1.49.0 #

Released on: 31 December, 2020
Branched from master on: 13 November, 2020

Language #

Unions can now implement Drop, and you can now have a field in a union with ManuallyDrop<T>.
You can now cast uninhabited enums to integers.

You can now bind by reference and by move in patterns. This allows you to selectively borrow individual components of a type. E.g.

#[derive(Debug)]
struct Person {
    name: String,
    age: u8,
}

let person = Person {
    name: String::from("Alice"),
    age: 20,
};

// `name` is moved out of person, but `age` is referenced.
let Person { name, ref age } = person;
println!("{} {}", name, age);

Compiler #

* Refer to Rust’s [platform support page][platform-support-doc] for more information on Rust’s tiered platform support.

Libraries #

Stabilized APIs #

The following previously stable methods are now const.

Cargo #

Building a crate with cargo-package should now be independently reproducible.
cargo-tree now marks proc-macro crates.
Added CARGO_PRIMARY_PACKAGE build-time environment variable. This variable will be set if the crate being built is one the user selected to build, either with -p or through defaults.
You can now use glob patterns when specifying packages & targets.

Compatibility Notes #

Demoted i686-unknown-freebsd from host tier 2 to target tier 2 support.
Macros that end with a semi-colon are now treated as statements even if they expand to nothing.
Rustc will now check for the validity of some built-in attributes on enum variants. Previously such invalid or unused attributes could be ignored.
Leading whitespace is stripped more uniformly in documentation comments, which may change behavior. You read this post about the changes for more details.
Trait bounds are no longer inferred for associated types.

Internal Only #

These changes provide no direct user facing benefits, but represent significant improvements to the internals and overall performance of rustc and related tools.