Announcing Rust 1.32.0

Jan. 17, 2019 · The Rust Release Team

The Rust team is happy to announce a new version of Rust, 1.32.0. Rust is a programming language that is empowering everyone to build reliable and efficient software.

If you have a previous version of Rust installed via rustup, getting Rust 1.32.0 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 1.32.0 on GitHub.

As a small side note, rustup has seen some new releases lately! To update rustup itself, run rustup self update.

What's in 1.32.0 stable

Rust 1.32.0 has a few quality of life improvements, switches the default allocator, and makes additional functions const. Read on for a few highlights, or see the detailed release notes for additional information.

The dbg macro

First up, a quality of life improvement. Are you a "print debugger"? If you are, and you've wanted to print out some value while working on some code, you have to do this:

let x = 5;

println!("{:?}", x);

// or maybe even this
println!("{:#?}", x);

This isn't the largest speed bump, but it is a lot of stuff to simply show the value of x. Additionally, there's no context here. If you have several of these println!s, it can be hard to tell which is which, unless you add your own context to each invocation, requiring even more work.

In Rust 1.32.0, we've added a new macro, dbg!, for this purpose:

fn main() {
    let x = 5;

If you run this program, you'll see:

[src/] x = 5

You get the file and line number of where this was invoked, as well as the name and value. Additionally, println! prints to the standard output, so you really should be using eprintln! to print to standard error. dbg! does the right thing and goes to stderr.

It even works in more complex circumstances. Consider this factorial example:

fn factorial(n: u32) -> u32 {
    if n <= 1 {
    } else {
        n * factorial(n - 1)

If we wanted to debug this, we might write it like this with eprintln!:

fn factorial(n: u32) -> u32 {
    eprintln!("n: {}", n);

    if n <= 1 {
        eprintln!("n <= 1");

    } else {
        let n = n * factorial(n - 1);

        eprintln!("n: {}", n);


We want to log n on each iteration, as well as have some kind of context for each of the branches. We see this output for factorial(4):

n: 4
n: 3
n: 2
n: 1
n <= 1
n: 2
n: 6
n: 24

This is servicable, but not particularly great. Maybe we could work on how we print out the context to make it more clear, but now we're not debugging our code, we're figuring out how to make our debugging code better.

Consider this version using dbg!:

fn factorial(n: u32) -> u32 {
    if dbg!(n <= 1) {
    } else {
        dbg!(n * factorial(n - 1))

We simply wrap each of the various expressions we want to print with the macro. We get this output instead:

[src/] n <= 1 = false
[src/] n <= 1 = false
[src/] n <= 1 = false
[src/] n <= 1 = true
[src/] 1 = 1
[src/] n * factorial(n - 1) = 2
[src/] n * factorial(n - 1) = 6
[src/] n * factorial(n - 1) = 24
[src/] factorial(4) = 24

Because the dbg! macro returns the value of what it's debugging, instead of eprintln! which returns (), we need to make no changes to the structure of our code. Additionally, we have vastly more useful output.

That's a lot to say about a little macro, but we hope it improves your debugging experience! We are contining to work on support for gdb and friends as well, of course.

jemalloc is removed by default

Long, long ago, Rust had a large, Erlang-like runtime. We chose to use jemalloc instead of the system allocator, because it often improved performance over the default system one. Over time, we shed more and more of this runtime, and eventually almost all of it was removed, but jemalloc was not. We didn't have a way to choose a custom allocator, and so we couldn't really remove it without causing a regression for people who do need jemalloc.

Also, saying that jemalloc was always the default is a bit UNIX-centric, as it was only the default on some platforms. Notably, the MSVC target on Windows has shipped the system allocator for a long time.

Finally, while jemalloc usually has great performance, that's not always the case. Additionally, it adds about 300kb to every Rust binary. We've also had a host of other issues with jemalloc in the past. It has also felt a little strange that a systems language does not default to the system's allocator.

For all of these reasons, once Rust 1.28 shipped a way to choose a global allocator, we started making plans to switch the default to the system allocator, and allow you to use jemalloc via a crate. In Rust 1.32, we've finally finished this work, and by default, you will get the system allocator for your programs.

If you'd like to continue to use jemalloc, use the jemallocator crate. In your Cargo.toml:

jemallocator = "0.1.8"

And in your crate root:

static ALLOC: jemallocator::Jemalloc = jemallocator::Jemalloc;

That's it! If you don't need jemalloc, it's not forced upon you, and if you do need it, it's a few lines of code away.

Final module improvements

In the past two releases, we announced several improvements to the module system. We have one last tweak landing in 1.32.0 and the 2018 edition. Nicknamed "uniform paths", it permits previously invalid import path statements to be resolved exactly the same way as non-import paths. For example:

enum Color { Red, Green, Blue }

use Color::*;

This code did not previously compile, as use statements had to start with super, self, or crate. Now that the compiler supports uniform paths, this code will work, and do what you probably expect: import the variants of the Color enum defined above the use statement.

With this change in place, we've completed our efforts at revising the module system. We hope you've been enjoying the simplified system so far!

Macro improvements

A few improvements to macros have landed in Rust 1.32.0. First, a new literal matcher was added:

macro_rules! m {
    ($lt:literal) => {};

fn main() {
    m!("some string literal");

literal matches against literals of any type; string literals, numeric literals, char literals.

In the 2018 edition, macro_rules macros can also use ?, like this:

macro_rules! bar {
    ($(a)?) => {}

The ? will match zero or one repetitions of the pattern, similar to the already-existing * for "zero or more" and + for "one or more."

Library stabilizations

We talked above about the dbg! macro, which is a big library addition. Beyond that, 19 functions were made const fns, and all integral numeric primitives now provide conversion functions to and from byte-arrays with specified endianness. These six functions are named to_<endian>_bytes and from_<endian>_bytes, where <endian> is one of:

  • ne - native endianness
  • le - little endian
  • be - big endian

See the detailed release notes for more details.

Cargo features

Cargo gained cargo c as an alias for cargo check, and now allows usernames in registry URLs.

See the detailed release notes for more.

Contributors to 1.32.0

Many people came together to create Rust 1.32.0. We couldn't have done it without all of you. Thanks!