Announcing Rust 1.56.0 and Rust 2021

posts/2021-10-21-Rust-1.56.0.md

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+---
+layout: post
+title: "Announcing Rust 1.56.0 and Rust 2021"
+author: The Rust Release Team
+release: true
+---
+
+The Rust team is happy to announce a new version of Rust, 1.56.0. This stabilizes the 2021 edition as well.
+Rust is a programming language empowering everyone to build reliable and efficient software.
+
+If you have a previous version of Rust installed via rustup, getting Rust 1.56.0 is as easy as:
+
+```console
+rustup update stable
+```
+
+If you don't have it already, you can [get `rustup`][install]
+from the appropriate page on our website, and check out the
+[detailed release notes for 1.56.0][notes] on GitHub.
+
+[install]: https://www.rust-lang.org/install.html
+[notes]: https://github.com/rust-lang/rust/blob/master/RELEASES.md#version-1560-2021-10-21
+
+## What's in 1.56.0 stable
+
+### Rust 2021
+
+We wrote about plans for the Rust 2021 Edition [in May](https://blog.rust-lang.org/2021/05/11/edition-2021.html).
+Editions are a mechanism for opt-in changes that may otherwise pose backwards compatibility risk. See [the edition guide](https://doc.rust-lang.org/stable/edition-guide/editions/index.html) for details on how this is achieved.
+This a smaller edition, especially compared to 2018, but there
+are still some nice quality-of-life changes that require an edition opt-in to
+avoid breaking some corner cases in existing code. See the new chapters of the
+edition guide below for more details on each new feature and guidance for
+migration.
+
+* [Disjoint capture](https://doc.rust-lang.org/edition-guide/rust-2021/disjoint-capture-in-closures.html): closures now capture individual named fields rather than always capturing whole identifiers.
+* [`IntoIterator` for arrays](https://doc.rust-lang.org/edition-guide/rust-2021/IntoIterator-for-arrays.html): `array.into_iter()` now iterates over items by value instead of by reference.
+* [Or patterns in macro-rules](https://doc.rust-lang.org/edition-guide/rust-2021/or-patterns-macro-rules.html) now match top-level `A|B` in `:pat`.
+* [Default Cargo feature resolver](https://doc.rust-lang.org/edition-guide/rust-2021/default-cargo-resolver.html) is now version 2.
+* [Additions to the prelude](https://doc.rust-lang.org/edition-guide/rust-2021/prelude.html): `TryInto`, `TryFrom`, and `FromIterator` are now in scope by default.
+* [Panic macros](https://doc.rust-lang.org/edition-guide/rust-2021/panic-macro-consistency.html) now always expect format strings, just like `println!()`.
+* [Reserving syntax](https://doc.rust-lang.org/edition-guide/rust-2021/reserving-syntax.html) for `ident#`, `ident"..."`, and `ident'...'`.
+* [Warnings promoted to errors](https://doc.rust-lang.org/edition-guide/rust-2021/warnings-promoted-to-error.html): `bare_trait_objects` and `ellipsis_inclusive_range_patterns`.
+
+#### Disjoint capture in closures
+
+Closures automatically capture values or references to identifiers that are
+used in the body, but before 2021, they were always captured as a whole. The new
+disjoint-capture feature will likely simplify the way you write closures, so
+let's look at a quick example:
+
+```rust
+// 2015 or 2018 edition code
+let a = SomeStruct::new();
+
+// Move out of one field of the struct
+drop(a.x);
+
+// Ok: Still use another field of the struct
+println!("{}", a.y);
+
+// Error: Before 2021 edition, tries to capture all of `a`
+let c = || println!("{}", a.y);
+c();
+```
+
+To fix this, you would have had to extract something like `let y = &a.y;`
+manually before the closure to limit its capture. Starting in Rust 2021,
+closures will automatically capture only the fields that they use, so the
+above example will compile fine!
+
+This new behavior is only activated in the new edition, since it can change
+the order in which fields are dropped. As for all edition changes, an
+automatic migration is available, which will update your closures for which
+this matters by inserting `let _ = &a;` inside the closure to force the
+entire struct to be captured as before.
+
+#### Migrating to 2021
+
+The guide includes migration instructions for all new features, and in general
+[transitioning an existing project to a new edition](https://doc.rust-lang.org/edition-guide/editions/transitioning-an-existing-project-to-a-new-edition.html).
+In many cases `cargo fix` can automate the necessary changes. You may even
+find that no changes in your code are needed at all for 2021!
+
+However small this edition appears on the surface, it's still the product
+of a lot of hard work from many contributors: see our dedicated
+[celebration and thanks](https://github.com/rust-lang/rust/issues/88623) tracker!
+
+### Cargo `rust-version`
+
+`Cargo.toml` now supports a `[package]` [`rust-version`] field to specify
+the minimum supported Rust version for a crate, and Cargo will exit with an
+early error if that is not satisfied. This doesn't currently influence the
+dependency resolver, but the idea is to catch compatibility problems before
+they turn into cryptic compiler errors.
+
+[`rust-version`]: https://doc.rust-lang.org/cargo/reference/manifest.html#the-rust-version-field
+
+### New bindings in `binding @ pattern`
+
+Rust pattern matching can be written with a single identifier that binds
+the entire value, followed by `@` and a more refined structural pattern,
+but this has not allowed additional bindings in that pattern -- until now!
+
+```rust
+struct Matrix {
+    data: Vec<f64>,
+    row_len: usize,
+}
+
+// Before, we need separate statements to bind
+// the whole struct and also read its parts.
+let matrix = get_matrix();
+let row_len = matrix.row_len;
+// or with a destructuring pattern:
+let Matrix { row_len, .. } = matrix;
+
+// Rust 1.56 now lets you bind both at once!
+let matrix @ Matrix { row_len, .. } = get_matrix();
+```
+
+This actually was allowed in the days before Rust 1.0, but that was removed
+due to known [unsoundness](https://github.com/rust-lang/rust/pull/16053) at
+the time. With the evolution of the borrow checker since that time, and with
+heavy testing, the compiler team determined that this was safe to finally
+allow in stable Rust!
+
+### Stabilized APIs
+
+The following methods and trait implementations were stabilized.
+
+- [`std::os::unix::fs::chroot`]
+- [`UnsafeCell::raw_get`]
+- [`BufWriter::into_parts`]
+- [`core::panic::{UnwindSafe, RefUnwindSafe, AssertUnwindSafe}`]\
+  \(previously only in `std`)
+- [`Vec::shrink_to`]
+- [`String::shrink_to`]
+- [`OsString::shrink_to`]
+- [`PathBuf::shrink_to`]
+- [`BinaryHeap::shrink_to`]
+- [`VecDeque::shrink_to`]
+- [`HashMap::shrink_to`]
+- [`HashSet::shrink_to`]
+
+The following previously stable functions are now `const`.
+
+- [`std::mem::transmute`]
+- [`[T]::first`][`slice::first`]
+- [`[T]::split_first`][`slice::split_first`]
+- [`[T]::last`][`slice::last`]
+- [`[T]::split_last`][`slice::split_last`]
+
+[`std::os::unix::fs::chroot`]: https://doc.rust-lang.org/stable/std/os/unix/fs/fn.chroot.html
+[`UnsafeCell::raw_get`]: https://doc.rust-lang.org/stable/std/cell/struct.UnsafeCell.html#method.raw_get
+[`BufWriter::into_parts`]: https://doc.rust-lang.org/stable/std/io/struct.BufWriter.html#method.into_parts
+[`core::panic::{UnwindSafe, RefUnwindSafe, AssertUnwindSafe}`]: https://github.com/rust-lang/rust/pull/84662
+[`Vec::shrink_to`]: https://doc.rust-lang.org/stable/std/vec/struct.Vec.html#method.shrink_to
+[`String::shrink_to`]: https://doc.rust-lang.org/stable/std/string/struct.String.html#method.shrink_to
+[`OsString::shrink_to`]: https://doc.rust-lang.org/stable/std/ffi/struct.OsString.html#method.shrink_to
+[`PathBuf::shrink_to`]: https://doc.rust-lang.org/stable/std/path/struct.PathBuf.html#method.shrink_to
+[`BinaryHeap::shrink_to`]: https://doc.rust-lang.org/stable/std/collections/struct.BinaryHeap.html#method.shrink_to
+[`VecDeque::shrink_to`]: https://doc.rust-lang.org/stable/std/collections/struct.VecDeque.html#method.shrink_to
+[`HashMap::shrink_to`]: https://doc.rust-lang.org/stable/std/collections/hash_map/struct.HashMap.html#method.shrink_to
+[`HashSet::shrink_to`]: https://doc.rust-lang.org/stable/std/collections/hash_set/struct.HashSet.html#method.shrink_to
+[`std::mem::transmute`]: https://doc.rust-lang.org/stable/std/mem/fn.transmute.html
+[`slice::first`]: https://doc.rust-lang.org/stable/std/primitive.slice.html#method.first
+[`slice::split_first`]: https://doc.rust-lang.org/stable/std/primitive.slice.html#method.split_first
+[`slice::last`]: https://doc.rust-lang.org/stable/std/primitive.slice.html#method.last
+[`slice::split_last`]: https://doc.rust-lang.org/stable/std/primitive.slice.html#method.split_last
+
+### Other changes
+
+There are other changes in the Rust 1.56.0 release: check out what changed in
+[Rust](https://github.com/rust-lang/rust/blob/master/RELEASES.md#version-1560-2021-10-21),
+[Cargo](https://github.com/rust-lang/cargo/blob/master/CHANGELOG.md#cargo-156-2021-10-21),
+and [Clippy](https://github.com/rust-lang/rust-clippy/blob/master/CHANGELOG.md#rust-156).
+
+### Contributors to 1.56.0
+
+Many people came together to create Rust 1.56.0 and the 2021 edition.
+We couldn't have done it without all of you.
+[Thanks!](https://thanks.rust-lang.org/rust/1.56.0/)