MIR-based borrowck is almost here

31 October 2018

Now that the final Rust 2018 Release Candidate has shipped, I thought it would be a good idea to do another update on the state of the MIR-based borrow check (aka NLL). The last update was in June, when we were still hard at work on getting things to work.

Rust 2018 will use NLL now

Let’s get the highlights out of the way. Most importantly, Rust 2018 crates will use NLL by default. Once the Rust 2018 release candidate becomes stable, we plan to switch Rust 2015 crates to use NLL as well, but we’re holding off until we have some more experience with people using it in the wild.

NLL is awesome

I’ve been using NLL in practice for quite some time now, and I can’t imagine going back. Recently I’ve been working in my spare time on the salsa crate1, which uses Rust 2018, and I’ve really noticed how NLL makes a lot of “complex” borrowing interactions work out quite smoothly. These are all instances of the problem cases #1 and #2 I highlighted way back when2, but they interact in interesting ways I did not fully anticipate.

Let me give you a hypothetical example. Imagine I am writing some bit of code that routes messages, which look like this:

enum Message {
    Letter { recipient: String, data: String },
    // ... maybe other cases here ...

When I receive a letter, I want to inspect its recipient. If that matches my name, I will process the data using process:

fn process(data: &str) { .. }

but otherwise I’ll forward it along to the next person in the chain. Using NLL, I can write this code like so (playground):

fn router(me: &str, rx: Receiver<Message>, tx: Sender<Message>) {
  for message in rx {
    match &message {
      Message::Letter { recipient, data } => {
        if recipient != me {
        } else {

      // ... maybe other cases here ...

What’s interesting about this code is how uninteresting it is – it basically just does what you expect, and didn’t require any special action to please the borrow checker3. But the borrowing patterns are actually sort of complex: it starts as we enter the match (match &message) and continues into the match arm. On the else branch of the match, the borrow is still in use (in the form of the data variable), but in the if branch, it is not (and hence we can call tx.send(message) and move the message). Before NLL, this would have required some significant contortions to achieve (try it yourself if you like – that’s a link to the same code, but with Rust 2015 edition set).

Diagnostics, migration, and performance

We’ve also put a lot of effort into NLL diagnostics and I think that by and large they are even better than the old borrow checker (which were already quite good). This is particularly true for the ’lifetime error messages’. Unfortunately, you won’t see all of those improvements yet on Rust 2018 – the reason has to do with migration.

What is this migration you ask? Well, it’s our way of dealing with the fact that the new MIR-based borrow checker has fixed a ton of soundness bugs from the old checker. Unfortuantely, in practice, that means that some existing code will not compile anymore (because it never should have compiled in the first place!). To give people time to make that transition, we are running the NLL code in “migration mode”, which means that if you have code that used to compile, but no longer does, we issue warnings instead of errors. This migration mode will eventually change to issue hard errors instead (probably in a few releases, but that depends a bit on what we find in the wild).

One downside of migration mode is that it requires keeping around the older code. In some cases, this older code can produce errors that wind up masking the newer, nicer errors that are produced by the MIR-based checker. The good news is that once we finish the migration, this means that errors will just get better.

Finally, those of you who read the previous posts may remember that compilation times when using the NLL checker was a big stumbling block. I’m happy to report that the performance issues were largely addressed: there remains some slight overhead to using NLL, but it is largely not noticeable in practice, and I expect we’ll continue to improve it over time.

What next?

So, now that NLL is shipping, what is next for ownership and borrowing in Rust? That’s a big question, and it has a few different answers, depending on the “scale” of time we are looking at. The immediate answer is that we’ve still got some bugs to nail down (small ones) and of course we expect that once more people start banging on the new code, they’ll encounter new problems that have to be fixed. In addition, we’ve got to put some energy into writing up documentation for how the new checker works and similar things (we wound up deviating from the RFC analysis in various ways, and it’d be nice to document those).

In the medium term, the plan is to push more on the Polonius formulation of NLL that I described here. In addition to offering a crisp formalization of our analysis, Polonius promises to fix the Problem Case #3 that I identified in the original NLL introduction, along with some other cases where the current analysis falls short.

In the longer term, well, that’s an open question, and one where I would like to hear from you, dear reader. Over the next week or so, I am planning to write up a series of blog posts. Each will describe what I consider to be a common “tricky scenario” where people hit problems with the borrow checker, and none of which are solved by NLL. I’ll also describe the current fixes required. Then I hope to do a survey, trying to get a picture of which of these challenges cause the most problems for folks, so that we can try to decide how to prioritize future improvements to Rust.


  1. Did you see how smoothly I worked in that plug for salsa? I’ll write a post about it soon, I promise. ↩︎

  2. Note that the current NLL implementation does not solve Problem Case #3. See [the “What Next?” section][wn] for more. [wn]: #what-next ↩︎

  3. Interestingly, I remember an example almost exactly like this being shown to me by a Servo intern – I forget which one – many years ago. At the time, it didn’t seem like a big deal to do the workarounds, but I realize now I was wrong about that. Ah well. ↩︎