news:: rust_1_27:: Script
Hello, I’m Chris Krycho and this is New Rustacean, a show about the Rust Programming Language and the people who use it. This is a news episode for Rust 1.27.
Rust 1.27 is another big release – if not quite as big as 1.26 was, still very significant.
Let’s start by digging into the stabilizations for the language itself.
The first big new stable feature is one I already discussed briefly in the news episode for Rust 1.25: SIMD coming to stable Rust. Back in Rust 1.25, the old feature flags were deprecated, in preparation for stable SIMD landing – and the first step of that happens now with Rust 1.27. For a basic intro to SIMD itself, you can go back to the 1.25 episode, and for the full stabilization plan, you can look at RFC #2325 for the master plan. Here I’m focusing on what actually landed in Rust 1.27, which is a subset of the full available set of features SIMD exposes.
The Rust compiler, via LLVM, already does a lot of the ‘vectorization’ that SIMD gives you – the parallelization of the process across all available processing units, because everything in the vector in question can safely be processed in parallel. (Note that “vector” is used here in the fundamental sense, not in the Rust data type sense, though Rust’s
Vec type is certainly a candidate for this kind of parallelization.) However, rustc and LLVM cannot always tell that something is safe to vectorize, so some functions, loops, etc. which could be vectorized aren’t.
The newly stabilized
std::arch standard library module exposes primitives which let you access the “SIMD intrinsics” for a number of platforms. An “intrinsic” is a function that is intrinsic, or built into, to the language or compiler. In this case, the intrinsics are actually built into the architecture, all the way down at the processor level; they’re intrinsic to the CPU.
You can use the currently-stabilized set of SIMD intrinsics with the
#[cfg(...)] attribute applied to
use statements to get specific versions of a given function at compile time, or on the declaration of a function to choose at runtime for when a given architecture may or may not support the intrinsics in question depending on the age of the machine in question (like the x86 line). The module also exposes a number of convenience helpers, like the
is_<arch>_feature_detected! macro, which lets you generate code with fallback for when you want to use a feature if it’s available on your architecture.
The upshot of all of this, for most Rust users, is that a bunch of the libraries you use are going to get faster. You can use these yourself where it makes sense, of course, but it’s not a most-programs-most-of-the-time concern. At the same time, there are plans to expose higher-level APIs on top of the low-level primitives now available in the
std::arch module in the future, which should make it that much easier to use these for average developers. And those kinds of changes have the possibility to make it relatively easy for Rust to go even faster than it does today.
The other big feature,
dyn Trait, is a complement to the
impl Trait feature released in 1.26.
dyn Trait is the official replacement for the bare
Trait syntax. Wherever you might have written something like
&Iterator in the past, now you’d write
Box<dyn Iterator> or
&dyn Iterator instead.
The motivation for this is two-fold: one is that, in combination with
impl Trait it makes static vs. dynamic dispatch both explicit and symmetric. That is, you can always tell when looking at a given reference to a trait whether it’ll be dynamically dispatched (that is, at runtime), or statically dispatched (that is, “monomorphized” into distinct functions at compile time).
dyn Trait will be dynamically dispatched – thus the name! – and
impl Trait will be statically dispatched.
The fact that it’s symmetric is nice – both forms have a short keyword in front of the trait – but the fact that it’s explicit takes us to the second reason: the bare trait version we had historically was something of a footgun for people – a place where it was easy to shoot yourself in the foot because you didn’t realize the consequences of what you might normally write.
&Something are ambiguous, and their performance characteristics are ambiguous as well: they could refer to a pointer to a normal type (a
enum), or they could refer to a trait object.
We’ll talk more about trait objects and the related concept of object safety in the upcoming Traits Deep Dive, Part 3 episode. Here, it’s enough to note that that ambiguity – is this a normal reference or a trait object? And should I write
impl SomeTrait for SomeOtherTrait, or should I be writing
impl<T> SomeTrait for T where T: SomeOtherTrait? Because those are two different things! One is implementing things for a trait object, the other for a constrained generic, but the shorter one that you’re more likely to write is usually not what you want (because you more often want the generic, not the trait object).
There’s a lot more to say here, which is why there’s another episode coming up to cover it in more detail. The takeaway for today on this particular change is that anywhere you had a bare trait name in argument or return position before, you now need to change it to be
dyn Trait instead – at least, assuming you want to keep the same dynamic-dispatch dynamics. Because you now have both
dyn Trait and
impl Trait at your disposal, though, it’s much easier to switch between the two as makes sense in the specific context of your program.
The last couple language changes of interest that landed in 1.27 were all to do with attributes. The first of these is that you can now put the
#[must_use] attribute on any function – not just any type. With this change, you can require-via-lint that the result of your function be checked even when you might not want to require that for every use of the type (or, if it’s a third-party type, might not be able to). The other is that you can now put attributes on generic parameters like lifetimes and types. This is a fairly obscure feature for the moment, but it’s likely to be useful for procedural macros, which are the main place you see heavy use of attributes anyway.
Standard library stabilizations
Now let’s turn our attention to standard library stabilizations. The biggest of these in 1.27 is
std:arch, which we already talked about: it’s the module which exposes the SIMD tooling! Others were more the normal gamut of small-niceties. A few that caught my eye – you should see the full release notes for the rest –
- The new
Iterator::try_for_eachmethods are both short-circuiting versions of their non-
try_equivalents. So where
foldrequires that you have a non-fallible function and are responsible to manage the case where you failed on an earlier iteration yourself,
try_foldwill stop and return immediately if there’s a failure. The same thing goes for
for_eachkeeps applying the closure no matter what, so you have to do some extra work yourself if you don’t want to operate if you’ve encountered a failure;
try_for_eachjust returns immediately if you return an error. These are super handy for fairly common real-world scenarios with iterator operations!
String::replace_rangeis exactly what it sounds like: it lets you replace a specified range within a string. Note that you’re responsible for making sure you’re on a character boundary, though; it’ll panic if your range starts or ends in the middle of a character. UTF problems!
Option::filteris just like
Iterator::filter(which is implemented for
Option!), but you don’t have to call
.collectto get the result. So it’s at least potentially lower overhead and it’s definitely more convenient.
Again, for the others, see the release notes!
There are also some nice updates to the Rust documentation. First, and something I’ve desperately wanted for a couple of years, is the ability to search all the Rust documentation. You’ve been able to search the API docs for a long time, but not the Rust books. This has been a constant source of low-level pain for everyone – and it has driven me nuts whenever I’m trying to find the right page to link in show notes! – so this change is most welcome.
Second is that there’s now a guide for invoking
rustc directly: it has its own dedicated book. Most of us use Cargo for all of our build interactions most of the time, but there are lots of times when you want to use
rustc directly – particularly for including it in other build pipelines where the ongoing work to make Cargo integrate easily isn’t ready yet. This guide should be very helpful for those purposes!
Rust 2018 Preview
Talking about documentation makes for a nice segue into talking about our next topic: the Rust 2018 Edition preview! We’re getting close to the target release for the Edition! As such, there’s an early alpha release out for testing. If you want to make sure the Rust 2018 Edition release is as solid as it possibly can be, you should set up the release preview using the
rust_2018_preview crate-level feature flag on nightly and try out the various tweaks and improvements landing in the next few months. (To do that, you just add
#[feature(rust_2018_preview)] to the top of your
There’s an announcement post in the internals forum and a Rust 2018 Edition guide book in the Rust docs, both of which are linked in the show notes, which give you the current status of the preview and guide you through the changes. You’ll also want to install the
rustfix tool for some nice automatic code-modification changes: you can just
cargo install cargo-fix and then run
dyn Trait change I mentioned a couple minutes ago – it will make that fix for you!
The current set of Rust 2018 features which work include the stable changes we talked about earlier in this episode and in the Rust 1.26 news episode:
dyn Trait, SIMD, improved match ergonomics, and inclusive ranges and slice patterns. They also include some done-but-not-yet-stable features, which I’ll cover when they stabilize: module system improvements, simplified lifetime declarations, and the new rustfix tool I mentioned above. The non-lexical lifetimes and inference of struct lifetimes from the struct fields are still works-in-progress – more on those soon as well!
I did not have time to cover any community updates; there’s too much happening! So once again I’ll just commend This Week in Rust and the Rusty Spike podcast for weekly updates covering a much broader swath! Also check out Matthias Endler’s Hello Rust for awesome video learning materials!
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You can find the notes for this episode at <newrustacean.com/show_notes/news/rust_1_27/>. The website also has scripts and code samples for most of the teaching episodes and transcripts for many of the interviews.
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Until next time, happy coding!
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