AiC: Unbounded queues and lang design

10 July 2019

I have been thinking about how language feature development works in Rust1. I wanted to write a post about what I see as one of the key problems: too much concurrency in our design process, without any kind of “back-pressure” to help keep the number of “open efforts” under control. This setup does enable us to get a lot of things done sometimes, but I believe it also leads to a number of problems.

Although I don’t make any proposals in this post, I am basically advocating for changes to our process that can help us to stay focused on a few active things at a time. Basically, incorporating a notion of capacity such that, if we want to start something new, we either have to finish up with something or else find a way to grow our capacity.

The feature pipeline

Consider how a typical language feature gets introduced today:

  • Initial design in the form of an RFC. This is done by the lang team.
  • Initial implementation is done. This work is overseen by the compiler team, but often it is done by a volunteer contributor who is not themselves affiliated.
  • Documentation work is done, again often by a contributor, overseen by the docs team.
  • Experimentation in nightly takes places, often leading to changes in the design. (These changes have their own FCP periods.)
  • Finally, at some point, we stabilize the feature. This involves a stabilization report that summarizes what has changed, known bugs, what tests exist, and other details. This decision is made by the lang team.

At any given time, therefore, we have a number of features at each point in the pipeline – some are being designed, some are waiting for an implementor to show up, etc.

Today we have unbounded queues

One of the challenges is that the “links” between these pipeline are effectively unbounded queues. It’s not uncommon that we get an RFC for a piece of design that “seems good”. The RFC gets accepted. But nobody is really driving that work – as a result, it simply languishes. To me, the poster child for this is RFC 66 – a modest change to our rules around the lifetime of temporary values. I still think the RFC is a good idea (although its wording is very imprecise and it needs to be rewritten to be made precise). But it’s been sitting around unimplemented since June of 2014. At this point, is the original decision approving the RFC even still valid? (I sort of think no, but we don’t have a formal rule about that.)

How can an RFC sit around for 5 years?

Why did this happen? I think the reason is pretty clear: the idea was good, but it didn’t align with any particular priority. We didn’t have resources lined up behind it. It needed somebody from the lang team (probably me) to rewrite its text to be actionable and precise2. It needed somebody from the compiler team (maybe me again) to either write a PR or mentor somebody through it. And all those people were busy doing other things. So why did we accept the PR in the first place? Well, why wouldn’t we? Nothing in the process states that we should consider available resources when making an RFC decision.

Unbounded queues lead to confusion for users

So why does it matter when things sit around? I think it has a number of negative effects. The most obvious is that it sends really confusing signals to people trying to follow along with Rust’s development. It’s really hard to tell what the current priorities are; it’s hard to tell when a given feature might actually appear. Some of this we can help resolve just by better labeling and documentation.

Unbounded queues make it harder for teams

But there are other, more subtle effects. Overall, it makes it much harder for the team itself to stay organized and focused and that in turn can create a lot of stress. Stress in turn magnifies all other problems.

How does it make it harder to stay organized? Under the current setup, people can add new entries into any of these queues at basically any time. This can come in many forms, such as new RFCs (new design work and discussion), proposed changes to an existing design (new design or implementation work), etc.

Just having a large number of existing issues means that, in a very practical sense, it becomes challenging to follow GitHub notifications or stay on top of all the things going on. I’ve lost count of the number of attempts I’ve made at this personally.

Finally, the fact that design work stretches over such long periods (frequently years!) makes it harder to form stable communities of people that can dig deeply into an issue, develop a rapport, and reach a consensus.

Leaving room for serendipity?

Still, there’s a reason that we setup the system the way we did. This setup can really be a great fit for an open source project. After all, in an open source project, it can be really hard for us to figure out how many resources we actually have. It’s certainly more than the number of folks on the teams. It happens pretty regularly that people appear out of the blue with an amazing PR implementing some feature or other – and we had no idea they were working on it!

In the 2018 RustConf keynote, we talked about the contrast between OSS by serendipity and OSS on purpose. We were highlighting exactly this tension: on the one hand, Rust is a product, and like any product it needs direction. But at the same time, we want to enable people to contribute as much as we can.

Reviewing as the limited resource

Still, while the existing setup helps ensure that there are many opportunities for people to get involved, it also means that people who come with a new idea, PR, or whatever may wind up waiting a long time to get a response. Often the people who are supposed to answer are just busy doing other things. Sometimes, there is a (often unspoken) understanding that a given issue is just not high enough priority to worry about.

In an OSS project, therefore, I think that the right way to measure capacity is in terms of reviewer bandwidth. Here I mean “reviewer” in a pretty general way. It might be someone who reviews a PR, but it might also be a lang team member who is helping to drive a particular design forward.

Leaving room for new ideas?

One other thing I’ve noticed that’s worth highlighting is that, sometimes, hard ideas just need time to bake. Trying to rush something through the design process can be a bad idea.

Consider specialization: On the one hand, this feature was first proposed in July of 2015. We had a lot of really important debate at the time about the importance of parametricity and so forth. We have an initial implementation. But there was one key issue that never got satisfactorily resolved, a technical soundness concern around lifetimes and traits. As such, the issue has sat around – it would get periodically discussed but we never came to a satisfactory conclusion. Then, in Feb of 2018, I had an idea which aturon then extended in April. It seems like these ideas have basically solved the problem, but we’ve been busy in the meantime and haven’t had time to follow up.

This is a tricky case: maybe if we had tried to push specialization all the way to stabilization, we would have had these same ideas. But maybe we wouldn’t have. Overall, I think that deciding to wait has worked out reasonably well for us, but probably not optimally. I think in an ideal world we would have found some useful subset of specialization that we could stabilize, while deferring the tricky questions.

Tabling as an explicit action

Thinking about specialization leads to an observation: one of the things we’re going to have to figure out is how to draw good boundaries so that we can push out a useful subset of a feature (an “MVP”, if you will) and then leave the rest for later. Unlike today, though, I think should be an explicit process, where we take the time to document the problems we still see and our current understanding of the space, and then explicitly “table” the remainder of the work for another time.

People need help to set limits

One of the things I think we should put into our system is some kind of hard cap on the number of things you can do at any given time. I’d like this cap to be pretty small, like one or two. This will be frustrating. It will be tempting to say “sure I’m working on X, but I can make a little time for Y too”. It will also slow us down a bit.

But I think that’s ok. We can afford to do a few less things. Or, if it seems like we can’t, that’s probably a sign that we need to grow that capacity: find more people we trust to do the reviews and lead the process. If we can’t do that, then we have to adjust our ambitions.

In other words, in the absence of a cap, it is very easy to “stretch” to achieve our goals. That’s what we’ve done often in the past. But you can only stretch so far and for so long.


As I wrote in the beginning, I’m not making any proposals in this post, just sharing my current thoughts. I’d like to hear if you think I’m onto something here, or heading in the wrong direction. Here is a link to the Adventures in Consensus thread on internals.

One thing that has been pointed out to me is that these ideas resemble a number of management philosophies, most notably kanban. I don’t have much experience with that personally but it makes sense to me that others would have tried to tackle similar issues.


  1. I’m coming at this from the perspective of the lang team, but I think a lot of this applies more generally. ↩︎

  2. For that matter, it would be helpful if there were a spec of the current behavior for it to build off of. ↩︎