Announcing Rust 1.28

Aug. 2, 2018 · The Rust Core Team

The Rust team is happy to announce a new version of Rust, 1.28.0. 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 1.28.0 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 1.28.0 on GitHub.

What's in 1.28.0 stable

Global Allocators

Allocators are the way that programs in Rust obtain memory from the system at runtime. Previously, Rust did not allow changing the way memory is obtained, which prevented some use cases. On some platforms, this meant using jemalloc, on others, the system allocator, but there was no way for users to control this key component. With 1.28.0, the #[global_allocator] attribute is now stable, which allows Rust programs to set their allocator to the system allocator, as well as define new allocators by implementing the GlobalAlloc trait.

The default allocator for Rust programs on some platforms is jemalloc. The standard library now provides a handle to the system allocator, which can be used to switch to the system allocator when desired, by declaring a static and marking it with the #[global_allocator] attribute.

use std::alloc::System;

static GLOBAL: System = System;

fn main() {
    let mut v = Vec::new();
    // This will allocate memory using the system allocator.

However, sometimes you want to define a custom allocator for a given application domain. This is also relatively easy to do by implementing the GlobalAlloc trait. You can read more about how to do this in the documentation.

Improved error message for formatting

Work on diagnostics continues, this time with an emphasis on formatting:

format!("{_foo}", _foo = 6usize);

Previously, the error message emitted here was relatively poor:

error: invalid format string: expected `'}'`, found `'_'`
2 |     format!("{_foo}", _foo = 6usize);
  |             ^^^^^^^^

Now, we emit a diagnostic that tells you the specific reason the format string is invalid:

error: invalid format string: invalid argument name `_foo`
2 |     let _ = format!("{_foo}", _foo = 6usize);
  |                       ^^^^ invalid argument name in format string
  = note: argument names cannot start with an underscore

See the detailed release notes for more.

Library stabilizations

We've already mentioned the stabilization of the GlobalAlloc trait, but another important stabilization is the NonZero number types. These are wrappers around the standard unsigned integer types: NonZeroU8, NonZeroU16, NonZeroU32, NonZeroU64, NonZeroU128, and NonZeroUsize.

This allows for size optimization, for example, Option<u8> is two bytes large, but Option<NonZeroU8> is just one byte large. Note that this optimization remains even when NonZeroU8 is wrapped inside another struct; the example below illustrates that Door is still 1 byte large despite being placed inside an Option. This optimization applies to user-defined enums as well: Option is not special.

use std::mem;
use std::num::NonZeroU8;

struct Key(NonZeroU8);

struct Door {
    key: Key,

fn main() {
    assert_eq!(mem::size_of::<Door>(), 1);
    assert_eq!(mem::size_of::<Option<Door>>(), 1);

A number of other libraries have also been stabilized: you can see the more detailed release notes for full details.

Cargo features

Cargo will now no longer allow you to publish crates with build scripts that modify the src directory. The src directory in a crate should be considered to be immutable.

Contributors to 1.28.0

Many people came together to create Rust 1.28. We couldn't have done it without all of you. Thanks!