Rust 1.27

_posts/2018-06-21-Rust-1.27.md

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+---
+layout: post
+title: "Announcing Rust 1.27"
+author: The Rust Core Team
+---
+
+The Rust team is happy to announce a new version of Rust, 1.27.0. Rust is a
+systems programming language focused on safety, speed, and concurrency.
+
+If you have a previous version of Rust installed via rustup, getting Rust
+1.27.0 is as easy as:
+
+```bash
+$ rustup update stable
+```
+
+If you don't have it already, you can [get `rustup`][install] from the
+appropriate page on our website, and check out the [detailed release notes for
+1.27.0][notes] on GitHub.
+
+[install]: https://www.rust-lang.org/install.html
+[notes]: https://github.com/rust-lang/rust/blob/master/RELEASES.md#version-1270-2018-06-21
+
+Additionally, we would like to draw attention to something: just before the
+release of 1.27.0, we found [a
+bug](https://github.com/rust-lang/rust/pull/51686) in the 'default match
+bindings' feature introduced in 1.26.0 that can possibly introduce unsoundness.
+Since it was discovered very late in the release process, and has been present
+since 1.26.0, we decided to stick to our release train model. We expect to put
+out a 1.27.1 with that fix applied soon, and if there's demand, possibly a
+1.26.3 as well. More information about the specifics here will come in that
+release announcement.
+
+## What's in 1.27.0 stable
+
+This release has two big language features that people have been waiting for.
+But first, a small comment on documentation: All books in [the Rust
+Bookshelf] are [now searchable]! For example, here's [a search of "The Rust
+Programming Language" for
+'borrow'](https://doc.rust-lang.org/book/second-edition/?search=borrow).
+This will hopefully make it much easier to find what you're looking for.
+Additionally, there's one new book: [the `rustc` Book]. This book explains
+how to use `rustc` directly, as well as some other useful information, like a
+list of all lints.
+
+[the Rust Bookshelf]: https://doc.rust-lang.org/
+[now searchable]: https://github.com/rust-lang/rust/pull/49623/
+[the `rustc` Book]: https://github.com/rust-lang/rust/pull/49707/
+
+### SIMD
+
+Okay, now for the big news: the [basics of SIMD] are now available! SIMD
+stands for "single instruction, multiple data." Consider a function
+like this:
+
+```rust
+pub fn foo(a: &[u8], b: &[u8], c: &mut [u8]) {
+    for ((a, b), c) in a.iter().zip(b).zip(c) {
+        *c = *a + *b;
+    }
+}
+```
+
+[basics of SIMD]: https://github.com/rust-lang/rust/pull/49664/
+
+Here, we're taking two slices, and adding the numbers together, placing the
+result in a third slice. The simplest possible way to do this would be to do
+exactly what the code does, and loop through each set of elements, add them
+together, and store it in the result. However, compilers can often do better.
+LLVM will often "autovectorize" code like this, which is a fancy term for
+"use SIMD." Imagine that `a` and `b` were both 16 elements long. Each element
+is a `u8`, and so that means that each slice would be 128 bits of data. Using
+SIMD, we could put *both* `a` and `b` into 128 bit registers, add them
+together in a `*single*` instruction, and then copy the resulting 128 bits
+into `c`. That'd be much faster!
+
+While stable Rust has always been able to take advantage of
+autovectorization, sometimes, the compiler just isn't smart enough to realize
+that we can do something like this. Additionally, not every CPU has these
+features, and so LLVM may not use them so your program can be used on a wide
+variety of hardware. So, in Rust 1.27, the addtion of [the `std::arch`
+module] allows us to use these kinds of instructions *directly*, which
+means we don't need to rely on a smart compiler. Additionally, it includes
+some features that allow us to choose a particular implementation based
+on various criteria. For example:
+
+[the `std::arch` module]: https://doc.rust-lang.org/stable/std/arch/
+
+```rust
+#[cfg(all(any(target_arch = "x86", target_arch = "x86_64"),
+      target_feature = "avx2"))]
+fn foo() {
+    #[cfg(target_arch = "x86")]
+    use std::arch::x86::_mm256_add_epi64;
+    #[cfg(target_arch = "x86_64")]
+    use std::arch::x86_64::_mm256_add_epi64;
+
+    unsafe {
+        _mm256_add_epi64(...);
+    }
+}
+```
+
+Here, we use `cfg` flags to choose the correct version based on the machine
+we're targetting; on `x86` we use that version, and on `x86_64` we use
+its version. We can also choose at runtime:
+
+```rust
+fn foo() {
+    #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]
+    {
+        if is_x86_feature_detected!("avx2") {
+            return unsafe { foo_avx2() };
+        }
+    }
+
+    foo_fallback();
+}
+```
+
+Here, we have two versions of the function: one which uses `AVX2`, a specific
+kind of SIMD feature that lets you do 256-bit operations. The
+`is_x86_feature_detected!` macro will generate code that detects if your CPU
+supports AVX2, and if so, calls the `foo_avx2` function. If not, then we fall
+back to a non-AVX implementation, `foo_fallback`. This means that our code
+will run super fast on CPUs that support AVX2, but still work on ones that
+don't, albeit slower.
+
+If all of this seems a bit low-level and fiddly, well, it is! `std::arch` is
+specifically *primitives* for building these kinds of things. We hope to
+eventually stabilize a `std::simd` module with higher-level stuff in the
+future. But landing the basics now lets the ecosystem experiment with higher
+level libraries starting today. For example, check out the
+[faster](https://github.com/AdamNiederer/faster) crate. Here's a code
+snippet with no SIMD:
+
+```rust
+let lots_of_3s = (&[-123.456f32; 128][..]).iter()
+    .map(|v| {
+        9.0 * v.abs().sqrt().sqrt().recip().ceil().sqrt() - 4.0 - 2.0
+    })
+    .collect::<Vec<f32>>();
+```
+
+To use SIMD with this code via `faster`, you'd change it to this:
+
+```rust
+let lots_of_3s = (&[-123.456f32; 128][..]).simd_iter()
+    .simd_map(f32s(0.0), |v| {
+        f32s(9.0) * v.abs().sqrt().rsqrt().ceil().sqrt() - f32s(4.0) - f32s(2.0)
+    })
+    .scalar_collect();
+```
+
+It looks almost the same: `simd_iter` instead of `iter`, `simd_map` instead
+of `map`, `f32s(2.0)` instead of `2.0`. But you get a SIMD-ified version
+generated for you.
+
+Beyond *that*, you may never write any of this yourself, but as always, the
+libraries you depend on may. For example, the [regex crate has already added
+support](https://github.com/rust-lang/regex/pull/456), and a new release
+will contain these SIMD speedups without you needing to do anything at all!
+
+### `dyn Trait`
+
+Rust's trait object syntax is one that we ultimately regret. If you'll recall,
+given a trait `Foo`, this is a trait object:
+
+```rust
+Box<Foo>
+```
+
+However, if `Foo` were a struct, it'd just be a normal struct placed inside a
+`Box<T>`. When designing the language, we though that the similarity here was
+a good thing, but experience has demonstrated that it is confusing. And it's
+not just for the `Box<Trait>` case; `impl SomeTrait for SomeOtherTrait` is
+also technically valid syntax, but you almost always want to write `impl<T>
+SomeTrait for T where T: SomeOtherTrait` instead. Same with `impl SomeTrait`,
+which looks like it would add methods or possibly default implementations
+but in fact adds inherent methods to a trait object. Finally, with the recent
+addition of `impl Trait` syntax, it's `impl Trait` vs `Trait` when explaining
+things, and so that feels like `Trait` is what you should use, given that it's
+shorter, but in reality, that's not always true.
+
+As such, in Rust 1.27, we have stabilized a new syntax, [`dyn Trait`]. A
+trait object now looks like this:
+
+```rust
+// old => new
+Box<Foo> => Box<dyn Foo>
+&Foo => &dyn Foo
+&mut Foo => &mut dyn Foo
+```
+
+And similarly for other pointer types, `Arc<Foo>` is now `Arc<dyn Foo>`, etc.
+Due to backwards compatibility, we cannot remove the old syntax, but we have
+included a lint, which is set to allow by default, called [`bare-trait-object`].
+If you want to lint against the older syntax, you can turn it on. We thought that
+it would throw far too many warnings to turn on by default at present.
+
+> Incidentally, we're working on a tool called `rustfix` that can automatically
+> upgrade your code to newer idioms. It uses these sorts of lints to do so.
+> Expect to hear more about `rustfix` in a future announcement.
+
+[`dyn Trait`]: https://github.com/rust-lang/rfcs/blob/master/text/2113-dyn-trait-syntax.md
+[`bare-trait-object`]: https://doc.rust-lang.org/rustc/lints/listing/allowed-by-default.html#bare-trait-object
+
+### `#[must_use]` on functions
+
+Finally, the `#[must_use]` attribute is getting an upgrade: [it can now be
+used on functions](https://github.com/rust-lang/rust/pull/48925/).
+
+Previously, it only applied to types, like `Result<T, E>`. But now, you can
+do this:
+
+```rust
+#[must_use]
+fn double(x: i32) -> i32 {
+    2 * x
+}
+
+fn main() {
+    double(4); // warning: unused return value of `double` which must be used
+
+    let _ = double(4); // (no warning)
+}
+```
+
+We've also [enhanced several bits of the standard
+library](https://github.com/rust-lang/rust/pull/49533/) to make use of this;
+`Clone::clone`, `Iterator::collect`, and `ToOwned::to_owned` will all start
+warning if you don't use their results, helping you notice expensive operations
+you may be throwing away by accident.
+
+See the [detailed release notes][notes] for more.
+
+### Library stabilizations
+
+Several new APIs were stabilized this release:
+
+- [`DoubleEndedIterator::rfind`]
+- [`DoubleEndedIterator::rfold`]
+- [`DoubleEndedIterator::try_rfold`]
+- [`Duration::from_micros`]
+- [`Duration::from_nanos`]
+- [`Duration::subsec_micros`]
+- [`Duration::subsec_millis`]
+- [`HashMap::remove_entry`]
+- [`Iterator::try_fold`]
+- [`Iterator::try_for_each`]
+- [`NonNull::cast`]
+- [`Option::filter`]
+- [`String::replace_range`]
+- [`Take::set_limit`]
+- [`hint::unreachable_unchecked`]
+- [`os::unix::process::parent_id`]
+- [`process::id`]
+- [`ptr::swap_nonoverlapping`]
+- [`slice::rsplit_mut`]
+- [`slice::rsplit`]
+- [`slice::swap_with_slice`]
+
+[`DoubleEndedIterator::rfind`]: https://doc.rust-lang.org/std/iter/trait.DoubleEndedIterator.html#method.rfind
+[`DoubleEndedIterator::rfold`]: https://doc.rust-lang.org/std/iter/trait.DoubleEndedIterator.html#method.rfold
+[`DoubleEndedIterator::try_rfold`]: https://doc.rust-lang.org/std/iter/trait.DoubleEndedIterator.html#method.try_rfold
+[`Duration::from_micros`]: https://doc.rust-lang.org/std/time/struct.Duration.html#method.from_micros
+[`Duration::from_nanos`]: https://doc.rust-lang.org/std/time/struct.Duration.html#method.from_nanos
+[`Duration::subsec_micros`]: https://doc.rust-lang.org/std/time/struct.Duration.html#method.subsec_micros
+[`Duration::subsec_millis`]: https://doc.rust-lang.org/std/time/struct.Duration.html#method.subsec_millis
+[`HashMap::remove_entry`]: https://doc.rust-lang.org/std/collections/struct.HashMap.html#method.remove_entry
+[`Iterator::try_fold`]: https://doc.rust-lang.org/std/iter/trait.Iterator.html#method.try_fold
+[`Iterator::try_for_each`]: https://doc.rust-lang.org/std/iter/trait.Iterator.html#method.try_for_each
+[`NonNull::cast`]: https://doc.rust-lang.org/std/ptr/struct.NonNull.html#method.cast
+[`Option::filter`]: https://doc.rust-lang.org/std/option/enum.Option.html#method.filter
+[`String::replace_range`]: https://doc.rust-lang.org/std/string/struct.String.html#method.replace_range
+[`Take::set_limit`]: https://doc.rust-lang.org/std/io/struct.Take.html#method.set_limit
+[`slice::rsplit_mut`]: https://doc.rust-lang.org/std/primitive.slice.html#method.rsplit_mut
+[`slice::rsplit`]: https://doc.rust-lang.org/std/primitive.slice.html#method.rsplit
+[`slice::swap_with_slice`]: https://doc.rust-lang.org/std/primitive.slice.html#method.swap_with_slice
+[`hint::unreachable_unchecked`]: https://doc.rust-lang.org/std/hint/fn.unreachable_unchecked.html
+[`os::unix::process::parent_id`]: https://doc.rust-lang.org/std/os/unix/process/fn.parent_id.html
+[`ptr::swap_nonoverlapping`]: https://doc.rust-lang.org/std/ptr/fn.swap_nonoverlapping.html
+[`process::id`]: https://doc.rust-lang.org/std/process/fn.id.html
+
+See the [detailed release notes][notes] for more.
+
+### Cargo features
+
+Cargo has two small upgrades this release. First, it now [takes a
+`--target-dir` flag](https://github.com/rust-lang/cargo/pull/5393/) if you'd
+like to change the target directory for a given invocation.
+
+Additionally, a tweak to the way Cargo deals with targets has landed. Cargo
+will attempt to automatically discover tests, examples, and binaries within
+your project. However, sometimes explicit configuration is needed. But the
+initial implementation had a problem: let's say that you have two examples,
+and Cargo is discovering them both. You want to tweak one of them, and so
+you add a `[[example]]` to your `Cargo.toml` to configure its settings.
+Cargo currently sees that you've set one explicitly, and therefore, doesn't
+attempt to do any autodetection for the others. That's quite surprising.
+
+As such, we've [added several 'auto' keys to
+`Cargo.toml`](https://github.com/rust-lang/cargo/pull/5335/) We can't fix
+this behavior without possibly breaking projects that may have inadvertently
+been relying on it, and so, if you'd like to configure some targets, but not
+others, you can set the `autoexamples` key to `true` in the `[package]`
+section.
+
+See the [detailed release notes][notes] for more.
+
+## Contributors to 1.27.0
+
+Many people came together to create Rust 1.27. We couldn't have done it
+without all of you. [Thanks!](https://thanks.rust-lang.org/rust/1.27.0)