Announcing Rust 1.64.0

Rust 1.64 stabilizes the IntoFuture trait. IntoFuture is a trait similar to IntoIterator, but rather than supporting for ... in ... loops, IntoFuture changes how .await works. With IntoFuture, the .await keyword can await more than just futures; it can await anything which can be converted into a Future via IntoFuture - which can help make your APIs more user-friendly!

Take for example a builder which constructs requests to some storage provider over the network:

pub struct Error { ... }
pub struct StorageResponse { ... }:
pub struct StorageRequest(bool);

impl StorageRequest {
    /// Create a new instance of `StorageRequest`.
    pub fn new() -> Self { ... }
    /// Decide whether debug mode should be enabled.
    pub fn set_debug(self, b: bool) -> Self { ... }
    /// Send the request and receive a response.
    pub async fn send(self) -> Result<StorageResponse, Error> { ... }
}

Typical usage would likely look something like this:

let response = StorageRequest::new()  // 1. create a new instance
    .set_debug(true)                  // 2. set some option
    .send()                           // 3. construct the future
    .await?;                          // 4. run the future + propagate errors

This is not bad, but we can do better here. Using IntoFuture we can combine “construct the future” (line 3) and “run the future” (line 4) into a single step:

let response = StorageRequest::new()  // 1. create a new instance
    .set_debug(true)                  // 2. set some option
    .await?;                          // 3. construct + run the future + propagate errors

We can do this by implementing IntoFuture for StorageRequest. IntoFuture requires us to have a named future we can return, which we can do by creating a “boxed future” and defining a type alias for it:

// First we must import some new types into the scope.
use std::pin::Pin;
use std::future::{Future, IntoFuture};

pub struct Error { ... }
pub struct StorageResponse { ... }
pub struct StorageRequest(bool);

impl StorageRequest {
    /// Create a new instance of `StorageRequest`.
    pub fn new() -> Self { ... }
    /// Decide whether debug mode should be enabled.
    pub fn set_debug(self, b: bool) -> Self { ... }
    /// Send the request and receive a response.
    pub async fn send(self) -> Result<StorageResponse, Error> { ... }
}

// The new implementations:
// 1. create a new named future type
// 2. implement `IntoFuture` for `StorageRequest`
pub type StorageRequestFuture = Pin<Box<dyn Future<Output = Result<StorageResponse, Error>> + Send + 'static>>
impl IntoFuture for StorageRequest {
    type IntoFuture = StorageRequestFuture;
    type Output = <StorageRequestFuture as Future>::Output;
    fn into_future(self) -> Self::IntoFuture {
        Box::pin(self.send())
    }
}

This takes a bit more code to implement, but provides a simpler API for users.

In the future, the Rust Async WG hopes to simplify the creating new named futures by supporting impl Trait in type aliases (Type Alias Impl Trait or TAIT). 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.

When calling or being called by C ABIs, Rust code can use type aliases like c_uint or c_ulong to match the corresponding types from C on any target, without requiring target-specific code or conditionals.

Previously, these type aliases were only available in std, so code written for embedded targets and other scenarios that could only use core or alloc could not use these types.

Rust 1.64 now provides all of the 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 is now 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. It is available as a rustup component which can be installed with:

rustup component add rust-analyzer

At this time, to run the rustup-installed version, you need to invoke it this way:

rustup run stable rust-analyzer

The next release of rustup will provide a built-in proxy so that running the executable rust-analyzer will launch the appropriate version.

Most users should continue to use the releases provided by the rust-analyzer team (available on the rust-analyzer releases page), which are published more frequently. Users of the official VSCode extension are not affected since it automatically downloads and updates releases in the background.

When working with collections of related libraries or binary crates in one Cargo workspace, you can now avoid duplication of common field values between crates, such as common version numbers, repository URLs, or rust-version. This also helps keep these values in sync between crates when updating them. For more details, see workspace.package, workspace.dependencies, and “inheriting a dependency from a workspace”.

When building for multiple targets, you can now pass multiple --target options to cargo build, to build all of those targets at once. You can also set build.target to an array of multiple targets in .cargo/config.toml to build for multiple targets by default.