Setting our vision for the 2017 cycle

First: There are over 150 replies to this thread so far. I’ve skimmed them and nothing jumped out that I felt I was duplicating, but if I have, my apologies. Please point me to the correct existing conversations.


Ultimately, the goals listed in the original post here feel like “How does Rust compete with Go”, instead of “How does rust replace C and C++”. That worries me. Mostly because I’m in that vague “systems programming” space, and I want a better language; but also because I work closely with people impacted by all the easy-to-prevent security vulnerabilities in today’s OSes and devices.

At first blush, the list of non-goals seems like the laundry-list of “things that would help get Rust code into places currently using C and C++”:

  • Making Cargo work well in more contexts (e.g. big existing build systems)
  • Adding compiler/platform targets
  • Producing more API bindings for core platform libraries
  • Improving our story for programming embedded systems/OSes

I worry, I guess, that the direction of the project is being overly impacted by comparisons laid forth in that orange forum that many of us spend time on. There’s an absolute bias there towards web-focused development and SaaS applications, which doesn’t necessarily map onto Rusts strengths.

Rust, in my mind, can do the most good in embedded, OS, and other “systems” development. There are already very good languages for writing fast, correct servers. Why push Rust in that space, when it has an opportunity to stand on its own in an area where there’s been no serious competition for decades?


You may have misinterpreted that heading. It was listed as “Other possibilities, or potential non-goals” – the emphasis was supposed to be on the first part (“Other possibilities”) and not the second part (“non-goals”). In other words, these are things that seem very important – are they important enough to bump to the top of the list?

There have been several replies in this area. I’d like to discuss the needs of the “embedded” space a bit more deeply. I am thinking of opening a separate thread though for that so I’ll hold off on saying much just this second.

So. There’s a lot here. Yesterday and today I sat down and re-read through all the comments on this thread. Let me say up front: @dikaiosune, your summary comments are invaluable, thank you so much for that!

One thing I noticed is that a lot of the ideas here already fall under some of the existing priorities (which is great). In many cases you can see that from the summary that @dikaiosune wrote, where he linked things to [root]. I think there are also a lot of cases (e.g., “Rust needs to continue improving ergonomics”) that he didn’t link to [root] but which (to my mind) are already covered (e.g., under “improve learning curve”).

Digging deeper into some areas

In terms of moving the discussion forward, I wanted to try a thing. I branched off a thread to dig deeper into the needs for embedded development. I would like to see threads branched off for a few other areas that I think merit some deeper attention:

  • Productivity: learning curve and expressiveness
    • I think we all agree this is important, but I’ve been wanting to start some threads trying to expound on the right ways to do this, and in particular with a focus on “positive sum” outcomes where we don’t pit new vs experienced users against one another
  • HPC / numerical computing.
  • Seamless FFI.
  • Game development, perhaps? I didn’t see a lot of citations here.
  • Windows development needs (I hear mixed stories here?)
  • Dashboard and visibility
    • visibility into the state of things has been very much on our minds, and part of the point of the roadmap process was to help lay the framing for that
    • @dikaiosune and @brson have been doing great work here with
  • First contact: @nrc mentioned ways to make it easier to experiment with Rust and cited React/Angular, I’d like to hear more about what he has in mind

I am going to try and fork off some of these threads, but I may not get to all of them today. Feel free to start up a thread yourself. I think the main thing is that it would be really helpful if we can focus on the most burning problems and also on categorizing the space. A lot of domains (cough HPC, embedded cough) are really enormous and varied. What is a blocking requirement in one case may be a non-issue in others, and it’s helpful to clarify some of these distinctions for us noobs.

One thing I want to say: I feel like these threads are useful irrespective of whether we add some text about that particular topic to the roadmap. It’s awesome to dig into these areas in depth.

Possible non-goals

Now, one thing I also want to bring up are non-goals. I did see a number of topics raised that I think don’t rise to the level of “Rust project goals”. There are a couple of different reasons for this:

  • Often, the idea itself may be sound, but it is presented without motivation. I think we really want to focus on the problem we are trying to solve.
  • Sometimes, the idea doesn’t seem to require “community focus”, but might better be pursued independently.
  • Finally, some things just don’t seem as pressing as others.

Here are some candidates that I found. I’ll just quote from @dikaisune’s summary:

Rust needs to have a more expressive type system. (type level values, CTFE, variadics, HKT)

This is a classic case where I think we need more motivation and less mechanism. I don’t want (e.g.) HKT for HKT’s sake. I want to know what we plan to do with HKT (which may well affect the precise design of it). Basically I think these kinds of language extensions don’t belong on the roadmap, but the problems we aim to solve with them may. Anyway, my take on some popularly floated proposals:

  • const generics and CTFE seem like a valuable avenue for unlocking many high-performance applications (e.g., HPC, small vec, embedded spaces); we should aim for something minimal. @eddyb has already been pursuing a lot of the important refactoring as well of course work by @scott and others on miri.
  • HKT: the application heres are potentially widespread, e.g., Iterable traits, being generic over Rc vs Arc, but HKT as found in Haskell may not be the best solution (and raises a lot of open questions). I like RFC 1598 by @withoutboats quite a lot. It’s not HKT, but it’s a relatively small, natural extension to the language that can express HKT, and would address things like the Iterable trait and so forth. Not sure how to prioritize this: digging more into the applications may help!
  • variadics: the primary motivation to pursue variadics for me has been making it possible to implement the Fn traits. I think we can do this in other ways (e.g., adding syntax like impl Fn() for T) that would let us punt this a little further down the road. Given that I would want said syntax anyway, I’m sort of inclined to do that. =) (But maybe a good variadic solution is not so much work, not sure). In any case, I’m not sure just where this falls on the roadmap tbh, but I could imagine more power vis a vis Fn traits affecting a lot of areas a little bit (e.g., @carllerche raises this point to me regularly).

Rust needs to have data-driven feature prioritization.

The basic idea here is polling for what features people need etc. I am very much in favor of trying to get numerical data on what people are using and also trying to find out what people want. But I would not want to drive our decisions based on a purely numerical basis. For one thing, good polling is hard (citation: world events) – as soon as you turn “polls” into (effectively) “votes”, you raise incentives for people to do things like twitter-based vote drivers for their pet feature and so forth. I would rather that we try to evaluate things on the merits. But I do think that broad-based surveys are really useful for bringing things to awareness that were flying under the radar.

Rust needs a clearer specification of its semantics.

I think that as part of servicing other goals – e.g., mature ecosystem and tooling – we should focus on specifying the areas most urgently needed: unsafe code guidelines and grammar. Trying to formalize and specify all aspects of the type system seems like a big endeavor with very little payoff. It doesn’t seem to rise to the level of a roadmap priority for me.

That said, I am very interested in making it easier to be more formal in RFCs. We’ve done some academic collaborations (e.g., this formalization of the trait system done at UBC and of course the ongoing Rust Belt project), but these efforts tend to lag and subset the language. What is needed to help with RFCs is something much more complete. I think improving and simplifying our trait implementation would be a great starting point. I’ve been toying with ideas here and would love to dig into them (ah so many half-written blog posts).

Rust needs better publicly available information and guides about improving performance for common programs (i.e. hashing), or it needs to refocus its efforts on providing fast-by-default tools in std and common crates

This seems important, but I don’t know that it rises to the level of a roadmap point. Basically, if we find a good way to do it, I think we should just do it. But is hashing what’s really preventing Rust from making in-roads? (I guess maybe?)


Oh, things I learned recently: If you want to start a new thread based on an existing post, you can click on the “age indicator” (the same way you would get a permalink) and click on “+ New Topic”. This gives better metadata to discourse (which of course maybe we can employ for rustydash someday…). Know your tools! :smile:

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Plugins are definitely a life changer, but other than what’s already been said I don’t see anything to add.

I agree. Modelling DAGs in Rust is definitely not fun.

And lexical lifetimes…

Moreover, the dispatching can be made easier using my ifunky crate. A higher-level crate could combine it with a cpuid crate to do something closer to GCC function multiversioning (as opposed to bare ifuncs), though without the nice “automagically use the additional CPU features during optimization without requiring separate compilation.”

An RFC based on this discussion has now been opened!


One thing we do miss to really pitch Rust in our workplace is a good coverage story.

Right now it’s a wonky experience, and that’s something we’d need to push forward rust components in our stack (distributed servers, one of the main focus if I trust the RFC). I remember stating the same during the Rust survey, but since it wasn’t a default choice, there’s a big chance many people didn’t think about it.

We’re a linux shop so we’re not worried about lacking windows or OSX support, but I kind of get the feeling such a feature wouldn’t see the light of the day without a full support from tier1 platforms. Am I mistaken? I’d be willing to spend time getting that right for the *nix part.

I’d love to see built-in coverage support as well, to go with the built-in test support; a binary built with rustc --test could include built-in coverage analysis. That would make it easy to analyze the coverage of tests written using #[test] methods.

Built-in coverage analysis could do a much more accurate job than external coverage analysis, since it could work by statements/expressions rather than by lines.


I very much agree that Rust programs need to be able to recover from OOM. Returning Result from any function that may allocate would both have terrible ergonomics and massively break backwards compatibility, so the only reasonable thing to do in such circumstances is to panic. That, in turn, means that unwinding must never itself allocate.

It is expected that these panics, like any other, would be caught at rather high levels in the program, and handled generically, like any other unexpected error.

  • return an error code
  • 500 Internal Server Error
  • DB transaction rollback
  • save state to disc, then shutdown or restart
  • unload the image file
  • drop an RPC connection
  • kill the browser tab
  • etc

Or that unwinding must allocate a bounded total amount of memory, which Rust could reserve for that purpose.

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User defined hooks can be registered which will run when a panic happens.

Not to mention that user-defined Drop impls are called during unwinding.

A band-aid for that would be a lint that triggers if any function is called that may allocate. There has been some work on a #[deny(allocations)] lint by @llogiq


I like this idea. I think it should be Warn-by-default for destructors, but Allow-by-default elsewhere.

Note that if the issue is formatting the message, it is perfectly okay to give a generic message like “Out of memory when formatting panic message” (which can be in static storage) in that case.

Rust should have basic IDE support

I know that when I was first learning computer science having a working debugger in an IDE than I could step through my code and watch variables change over time really helped things “click” for me. As an intellij-rust user, there’s not yet debugger integration.

I’m curious if any of the planned work in the Rust community would make it easier for IDEs to have first-class debugging support or if at this point the work is mostly on the individual IDEs?

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One thing that could help “boost productivity” or “learn faster” would be a kind of interpreter, dont know if miri will cover all aspects of it, but I think that having introspection like dir(mything) and things like that could help a lot to not go to documentation even do a doc(mythin.y) will help I guess.

ALso debugging and tracing (maybe) could help… well maybe not, but I think that things like are nice to have at hand if needed.


After writing code, I’ve realized that the link_llvm_intrinsics feature is not a feature I want to use. Rather, I’d like to see explicit SIMD (internally based on the platform_intrinsics feature) stabilized instead.

The reason why I’d like to see explicit SIMD stabilized in 2017 (hopefully early 2017) is that I’d like to land encoding_rs in Firefox sooner rather than later, but the most performance-sensitive conversion pair, decoding 100% ASCII or almost 100% ASCII labeled as UTF-8 to UTF-16 (affects the Firefox startup path as well as CSS and JS on the Web even if the content isn’t English), would regress in performance without explicit SSE2. (Look for row “English, UTF-8” column “uconv” under “Decode” in the second table of the linked doc: only 63% of Gecko’s current decode speed if explicit SSE2 isn’t used by encoding_rs but over twice as fast as the current code if explicit SSE2 is used.)


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