Announcing Rust 1.42.0

Mar. 12, 2020 · The Rust Release Team

The Rust team is happy to announce a new version of Rust, 1.42.0. 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 1.42.0 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 1.42.0 on GitHub.

What's in 1.42.0 stable

The highlights of Rust 1.42.0 include: more useful panic messages when unwrapping, subslice patterns, the deprecation of Error::description, and more. See the detailed release notes to learn about other changes not covered by this post.

Useful line numbers in Option and Result panic messages

In Rust 1.41.1, calling unwrap() on an Option::None value would produce an error message looking something like this:

thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', /.../src/libcore/macros/

Similarly, the line numbers in the panic messages generated by unwrap_err, expect, and expect_err, and the corresponding methods on the Result type, also refer to core internals.

In Rust 1.42.0, all eight of these functions produce panic messages that provide the line number where they were invoked. The new error messages look something like this:

thread 'main' panicked at 'called `Option::unwrap()` on a `None` value', src/

This means that the invalid call to unwrap was on line 2 of src/

This behavior is made possible by an annotation, #[track_caller]. This annotation is not yet available to use in stable Rust; if you are interested in using it in your own code, you can follow its progress by watching this tracking issue.

Subslice patterns

In Rust 1.26, we stabilized "slice patterns," which let you match on slices. They looked like this:

fn foo(words: &[&str]) {
    match words {
        [] => println!("empty slice!"),
        [one] => println!("one element: {:?}", one),
        [one, two] => println!("two elements: {:?} {:?}", one, two),
        _ => println!("I'm not sure how many elements!"),

This allowed you to match on slices, but was fairly limited. You had to choose the exact sizes you wished to support, and had to have a catch-all arm for size you didn't want to support.

In Rust 1.42, we have expanded support for matching on parts of a slice:

fn foo(words: &[&str]) {
    match words {
        ["Hello", "World", "!", ..] => println!("Hello World!"),
        ["Foo", "Bar", ..] => println!("Baz"),
        rest => println!("{:?}", rest),

The .. is called a "rest pattern," because it matches the rest of the slice. The above example uses the rest pattern at the end of a slice, but you can also use it in other ways:

fn foo(words: &[&str]) {
    match words {
        // Ignore everything but the last element, which must be "!".
        [.., "!"] => println!("!!!"),

        // `start` is a slice of everything except the last element, which must be "z".
        [start @ .., "z"] => println!("starts with: {:?}", start),

        // `end` is a slice of everything but the first element, which must be "a".
        ["a", end @ ..] => println!("ends with: {:?}", end),

        rest => println!("{:?}", rest),

If you're interested in learning more, we published a post on the Inside Rust blog discussing these changes as well as more improvements to pattern matching that we may bring to stable in the future! You can also read more about slice patterns in Thomas Hartmann's post.


This release of Rust stabilizes a new macro, matches!. This macro accepts an expression and a pattern, and returns true if the pattern matches the expression. In other words:

// Using a match expression:
match self.partial_cmp(other) {
    Some(Less) => true,
    _ => false,

// Using the `matches!` macro:
matches!(self.partial_cmp(other), Some(Less))

You can also use features like | patterns and if guards:

let foo = 'f';
assert!(matches!(foo, 'A'..='Z' | 'a'..='z'));

let bar = Some(4);
assert!(matches!(bar, Some(x) if x > 2));

use proc_macro::TokenStream; now works

In Rust 2018, we removed the need for extern crate. But procedural macros were a bit special, and so when you were writing a procedural macro, you still needed to say extern crate proc_macro;.

In this release, if you are using Cargo, you no longer need this line when working with the 2018 edition; you can use use like any other crate. Given that most projects will already have a line similar to use proc_macro::TokenStream;, this change will mean that you can delete the extern crate proc_macro; line and your code will still work. This change is small, but brings procedural macros closer to regular code.


Stabilized APIs

Other changes

There are other changes in the Rust 1.42.0 release: check out what changed in Rust, Cargo, and Clippy.

Compatibility Notes

We have two notable compatibility notes this release: a deprecation in the standard library, and a demotion of 32-bit Apple targets to Tier 3.

Error::Description is deprecated

Sometimes, mistakes are made. The Error::description method is now considered to be one of those mistakes. The problem is with its type signature:

fn description(&self) -> &str

Because description returns a &str, it is not nearly as useful as we wished it would be. This means that you basically need to return the contents of an Error verbatim; if you wanted to say, use formatting to produce a nicer description, that is impossible: you'd need to return a String. Instead, error types should implement the Display/Debug traits to provide the description of the error.

This API has existed since Rust 1.0. We've been working towards this goal for a long time: back in Rust 1.27, we "soft deprecated" this method. What that meant in practice was, we gave the function a default implementation. This means that users were no longer forced to implement this method when implementing the Error trait. In this release, we mark it as actually deprecated, and took some steps to de-emphasize the method in Error's documentation. Due to our stability policy, description will never be removed, and so this is as far as we can go.

Downgrading 32-bit Apple targets

Apple is no longer supporting 32-bit targets, and so, neither are we. They have been downgraded to Tier 3 support by the project. For more details on this, check out this post from back in January, which covers everything in detail.

Contributors to 1.42.0

Many people came together to create Rust 1.42.0. We couldn't have done it without all of you. Thanks!