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‎_posts/2017-07-20-Rust-1.19.md

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---
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layout: post
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title: "Announcing Rust 1.19"
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author: The Rust Core Team
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---
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The Rust team is happy to announce the latest version of Rust, 1.19.0. Rust is a
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systems programming language focused on safety, speed, and concurrency.
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If you have a previous version of Rust installed, getting Rust 1.19 is as easy as:
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```bash
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$ rustup update stable
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```
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If you don't have it already, you can [get `rustup`][install] from the
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appropriate page on our website, and check out the [detailed release notes for
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1.19.0][notes] on GitHub.
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[install]: https://www.rust-lang.org/install.html
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[notes]: https://github.com/rust-lang/rust/blob/master/RELEASES.md#version-1190-2017-07-20
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### What's in 1.19.0 stable
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Rust 1.19.0 has some long-awaited features, but first, a note for our Windows
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users. On Windows, Rust relies on `link.exe` for linking, which you can get
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via the "Microsoft Visual C++ Build Tools." With the recent release of Visual
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Studio 2017, the directory structure for these tools has changed. As such, you
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had to stick with the 2015 tools to use Rust, or use a workaround (such as running
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`vcvars.bat`). In 1.19.0, `rustc` now knows how to find the 2017 tools, and so
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they work without a workaround.
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On to new features! Rust 1.19.0 is the first release that supports [`union`s]:
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```rust
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union MyUnion {
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f1: u32,
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f2: f32,
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}
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```
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Unions are kind of like `enum`s, but they are "untagged". Enums have a "tag"
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that stores which variant is the correct one at runtime; unions elide this tag.
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That means that reading or writing a union's field is unsafe:
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```rust
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let u = MyUnion { f1: 1 };
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unsafe { u.f1 = 5 };
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let value = unsafe { u.f1 };
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```
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Pattern matching works too:
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```rust
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fn f(u: MyUnion) {
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unsafe {
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match u {
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MyUnion { f1: 10 } => { println!("ten"); }
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MyUnion { f2 } => { println!("{}", f2); }
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}
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}
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}
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```
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When are unions useful? One major use-case is interoperability with C. C APIs
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can (and depending on the area, often do) expose unions, and so this makes writing
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API wrappers for those libraries significantly easier. Additionally, from [its RFC]:
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> A native union mechanism would also simplify Rust implementations of
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> space-efficient or cache-efficient structures relying on value
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> representation, such as machine-word-sized unions using the least-significant
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> bits of aligned pointers to distinguish cases.
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This feature has been long awaited, and there's still more improvements to come.
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For now, `union`s can only include `Copy` types, and may not implement `Drop`.
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We expect to lift these restrictions in the future.
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[`union`s]: https://github.com/rust-lang/rust/pull/42068
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[its RFC]: https://github.com/rust-lang/rfcs/blob/master/text/1444-union.md#motivation
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> As a side note, have you ever wondered how new features get added to Rust? This
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> feature was suggested by Josh Triplet, and he [gave a talk at RustConf
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> 2016](https://youtu.be/U8Gl3RTXf88?list=PLE7tQUdRKcybLShxegjn0xyTTDJeYwEkI)
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> about the process of getting `union`s into Rust. You should check it out!
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In other news, [`loop`s can now `break` with a value](https://github.com/rust-lang/rust/pull/42016):
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```rust
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// old code
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let x;
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loop {
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x = 7;
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break;
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}
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// new code
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let x = loop { break 7; };
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```
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Rust has tradititionally positioned itself as an "expression oriented language", that is,
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most things are expressions that evaluate to a value, rather than statements. `loop` stuck
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out as strange in this way, as it was previously a statement.
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What about other forms of loops? It's not yet clear. See [its
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RFC](https://github.com/rust-lang/rfcs/blob/master/text/1624-loop-break-value.md#extension-to-for-while-while-let)
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for some discussion around the open questions here.
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A smaller feature, closures that do not capture an environment [can now be coerced
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to a function pointer](https://github.com/rust-lang/rust/pull/42162):
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```rust
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let f: fn(i32) -> i32 = |x| x + 1;
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```
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We now produce [xz compressed tarballs](https://github.com/rust-lang/rust-installer/pull/57), and prefer them by default,
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making the data transfer smaller and faster. `gzip`'d tarballs are still produced,
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in case you can't use `xz` for some reason.
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The compiler can now [bootstrap on
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Android](https://github.com/rust-lang/rust/pull/41370). We've long supported Android
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in various ways, and this continues to improve our support.
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Finally, a compatibility note. Way back when we were running up to Rust 1.0, we did
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a huge push to verify everything that was being marked as stable, and as unstable.
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We overlooked one thing, however: `-Z` flags. The `-Z` flag to the compiler enables
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unstable flags. Unlike the rest of our stability story, you could still use `-Z` on
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stable Rust. Back in April of 2016, in Rust 1.8, we made the use of `-Z` on stable
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or beta produce a warning, and over a year later, we're fixing this hole in our
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stability story by [disallowing `-Z` on stable and beta](https://github.com/rust-lang/rust/pull/41751).
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See the [detailed release notes][notes] for more.
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#### Library stabilizations
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The largest new library feature is the [`eprint!` and `eprintln!` macros].
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These work exactly the same as `print!` and `printn!`, but instead write
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to standard error, as opposed to standard output.
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[`eprint!` and `eprintln!` macros]: [41192]: https://github.com/rust-lang/rust/pull/41192
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Other new features:
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- [`String` now implements `FromIterator<Cow<'a, str>>` and
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`Extend<Cow<'a, str>>`][41449]
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- [`Vec` now implements `From<&mut [T]>`][41530]
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- [`Box<[u8]>` now implements `From<Box<str>>`][41258]
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- [`SplitWhitespace` now implements `Clone`][41659]
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- [`[u8]::reverse` is now 5x faster and `[u16]::reverse` is now
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1.5x faster][41764]
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[41449]: https://github.com/rust-lang/rust/pull/41449
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[41530]: https://github.com/rust-lang/rust/pull/41530
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[41258]: https://github.com/rust-lang/rust/pull/41258
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[41659]: https://github.com/rust-lang/rust/pull/41659
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[41764]: https://github.com/rust-lang/rust/pull/41764
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And some freshly-stabilized APIs:
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- [`OsString::shrink_to_fit`]
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- [`cmp::Reverse`]
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- [`Command::envs`]
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- [`thread::ThreadId`]
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[`OsString::shrink_to_fit`]: https://doc.rust-lang.org/std/ffi/struct.OsString.html#method.shrink_to_fit
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[`cmp::Reverse`]: https://doc.rust-lang.org/std/cmp/struct.Reverse.html
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[`Command::envs`]: https://doc.rust-lang.org/std/process/struct.Command.html#method.envs
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[`thread::ThreadId`]: https://doc.rust-lang.org/std/thread/struct.ThreadId.html
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See the [detailed release notes][notes] for more.
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#### Cargo features
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Cargo mostly recieved small but valuable improvements in this relase. The
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largest is possibly that [Cargo no longer checks out a local working
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directory for the crates.io index][cargo/4026] This should provide smaller
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file size for the registry, and improve cloning times, especially on Windows
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machines.
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Other improvements:
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- [Build scripts can now add environment variables to the environment
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the crate is being compiled in.
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Example: `println!("cargo:rustc-env=FOO=bar");`][cargo/3929]
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- [Workspace members can now accept glob file patterns][cargo/3979]
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- [Added `--all` flag to the `cargo bench` subcommand to run benchmarks of all
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the members in a given workspace.][cargo/3988]
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- [Added an `--exclude` option for excluding certain packages when using the
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`--all` option][cargo/4031]
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- [The `--features` option now accepts multiple comma or space
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delimited values.][cargo/4084]
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- [Added support for custom target specific runners][cargo/3954]
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[cargo/3929]: https://github.com/rust-lang/cargo/pull/3929
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[cargo/3954]: https://github.com/rust-lang/cargo/pull/3954
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[cargo/3979]: https://github.com/rust-lang/cargo/pull/3979
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[cargo/3988]: https://github.com/rust-lang/cargo/pull/3988
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[cargo/4026]: https://github.com/rust-lang/cargo/pull/4026
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[cargo/4031]: https://github.com/rust-lang/cargo/pull/4031
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[cargo/4084]: https://github.com/rust-lang/cargo/pull/4084
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See the [detailed release notes][notes] for more.
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### Contributors to 1.19.0
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Many people came together to create Rust 1.19. We couldn't have done it without
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all of you. [Thanks!](https://thanks.rust-lang.org/rust/1.19.0)

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