Rust 1.37.0 releases with support for profile-guided optimization, built-in cargo vendor, and more

After releasing version 1.36.0 last month, the team behind Rust announced the release of Rust 1.37.0 yesterday. Among the highlights of this version are support for referring to enum variants via type aliases, built-in cargo vendor, unnamed const items, profile-guided optimization, and more.

Key updates in Rust 1.37.0

Referring to enum variants through type aliases

Starting with this release, you can refer to enum variants through type aliases in expression and pattern contexts. Since Self behaves like a type alias in implementations, you can also refer to enum variants with Self::Variant.

Built-in Cargo support for vendored dependencies

Until now the cargo vendor command was available as a separate crate to developers. Starting with Rust 1.37.0, it is integrated directly into Cargo, the Rust package manager, and crate host. This Cargo subcommand fetches all the crates.io and git dependencies for a project into the vendor/ directory. It also shows the configuration necessary to use the vendored code during builds.

Using unnamed const items for macros

Rust 1.37.0 allows you to create unnamed const items. So, instead of giving an explicit name to your constants, you can name them as ‘_’. This update will enable you to easily create ergonomic and reusable declarative and procedural macros for static analysis purposes.

Support for profile-guided optimization

Rust’s compiler, rustc now supports Profile-Guided Optimization (PGO) through the -C profile-generate and -C profile-use flags. PGO allows the compiler to optimize your code based on feedback for real workloads. It optimizes a program in the following two steps:

The program is first built with instrumentation inserted by the compiler. This is achieved by passing the -C profile-generate flag to rustc. This instrumented program is then run on sample data and the profiling data is written to a file.

The program is built again, however, this time the collected profiling data is fed into rustc by using the -C profile-use flag. This build will use the collected data to enable the compiler to make better decisions about code placement, inlining, and other optimizations.

Choosing a default binary in Cargo projects

The cargo run command allows you to run a binary or example of the local package enabling you to quickly test CLI applications. It often happens that there are multiple binaries present in the same packages. In such cases, developers need to explicitly mention the name of the binary they want to run with the --bin flag. This makes the cargo run command not as ergonomic, especially when we are calling a binary more often than the others.

To solve this issue, Rust 1.37.0 introduces a new key in Cargo.toml called default-run. Declaring this key in the [package] section will make the cargo run command default to the chosen binary if the --bin flag is not passed.

Developers have already started testing out this new release. A developer who used profile-guided optimization shared his experience on Hacker News, “The effect is very dependent on program structure and actual code running, but for a suitable application it's reasonable to expect anything from 5-15%, and sometimes much more (see e.g. Firefox reporting 18% here).”

Others also speculated that async/await will come in Rust 1.39. “Seems like async/await is going to slip into Rust 1.39 instead.” Another user said, “Congrats! Like many I was looking forward to async/await in this release but I'm happy they've taken some extra time to work through any existing issues before releasing it.”

Check out the official announcement by the Rust team to know more in detail.

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