Announcing Rust 1.25

Mar. 29, 2018 · The Rust Core Team

The Rust team is happy to announce a new version of Rust, 1.25.0. 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 1.25.0 is as easy as:

$ rustup update stable

If you don't have it already, you can get rustup from the appropriate page on our website, and check out the detailed release notes for 1.25.0 on GitHub.

What's in 1.25.0 stable

The last few releases have been relatively minor, but Rust 1.25 contains a bunch of stuff! The first one is straightforward: we've upgraded to LLVM 6 from LLVM 4. This has a number of effects, a major one being a step closer to AVR support.

A new way to write use statements has landed: nested import groups. If you've ever written a set of imports like this:

use std::fs::File;
use std::io::Read;
use std::path::{Path, PathBuf};

You can now write this:

// on one line
use std::{fs::File, io::Read, path::{Path, PathBuf}};

// with some more breathing room
use std::{

This can reduce some repetition, and make things a bit more clear.

There are two big documentation changes in this release: first, Rust By Example is now included on! We'll be redirecting the old domain there shortly. We hope this will bring more attention to a great resource, and you'll get a local copy with your local documentation.

Second, back in Rust 1.23, we talked about the change from Hoedown to pulldown-cmark. In Rust 1.25, pulldown-cmark is now the default. We have finally removed the last bit of C from rustdoc, and now properly follow the CommonMark spec.

Finally, in RFC 1358, #[repr(align(x))] was accepted. In Rust 1.25, it is now stable! This attribute lets you set the alignment of your structs:

struct Number(i32);

assert_eq!(std::mem::align_of::<Number>(), 4);
assert_eq!(std::mem::size_of::<Number>(), 4);

struct Align16(i32);

assert_eq!(std::mem::align_of::<Align16>(), 16);
assert_eq!(std::mem::size_of::<Align16>(), 16);

If you're working with low-level stuff, control of these kinds of things can be very important!

See the detailed release notes for more.

Library stabilizations

The biggest story in libraries this release is std::ptr::NonNull<T>. This type is similar to *mut T, but is non-null and covariant. This blog post isn't the right place to explain variance, but in a nutshell, NonNull<T>, well, guarantees that it won't be null, which means that Option<NonNull<T>> has the same size as *mut T. If you're building a data structure with unsafe code, NonNull<T> is often the right type for you!

libcore has gained a time module, containing the Duration type previously only available in libstd.

Additionally, the from_secs, and from_millis functions associated with Duration were made const fns, allowing them to be used to create a Duration as a constant expression.

See the detailed release notes for more.

Cargo features

Cargo's CLI has one really important change this release: cargo new will now default to generating a binary, rather than a library. We try to keep Cargo's CLI quite stable, but this change is important, and is unlikely to cause breakage.

For some background, cargo new accepts two flags: --lib, for creating libraries, and --bin, for creating binaries, or executables. If you don't pass one of these flags, in previous versions of Cargo, it would default to --lib. We made this decision because each binary (often) depends on many libraries, and so the library case is more common. However, this is incorrect; each library is depended upon by many binaries. Furthermore, when getting started, what you often want is a program you can run and play around with. It's not just new Rustaceans though; even very long-time community members have said that they find this default surprising. As such, we're changing it.

Similarly, cargo new previously would be a bit opinionated around the names of packages it would create. Specifically, if your package began with rust- or ended with -rs, Cargo would rename it. The intention was that well, it's a Rust package, this information is redundant. However, people feel quite strongly about naming, and when they bump into this, they're surprised and often upset. As such, we're not going to do that any more.

Many users love cargo doc, a way to generate local documentation for their Cargo projects. It's getting a huge speed boost in this release, as now, it uses cargo check, rather than a full cargo build, so some scenarios will get faster.

Additionally, checkouts of git dependencies should be a lot faster, thanks to the use of hard links when possible.

See the detailed release notes for more.

Contributors to 1.25.0

Many people came together to create Rust 1.25. We couldn't have done it without all of you.