Version 1.49.0 (2020-12-31)

Language

- [Unions can now implement `Drop`, and you can now have a field in a union

with `ManuallyDrop<T>`.][77547]

- [You can now cast uninhabited enums to integers.][76199]

- [You can now bind by reference and by move in patterns.][76119] This

allows you to selectively borrow individual components of a type. E.g.

```rust

#[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

- [Added tier 1\* support for `aarch64-unknown-linux-gnu`.][78228]

- [Added tier 2 support for `aarch64-apple-darwin`.][75991]

- [Added tier 2 support for `aarch64-pc-windows-msvc`.][75914]

- [Added tier 3 support for `mipsel-unknown-none`.][78676]

- [Raised the minimum supported LLVM version to LLVM 9.][78848]

- [Output from threads spawned in tests is now captured.][78227]

- [Change os and vendor values to "none" and "unknown" for some targets][78951]

\* Refer to Rust's [platform support page][forge-platform-support] for more

information on Rust's tiered platform support.

Libraries

- [`RangeInclusive` now checks for exhaustion when calling `contains` and indexing.][78109]

- [`ToString::to_string` now no longer shrinks the internal buffer in the default implementation.][77997]

- [`ops::{Index, IndexMut}` are now implemented for fixed sized arrays of any length.][74989]

Stabilized APIs

- [`slice::select_nth_unstable`]

- [`slice::select_nth_unstable_by`]

- [`slice::select_nth_unstable_by_key`]

The following previously stable methods are now `const`.

- [`Poll::is_ready`]

- [`Poll::is_pending`]

Cargo

- [Building a crate with `cargo-package` should now be independently reproducible.][cargo/8864]

- [`cargo-tree` now marks proc-macro crates.][cargo/8765]

- [Added `CARGO_PRIMARY_PACKAGE` build-time environment variable.][cargo/8758] 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.][cargo/8752]

Compatibility Notes

- [Demoted `i686-unknown-freebsd` from host tier 2 to target tier 2 support.][78746]

- [Macros that end with a semi-colon are now treated as statements even if they expand to nothing.][78376]

- [Rustc will now check for the validity of some built-in attributes on enum variants.][77015]

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][rustdoc-ws-post] for more details.

- [Trait bounds are no longer inferred for associated types.][79904]

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.

- [rustc's internal crates are now compiled using the `initial-exec` Thread

Local Storage model.][78201]

- [Calculate visibilities once in resolve.][78077]

- [Added `system` to the `llvm-libunwind` bootstrap config option.][77703]

- [Added `--color` for configuring terminal color support to bootstrap.][79004]

[75991]: https://github.com/rust-lang/rust/pull/75991

[78951]: https://github.com/rust-lang/rust/pull/78951

[78848]: https://github.com/rust-lang/rust/pull/78848

[78746]: https://github.com/rust-lang/rust/pull/78746

[78376]: https://github.com/rust-lang/rust/pull/78376

[78228]: https://github.com/rust-lang/rust/pull/78228

[78227]: https://github.com/rust-lang/rust/pull/78227

[78201]: https://github.com/rust-lang/rust/pull/78201

[78109]: https://github.com/rust-lang/rust/pull/78109

[78077]: https://github.com/rust-lang/rust/pull/78077

[77997]: https://github.com/rust-lang/rust/pull/77997

[77703]: https://github.com/rust-lang/rust/pull/77703

[77547]: https://github.com/rust-lang/rust/pull/77547

[77015]: https://github.com/rust-lang/rust/pull/77015

[76199]: https://github.com/rust-lang/rust/pull/76199

[76119]: https://github.com/rust-lang/rust/pull/76119

[75914]: https://github.com/rust-lang/rust/pull/75914

[74989]: https://github.com/rust-lang/rust/pull/74989

[79004]: https://github.com/rust-lang/rust/pull/79004

[78676]: https://github.com/rust-lang/rust/pull/78676

[79904]: https://github.com/rust-lang/rust/issues/79904

[cargo/8864]: https://github.com/rust-lang/cargo/pull/8864

[cargo/8765]: https://github.com/rust-lang/cargo/pull/8765

[cargo/8758]: https://github.com/rust-lang/cargo/pull/8758

[cargo/8752]: https://github.com/rust-lang/cargo/pull/8752

[`slice::select_nth_unstable`]: https://doc.rust-lang.org/nightly/std/primitive.slice.html#method.select_nth_unstable

[`slice::select_nth_unstable_by`]: https://doc.rust-lang.org/nightly/std/primitive.slice.html#method.select_nth_unstable_by

[`slice::select_nth_unstable_by_key`]: https://doc.rust-lang.org/nightly/std/primitive.slice.html#method.select_nth_unstable_by_key

[`hint::spin_loop`]: https://doc.rust-lang.org/stable/std/hint/fn.spin_loop.html

[`Poll::is_ready`]: https://doc.rust-lang.org/stable/std/task/enum.Poll.html#method.is_ready

[`Poll::is_pending`]: https://doc.rust-lang.org/stable/std/task/enum.Poll.html#method.is_pending

[rustdoc-ws-post]: https://blog.guillaume-gomez.fr/articles/2020-11-11+New+doc+comment+handling+in+rustdoc

`rustc` whether to link its own C runtime and libraries or to rely on a external

[54121]: https://github.com/rust-lang/rust/issues/54121/

export RUST_RELEASE_CHANNEL=stable