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Merge pull request #745 from steveklabnik/rust-1.49
Blog for Rust 1.49.0
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‎posts/2020-12-31-Rust-1.49.0.md

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---
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layout: post
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title: "Announcing Rust 1.49.0"
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author: The Rust Release Team
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release: true
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---
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The Rust team is happy to announce a new version of Rust, 1.49.0. Rust is a
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programming language that is empowering everyone to build reliable and
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efficient software.
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If you have a previous version of Rust installed via rustup, getting Rust
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1.49.0 is as easy as:
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```console
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rustup update stable
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```
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If you don't have it already, you can [get `rustup`][install] from the
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appropriate page on our website, and check out the [detailed release notes for
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1.49.0][notes] on GitHub.
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[install]: https://www.rust-lang.org/install.html
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[notes]: https://github.com/rust-lang/rust/blob/master/RELEASES.md#version-1490-2020-12-31
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## What's in 1.49.0 stable
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For this release, we have some new targets and an improvement to the test
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framework. See the [detailed release notes][notes] to learn about other
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changes not covered by this post.
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### 64-bit ARM Linux reaches Tier 1
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The Rust compiler supports [a wide variety of targets][platform-support], but
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the Rust Team can't provide the same level of support for all of them. To
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clearly mark how supported each target is, we use a tiering system:
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* Tier 3 targets are technically supported by the compiler, but we don't check
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whether their code build or passes the tests, and we don't provide any
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prebuilt binaries as part of our releases.
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* Tier 2 targets are guaranteed to build and we provide prebuilt binaries, but
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we don't execute the test suite on those platforms: the produced binaries
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might not work or might have bugs.
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* Tier 1 targets provide the highest support guarantee, and we run the full
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suite on those platforms for every change merged in the compiler. Prebuilt
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binaries are also available.
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Rust 1.49.0 promotes the `aarch64-unknown-linux-gnu` target to Tier 1 support,
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bringing our highest guarantees to users of 64-bit ARM systems running Linux!
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We expect this change to benefit workloads spanning from embedded to desktops
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and servers.
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This is an important milestone for the project, since it's the first time a
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non-x86 target has reached Tier 1 support: we hope this will pave the way for
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more targets to reach our highest tier in the future.
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Note that Android is not affected by this change as it uses a different Tier 2
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target.
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[platform-support]: https://doc.rust-lang.org/stable/rustc/platform-support.html
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### 64-bit ARM macOS and Windows reach Tier 2
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Rust 1.49.0 also features two targets reaching Tier 2 support:
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* The `aarch64-apple-darwin` target brings support for Rust on Apple M1 systems.
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* The `aarch64-pc-windows-msvc` target brings support for Rust on 64-bit ARM
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devices running Windows on ARM.
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Developers can expect both of those targets to have prebuilt binaries
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installable with `rustup` from now on! The Rust Team is not running the test
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suite on those platforms though, so there might be bugs or instabilities.
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### Test framework captures output in threads
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Rust's built-in testing framework doesn't have a ton of features, but that
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doesn't mean it can't be improved! Consider a test that looks like this:
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```rust
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#[test]
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fn thready_pass() {
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println!("fee");
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std::thread::spawn(|| {
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println!("fie");
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println!("foe");
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})
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.join()
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.unwrap();
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println!("fum");
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}
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```
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Here's what running this test looks like before Rust 1.49.0:
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```text
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❯ cargo test
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Compiling threadtest v0.1.0 (C:\threadtest)
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Finished test [unoptimized + debuginfo] target(s) in 0.38s
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Running target\debug\deps\threadtest-02f42ffd9836cae5.exe
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running 1 test
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fie
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foe
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test thready_pass ... ok
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test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
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Doc-tests threadtest
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running 0 tests
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test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out
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```
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You can see that the output from the thread is printed, which intermixes
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from the output of the test framework itself. Wouldn't it be nice
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if every `println!` worked like that one that prints "`fum`?" Well, [that's
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the behavior in Rust 1.49.0](https://github.com/rust-lang/rust/pull/78227):
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```text
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❯ cargo +nightly test
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Compiling threadtest v0.1.0 (C:\threadtest)
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Finished test [unoptimized + debuginfo] target(s) in 0.52s
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Running target\debug\deps\threadtest-40aabfaa345584be.exe
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running 1 test
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test thready_pass ... ok
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test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s
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Doc-tests threadtest
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running 0 tests
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test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s
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```
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But don't worry; if the test were to fail, you'll still see all of the
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output. By adding a `panic!` to the end of the test, we can see what failure
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looks like:
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```text
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❯ cargo +nightly test
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Compiling threadtest v0.1.0 (C:\threadtest)
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Finished test [unoptimized + debuginfo] target(s) in 0.52s
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Running target\debug\deps\threadtest-40aabfaa345584be.exe
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running 1 test
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test thready_pass ... FAILED
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failures:
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---- thready_pass stdout ----
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fee
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fie
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foe
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fum
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thread 'thready_pass' panicked at 'explicit panic', src\lib.rs:11:5
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```
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Specifically, the test runner makes sure to capture the output, and saves it
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in case the test fails.
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### Library changes
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In Rust 1.49.0, there are three new stable functions:
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- [`slice::select_nth_unstable`]
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- [`slice::select_nth_unstable_by`]
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- [`slice::select_nth_unstable_by_key`]
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And two functions were made `const`:
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- [`Poll::is_ready`]
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- [`Poll::is_pending`]
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See the [detailed release notes][notes] to learn about other changes.
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[`slice::select_nth_unstable`]: https://doc.rust-lang.org/nightly/std/primitive.slice.html#method.select_nth_unstable
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[`slice::select_nth_unstable_by`]: https://doc.rust-lang.org/nightly/std/primitive.slice.html#method.select_nth_unstable_by
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[`slice::select_nth_unstable_by_key`]: https://doc.rust-lang.org/nightly/std/primitive.slice.html#method.select_nth_unstable_by_key
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[`Poll::is_ready`]: https://doc.rust-lang.org/stable/std/task/enum.Poll.html#method.is_ready
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[`Poll::is_pending`]: https://doc.rust-lang.org/stable/std/task/enum.Poll.html#method.is_pending
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### Other changes
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[relnotes-cargo]: https://github.com/rust-lang/cargo/blob/master/CHANGELOG.md#cargo-149-2020-12-31
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[relnotes-clippy]: https://github.com/rust-lang/rust-clippy/blob/master/CHANGELOG.md#rust-149
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There are other changes in the Rust 1.49.0 release: check out what changed in
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[Rust][notes], [Cargo][relnotes-cargo], and [Clippy][relnotes-clippy].
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## Contributors to 1.49.0
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Many people came together to create Rust 1.49.0. We couldn't have done it
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without all of you. [Thanks!](https://thanks.rust-lang.org/rust/1.49.0/)

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