What’s new in the Rust programming language

Posted by paul_krill@idg.com,
What’s new in the Rust programming language

The unique approach of the Rust programming language results in better code with fewer compromises than C, C++, Go, and the other languages you probably use. It also gets updated regularly, often every month.

Where to download the latest Rust version

If you already have a previous version of Rust installed via rustup, you can access the latest version via the following command:

$ rustup update stable

The new features in Rust 1.68

Rust 1.68.0, announced March 9, stabilizes the “sparse” registry protocol for the Cargo package manager for reading the index of crates, along with infrastructure at http//index.crates.io/ for those published in the primary crates.io registry. The previous Git protocol, still the default, clones a repository that indexes all crates available in the registry. However, the Git protocol has begun to hit scaling limitations, with delays while updating the repository. The new protocol is expected to improve performance when accessing crates.io.

To use the sparse protocol with crates.io, set the environment variable CARGO_REGISTRIES_CRATES_IO_PROTOCOL=sparse, or edit your .cargo/config/toml file to add:

[registries.crates-io]protocol = "sparse"

The sparse protocol is set to become the default for crates.io in Rust 1.70.0, which is due in a few months.

Elsewhere in Rust 1.68.0, a new pin! macro constructs a Pin<&mut T> from a T expression, anonymously captured in local state. This often is called stack pinning, but that “stack” also could be the captured state of an async fn or block. This macro is similar to some crates, but the standard library can leverage Pin internals and temporary lifetime extension for a more expression-like macro.

Finally, Rust 1.68.0 stabilizes some APIs including {core, std}::pin::pin! and impl DerefMut for PathBuf. And Android platform support in Rust now targets NDK r25 toolset.

The new features in Rust 1.67

Rust 1.67, unveiled January 26, adds a compiler warning pertaining to #[must_use] and async fn. In Rust, async functions annotated with #[must_use] now apply that attribute to the output of the returned impl Future. The Future trait already is annotated with #[must_use], so types implementing [Future] are automatically #[must_use]. Previously there was no way to indicate that the output of the Future is itself significant and should be used in some way. In Rust 1.67, the compiler now will warn if the output is not used.

Also in Rust 1.67, the implementation of the multi-producer, single-consumer channel of the standard library has been updated. Rust’s standard library has had a multi-producer, single-consumer channel since before version 1.0. With Rust 1.67, the implementation has been switched out to be based on crossbeam-channel. The release contains no API changes but the new implementation fixes bugs and improves performance and maintainability of the implementation.

Rust 1.67 stabilizes several APIs such as {integer}::checked_ilog, {integer}::ilog, and NonZero*::BITS. A number of other APIs are now stable in const contexts including char::from_u32, char::from_digit, and char::to_digit. And invalid literals no longer are an error under cfg(FALSE).

Note: Rust 1.66.1 stable, released January 10, fixed a situation in which the Cargo package manager was not verifying SSH host keys when cloning dependencies or registry indexes with SSH. This vulnerability was tracked at cve.org, with more information in the advisory.

The new features in Rust 1.66

Introduced December 15, 2022, Rust 1.66 enables enums with integer representations to now use explicit discriminants, even when they have fields. Previously, developers could use explicit discriminants on enums with representations, but only if none of their variants had fields. Explicit discriminants are useful when passing values across language boundaries where the representation of the enum must match in both languages.

Also in Rust 1.66:

  • A newly stabilized black_box function takes a passed value and passes it right back. The compiler treats black_box as a function that could do anything with its input and return any value. This is useful for disabling optimizations when you don’t want them to occur, such as during benchmarking or when examining the machine code the compiler produces.
  • Developers can use cargo remove to remove dependencies. Rust 1.62 introduced cargo add, a command line utility to add dependencies to a project.
  • Developers now can use ..=x ranges in patterns.
  • Linux builds now optimize the rustc front end and LLVM back end with LTO and BOLT, respectively, improving runtime performance and memory usage.
  • APIs have been stabilized such as proc_macro::Span::source_text and Option::unzip.

The new features in Rust 1.65

Rust 1.65 was introduced November 3, 2022. With this release, generic associated types (GATs), a highly anticipated feature that has been in the works for several years, are finally introduced. GATs allow developers to define lifetime, type, and const generics on associated types. GATs enable patterns that were not previously possible in Rust.

Also in Rust 1.65:

  • A new type of let statement is introduced, let-else, with a refutable pattern and a diverging else block that executes when that pattern does not match.
  • Plain block expressions now can be labeled as a break target, terminating that block early.
  • To improve compilation, support for splitting debug information is now stable for use on Linux, after being supported on macOS since Rust 1.51. With this capability, -Csplit-debuginfo=unpacked will split debuginfo into multiple .dwo DWARF object files, while -Csplit-debuginfo=packed will produce a single .dwp DWARF package along with an output binary with all debuginfo packaged together.
  • APIs have been stabilized such as std::backtrace::Backtrace, Bound::as ref, and std::io::read_to_string.
  • MIR (mid-level intermediate representation) inlining now is enabled for optimized compilations, improving compile times for real world crates.
  • When scheduling builds, Cargo now sorts the queue of pending jobs, improving performance.

The new features in Rust 1.64

Rust 1.64.0, unveiled September 22, 2022, stabilizes the IntoFuture trait, to enhance .await and improve APIs. IntoFuture is similar to the IntoIterator trait, but instead of supporting for … in … loops, IntoFuture changes how .await works.

With IntoFuture, the .await keyword can await more than just features; it can await anything that can be converted into a Future via IntoFuture, to help make APIs more user-friendly. For the future, the developers of Rust hope to simplify development of new named futures by supporting impl Trait in type aliases. This should make implementing IntoFuture easier by simplifying the type alias signature and make it more performant by removing the Box from the type alias.

Also in Rust 1.64:

  • The language provides all c_* type aliases in core::ffi, as well as core::ffi::CStr, for working with C strings. Rust 1.64 also provides alloc::ffi::CString for working with owned C strings using only the alloc crate rather than the full std library.
  • rust-analyzer, an implementation of the Language Server protocol for Rust, now is included as part of the collection of tools included with Rust. This makes it easier to download and access rust-analyzer and makes it available on more platforms. The tool is available as a rustup component and can be installed with the command rustup component add rust_analyzer.
  • When working with collections of related libraries or binary crates in one Cargo workspace, developers now can avoid duplication of common field values between crates, such as common version numbers or repository URLs.
  • The memory layouts of Ipv6Addr, Ipv4Addr, SocketAddrV4, and SocketAddrV6 have been changed to be more memory efficient and compact.
  • Windows builds of the Rust compiler now use profile-guided optimization, improving performance.
  • A number of methods and trait implementations have been stabilized, including num::NonZero*::checked_mul, num::NonZero*::checked_pow, and many others.

The new features in Rust 1.63

Published August 11, 2022, Rust 1.63 adds scoped threads to the standard library. Scoped threads allow you to spawn a thread by borrowing from the local stack frame. The std::thread::scope API provides a guarantee that any spawned threads will have exited prior to its returning, allowing for safely borrowing data. Rust 1.63 also enables non-lexical lifetimes (NLL) by default; the feature is now fully stable. NLL is the second iteration of Rust’s borrow checker.

Also in Rust 1.63:

  • For I/O safety, wrapper types are provided such as BorrowedFD and OwnedFD, which are marked as #[repr(transparent)], meaning that extern "C" bindings can take these types to encode ownership semantics.
  • The Condvar::New, Mutex::New, and RwLock::new functions are now callable in const contexts, to avoid the use of crates such as lazy_static for creating global statics with Mutex, RwLock, or Condvar. This builds on work in Rust 1.62 to enable faster and thinner mutexes.
  • A number of APIs were stabilized including array::from_fn, Box::into_pin, and Path::try_exists.

The new features in Rust 1.62

Rust 1.62, which arrived June 30, 2022, lets developers add dependencies directly from the command line using cargo add. This command supports specifying versions and features and also can modify existing dependencies. Rust 1.62 also allows the use of #[derive(Default)] on enums if a default variant is specified.

Other new capabilities in Rust 1.62:

  • Rust’s standard library now ships with a raw futex-based implementation of locks on Linux, which is lightweight and does not carry any extra allocation. This addition is part of an effort to improve the efficiency of Rust lock types.
  • It is now easier to build OS-less binaries for x86_64, for example when writing a kernel. The x86_64-unknown-none target has been promoted to Tier 2 and can be installed with rustup.
  • A number of APIs have been stabilized including bool::then_some, f32::total_cmp, f64::total_cmp, and Stdin::lines.

The new features in Rust 1.61

Published May 19, 2022, Rust 1.61 highlights custom exit codes from main. Rust proponents said that in the beginning, Rust main functions only could return the unit type () either implicitly or explicitly, indicating success in the exit status, and if developers wanted otherwise, they had to call process::exit. 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. These alternate return types were unified by an unstable Termination trait. In this release, Termination trait is stable, along with a more-general ExitCode type that wraps platform-specific return types. The Termination trait also can be implemented for a developer’s own types, allowing for customization of reporting before converting to an ExitCode.

Also in Version 1.61:

  • Several incremental features have been stabilized to enable more functionality in const. Developers now can create, pass, and cast function pointers in a const fn, which could be useful to build compile-time function tables for an interpreter. But it is still not permitted to call fn pointers. Developers also now can write trait bounds on generic parameters to const fn, such as T: Copy, where previously only Sized was permitted. Also, const fn now can deal with trait objects, whereas arguments and return values for const fn can be opaque impl Trait types.
  • APIs have been stabilized such as Pin::static_mut, Pin;;static_ref, and Vec::retain_mut.
  • Previously, the creation of locked handles to stdin/stdlout/stderr would borrow the handles being locked, which prevented writing let out = std::io::stdout().lock(); because out would outlive the return value of stdout(). This code now works, eliminating a common pitfall affecting many Rust users.

The new features in Rust 1.60.0

Rust 1.60, introduced April 7, 2022, stabilizes support for LLVM-based coverage instrumentation in rustc. This provides for source-based code coverage. Developers can try this out by rebuilding their code with -Cinstrument-coverage. Afterward, running the resulting binary will produce a default.profraw file in the current directory.

The llvm-tools-preview component includes llvm-profdata for processing and merging raw profile output, llvm-profdata for processing raw file output, and llvm-cov for report generation. Baseline functionality is stable and will exist in all future Rust releases, but the specific output format and LLVM tools that produce it are subject to change. Developers should use the same version for both llvm-tools-preview and the rustc binary used to compile code.

Rust 1.60 also re-enables incremental compilation. The Rust team continues to work on fixing bugs in incremental but no problems causing widespread breakage are known at this time.

 Also in Rust 1.60:


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