The Rust team is happy to announce a new version of Rust, 1.65.0. Rust is a programming language empowering everyone to build reliable and efficient software.

Before going into the details of the new Rust release, we'd like to draw attention to the tragic death of Mahsa Amini and the death and violent suppression of many others, by the religious morality police of Iran. See https://en.wikipedia.org/wiki/Mahsa_Amini_protests for more details. We stand in solidarity with the people in Iran struggling for human rights.

If you have a previous version of Rust installed via rustup, you can get 1.65.0 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 1.65.0 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 1.65.0 stable

Generic associated types (GATs)

Lifetime, type, and const generics can now be defined on associated types, like so:

trait Foo {
    type Bar<'x>;
}

It's hard to put into few words just how useful these can be, so here are a few example traits, to get a sense of their power:

/// An `Iterator`-like trait that can borrow from `Self`
trait LendingIterator {
    type Item<'a> where Self: 'a;

    fn next<'a>(&'a mut self) -> Option<Self::Item<'a>>;
}

/// Can be implemented over smart pointers, like `Rc` or `Arc`,
/// in order to allow being generic over the pointer type
trait PointerFamily {
    type Pointer<T>: Deref<Target = T>;

    fn new<T>(value: T) -> Self::Pointer<T>;
}

/// Allows borrowing an array of items. Useful for
/// `NdArray`-like types that don't necessarily store
/// data contiguously.
trait BorrowArray<T> {
    type Array<'x, const N: usize> where Self: 'x;

    fn borrow_array<'a, const N: usize>(&'a self) -> Self::Array<'a, N>;
}

As you can see, GATs are quite versatile and enable a number of patterns that are not currently able to be written. For more information, check out the post announcing the push for stabilization published last year or the stabilization announcement post published last week. The former goes into a bit more depth of a couple of the examples above, while the latter talks about some of the known limitations of this stabilization.

More in depth reading can be found in the associated types section of the nightly reference or the original RFC (which was initially opened over 6.5 years ago!).

let-else statements

This introduces a new type of let statement with a refutable pattern and a diverging else block that executes when that pattern doesn't match.

let PATTERN: TYPE = EXPRESSION else {
    DIVERGING_CODE;
};

Normal let statements can only use irrefutable patterns, statically known to always match. That pattern is often just a single variable binding, but may also unpack compound types like structs, tuples, and arrays. However, that was not usable for conditional matches, like pulling out a variant of an enum -- until now! With let-else, a refutable pattern can match and bind variables in the surrounding scope like a normal let, or else diverge (e.g. break, return, panic!) when the pattern doesn't match.

fn get_count_item(s: &str) -> (u64, &str) {
    let mut it = s.split(' ');
    let (Some(count_str), Some(item)) = (it.next(), it.next()) else {
        panic!("Can't segment count item pair: '{s}'");
    };
    let Ok(count) = u64::from_str(count_str) else {
        panic!("Can't parse integer: '{count_str}'");
    };
    (count, item)
}
assert_eq!(get_count_item("3 chairs"), (3, "chairs"));

The scope of name bindings is the main thing that makes this different from match or if let-else expressions. You could previously approximate these patterns with an unfortunate bit of repetition and an outer let:

    let (count_str, item) = match (it.next(), it.next()) {
        (Some(count_str), Some(item)) => (count_str, item),
        _ => panic!("Can't segment count item pair: '{s}'"),
    };
    let count = if let Ok(count) = u64::from_str(count_str) {
        count
    } else {
        panic!("Can't parse integer: '{count_str}'");
    };

break from labeled blocks

Plain block expressions can now be labeled as a break target, terminating that block early. This may sound a little like a goto statement, but it's not an arbitrary jump, only from within a block to its end. This was already possible with loop blocks, and you may have seen people write loops that always execute only once, just to get a labeled break.

Now there's a language feature specifically for that! Labeled break may also include an expression value, just as with loops, letting a multi-statement block have an early "return" value.

let result = 'block: {
    do_thing();
    if condition_not_met() {
        break 'block 1;
    }
    do_next_thing();
    if condition_not_met() {
        break 'block 2;
    }
    do_last_thing();
    3
};

Splitting Linux debuginfo

Back in Rust 1.51, the compiler team added support for split debug information on macOS, and now this option is stable for use on Linux as well.

• -Csplit-debuginfo=unpacked will split debuginfo out into multiple .dwo DWARF object files.
• -Csplit-debuginfo=packed will produce a single .dwp DWARF package alongside your output binary with all the debuginfo packaged together.
• -Csplit-debuginfo=off is still the default behavior, which includes DWARF data in .debug_* ELF sections of the objects and final binary.

Split DWARF lets the linker avoid processing the debuginfo (because it isn't in the object files being linked anymore), which can speed up link times!

Other targets now also accept -Csplit-debuginfo as a stable option with their platform-specific default value, but specifying other values is still unstable.

Stabilized APIs

The following methods and trait implementations are now stabilized:

• std::backtrace::Backtrace
• Bound::as_ref
• std::io::read_to_string
• <*const T>::cast_mut
• <*mut T>::cast_const

Of particular note, the Backtrace API allows capturing a stack backtrace at any time, using the same platform-specific implementation that usually serves panic backtraces. This may be useful for adding runtime context to error types, for example.

These APIs are now usable in const contexts:

• <*const T>::offset_from
• <*mut T>::offset_from

Compatibility notes

• As the final step of the RLS deprecation, this release has replaced RLS with a small LSP server showing a deprecation warning, advising users to migrate to rust-analyzer.

Other changes

There are other changes in the Rust 1.65 release, including:

• MIR inlining is now enabled for optimized compilations. This provides a 3-10% improvement in compiletimes for real world crates.
• When scheduling builds, Cargo now sorts the queue of pending jobs to improve performance.

Check out everything that changed in Rust, Cargo, and Clippy.

Contributors to 1.65.0

Many people came together to create Rust 1.65.0. We couldn't have done it without all of you. Thanks!