Announcing Rust 1.61.0

May 19, 2022 · The Rust Release Team

The Rust team is happy to announce a new version of Rust, 1.61.0. Rust is a programming language empowering everyone to build reliable and efficient software.

If you have a previous version of Rust installed via rustup, you can get 1.61.0 with:

$ 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 1.61.0 on GitHub.

If you'd like to help us out by testing future releases, you might consider updating locally to use the beta channel (rustup default beta) or the nightly channel (rustup default nightly). Please report any bugs you might come across!

What's in 1.61.0 stable

Custom exit codes from main

In the beginning, Rust main functions could only return the unit type () (either implicitly or explicitly), always indicating success in the exit status, and if you wanted otherwise you had to call process::exit(code). Since Rust 1.26, main has been allowed to return a Result, where Ok translated to a C EXIT_SUCCESS and Err to EXIT_FAILURE (also debug-printing the error). Under the hood, these alternate return types were unified by an unstable Termination trait.

In this release, that Termination trait is finally stable, along with a more general ExitCode type that wraps platform-specific return types. That has SUCCESS and FAILURE constants, and also implements From<u8> for more arbitrary values. The Termination trait can also be implemented for your own types, allowing you to customize any kind of reporting before converting to an ExitCode.

For example, here's a type-safe way to write exit codes for a git bisect run script:

use std::process::{ExitCode, Termination};

pub enum GitBisectResult {
    Good = 0,
    Bad = 1,
    Skip = 125,
    Abort = 255,

impl Termination for GitBisectResult {
    fn report(self) -> ExitCode {
        // Maybe print a message here
        ExitCode::from(self as u8)

fn main() -> GitBisectResult {
    std::panic::catch_unwind(|| {
        todo!("test the commit")

More capabilities for const fn

Several incremental features have been stabilized in this release to enable more functionality in const functions:

  • Basic handling of fn pointers: You can now create, pass, and cast function pointers in a const fn. For example, this could be useful to build compile-time function tables for an interpreter. However, it is still not permitted to call fn pointers.

  • Trait bounds: You can now write trait bounds on generic parameters to const fn, such as T: Copy, where previously only Sized was allowed.

  • dyn Trait types: Similarly, const fn can now deal with trait objects, dyn Trait.

  • impl Trait types: Arguments and return values for const fn can now be opaque impl Trait types.

Note that the trait features do not yet support calling methods from those traits in a const fn.

See the Constant Evaluation section of the reference book to learn more about the current capabilities of const contexts, and future capabilities can be tracked in rust#57563.

Static handles for locked stdio

The three standard I/O streams -- Stdin, Stdout, and Stderr -- each have a lock(&self) to allow more control over synchronizing read and writes. However, they returned lock guards with a lifetime borrowed from &self, so they were limited to the scope of the original handle. This was determined to be an unnecessary limitation, since the underlying locks were actually in static storage, so now the guards are returned with a 'static lifetime, disconnected from the handle.

For example, a common error came from trying to get a handle and lock it in one statement:

// error[E0716]: temporary value dropped while borrowed
let out = std::io::stdout().lock();
//        ^^^^^^^^^^^^^^^^^       - temporary value is freed at the end of this statement
//        |
//        creates a temporary which is freed while still in use

Now the lock guard is 'static, not borrowing from that temporary, so this works!

Stabilized APIs

The following methods and trait implementations are now stabilized:

The following previously stable functions are now const:

Other changes

There are other changes in the Rust 1.61.0 release. Check out what changed in Rust, Cargo, and Clippy.

In a future release we're planning to increase the baseline requirements for the Linux kernel to version 3.2, and for glibc to version 2.17. We'd love your feedback in rust#95026.

Contributors to 1.61.0

Many people came together to create Rust 1.61.0. We couldn't have done it without all of you. Thanks!