenty of people who just enjoy coding and who don’t particularly want to do mentoring. So obviously we should ensure we always have a place for experienced devs who just want to code.

Worked mostly well: smaller working groups

First and foremost, a key part of our plan was breaking up tasks into working groups. A working group was meant to be a small set of people focused on a common goal. The hope was that having smaller groups would make it easier for people to get involved and would also encourage more collaboration.

I felt the working groups worked best when they had relatively clear focus and an active leader: the NLL group is a good example. It was great to see the people in the chatrooms working together and starting to help one another out when more experienced devs weren’t available.

Other working group divisions worked less well. For example, there were a few groups in the compiler that were not specific to particular tasks, but rather parts of the compiler pipeline: WG-compiler-front, WG-compiler-middle, etc. Lots of people participated in those groups, and a lot got done, but the division into groups felt a bit more arbitrary to me. It wasn’t always clear where to put the tasks.

Going forward, I continue to think there is a role for working groups, but I think we should try to keep them focused on goals, not on the parts of the project that they touch.

Worked well: clear mentoring instructions

I’ve noticed something: if you tag a bug on the Rust’s issue tracked as E-Easy and leave a comment like “ping me on IRC”, it can easily sit there for years and years. But if you write some mentoring instructions – that is, lay out the steps to take – it will be closed, often within hours.

This makes total sense. You want to make sure that all the tools people need to hack on Rust are ready and immediately available. This way, when somebody says “I have a few hours, let me see if I can fix a bug in rustc”, they can sieze the moment. If you say “ping me on IRC”, then it may well be that you are not available at that time. Or that may be intimidating. In general, every roadblock gives them a chance to get distracted.

Of course, ideally mentoring doesn’t stop at mentoring instructions. Especially for more complex projects, I often find myself scheduling times with people so that we can have an hour or two to discuss directly what is going on, often with screen sharing or a voice call. That doesn’t always work – timezones being what they are – but when it does, it can be a big win.

Clear problem: lack of leadership bandwidth

One problem we encountered is that there just weren’t enough experienced rustc developers who were willing and able to lead up working groups. Writing mentoring instructions is hard work. Breaking up a big task into smaller parts is hard work. This is a problem outside of the impl period too. It’s hard to balance all the maintenance, bug fixing, performance monitoring, and new feature development work that needs to get done.

I don’t see a real solution here other than growing the set of people who hack on rustc. I think this should be a top priority for us. I think we should try to incorporate the idea of “contributor accessibility” into our workflow wherever possible. In other words, we should have clear paths for (a) how to get started hacking on rustc and then (b) once you’ve gotten a few PRs under your belt, how to keep growing. The impl period focused on (a) and it’s clear we do pretty well there, but have room for improvement. Part (b) is harder, and I think we need to work on it.

Clear problem: rustc documentation

One problem that makes writing mentoring instructions very difficult is that the compiler is woefully underdocumented. At the start of the impl period, many of the basic idioms and concepts (e.g., what is “the HIR” or “the MIR”? what is this 'tcx I see everywhere?) were not written up at all. It’s somewhat better now, but not great.

We also lack documentation on common workflows. How do I build the compiler? How do I debug things and get debug logs? How do I run an individual test? Some of this exists, but not always in an easy-to-find place.

I think we really need to work on this. I’d like to form a working group and focus on it early this year – but more on that later. (If you’re interested in the idea of helping to document the compiler, though, please contact me, or stay tuned!)

Clear problem: some tasks are hard to subdivide

One thing we also found is that some tasks are just plain hard to subdivide. I think a good example of this was incremental compilation: it seems like, in principle, there ought to be a lot of things that can be done in parallel there. And we had some success with newcomers, for example, picking off tasks relating to testing and doing other refactorings. I think we need to work on better strategies here. Knowing how to structure tasks for massive participation is a skillset – not unrelated to coding, but clearly distinct from it. I don’t have answers yet, but I suspect we can gain experience with this as a community and find best practices.

In the case of NLL, the model that seemed to work best was to have one more experienced developer pushing on the “main trunk” of development (myself), but actively seeking places to spin out isolated tasks into issues that could be mentored. To avoid review and bors latecy from slowing us down, we used a dedicated feature branch on my repo (nll-master) and I would periodically open up pull requests containing a variety of commits. This seemed to work out pretty well – oh, and by the way, the job is not done. If you’re still hoping to get involved, we’ve still got plenty of work to do. =) (Though most of those issues do not yet have mentoring instructions.)

Mixed bag: gitter and dedicated chat rooms

One key part of our experiment was moving from a small number of chat rooms on IRC (e.g., #rustc) to dedicate rooms on Gitter, one per working group. I had mixed feelings about this.

Let me start with the pros of Gitter itself:

  • Gitter means everybody has a persistent connection. It is great to be able to send someone a message when they may or may not be online, and get an answer sometime later.
  • Gitter means everything can be easily linked from the web. I love being able to make a link to some conversation with one click and copy it into a GitHub issue. I love being able to link to a Gitter chat room very easily.
  • Gitter means single sign on and only one name to remember. I love that I can just use people’s GitHub names, which makes it easier for me to then correlate their pull requests, or checkout their fork of Rust, etc.

But there are some pretty big cons. Mostly having to do with Gitter being buggy. The android client doesn’t deliver notifications (and maybe others as well). The IRC bridge seems to mostly work, but sometimes people get funny names (e.g., I think the Discord bridge has only one user?) or we hit other arbitrary limits.

Similarly, I felt like having dedicated rooms had pros and cons. On the one hand, it was really helpful to me personally. I find it hard to keep up with #rustc on IRC. I liked that I could be sure to read every message in WG-compiler-nll, but I could just skim over groups like WG-compiler-const that I was not directly involved in.

On the other hand, a bigger room offers more opportunity for “cross talk”. People have told me that they like having the chance to hear something interesting. And others found it was hard to follow all the rooms they were interested in.

Finally, I found that I personally still wound up doing a lot of mentoring over private messages. This is not ideal, because it doesn’t offer visibility to the rest of the group, and you can wind up repeating things, but – particularly when you’re discussing asynchronously – it’s often the most natural way to set things up.

I don’t know what’s the ideal solution here, but I do think there’s going to be a role for smaller chat rooms (though probably not based on Gitter).


The impl period was awesome. We got a lot of things done. And I do mean we: the vast majority of that work was done by newcomers to the community, many of whom had never worked on a compiler before. I loved the overall enthusiasm that was in the air. To me, it felt like what open source is supposed to be like.

Of course, though, there are things we can do better. I hope to drill into these more in later posts (or perhaps forum discussion), but I think the most important thing is that we need to think carefully about how to enable mentoring and inclusion throughout our team structure. I think we do quite well, but we can do better – and in particular we should think more about how to help people who have already done a few PRs take the next step.

As you may have heard, we’re trying something new this year. We’re encouraging people to write blog posts about what they think Rust ought to focus on for 2018 – if you do it, you can either tweet about it with the hashtag #Rust2018, or else e-mail community@rust-lang.org. I’m pretty excited about this; I’ve been enjoying reading the posts that have arrived thus far, and I plan to write a few of my own!