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Blog post; 1.36.0 announcement. (#394)
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‎posts/2019-07-04-Rust-1.36.0.md

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---
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layout: post
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title: "Announcing Rust 1.36.0"
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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, 1.36.0.
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Rust is a programming language that is empowering everyone to build reliable and efficient software.
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If you have a previous version of Rust installed via rustup, getting Rust 1.36.0 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 appropriate page on our website,
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and check out the [detailed release notes for 1.36.0][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-1360-2019-07-04
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## What's in 1.36.0 stable
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This release brings many changes, including the stabilization of the [`Future`] trait,
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the [`alloc`][alloc-crate] crate, the [`MaybeUninit<T>`] type, [NLL for Rust 2015][felix-blog],
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a new `HashMap<K, V>` implementation, and [`--offline`] support in Cargo.
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Read on for a few highlights, or see the [detailed release notes][notes] for additional information.
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### The [`Future`] is here!
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[`Future`]: https://doc.rust-lang.org/std/future/trait.Future.html
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[pr-future]: https://github.com/rust-lang/rust/pull/59739
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In Rust 1.36.0 the long awaited [`Future`] trait has been [stabilized][pr-future]!
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With this stabilization, we hope to give important crates, libraries,
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and the ecosystem time to prepare for `async` / `.await`,
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which we'll tell you more about in the future.
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### The [`alloc`][alloc-crate] crate is stable
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[alloc-crate]: https://doc.rust-lang.org/alloc/index.html
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Before 1.36.0, the standard library consisted of the crates `std`, `core`, and `proc_macro`.
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The `core` crate provided core functionality such as `Iterator` and `Copy`
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and could be used in `#![no_std]` environments since it did not impose any requirements.
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Meanwhile, the `std` crate provided types like `Box<T>` and OS functionality
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but required a global allocator and other OS capabilities in return.
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Starting with Rust 1.36.0, the parts of `std` that depend on a global allocator, e.g. `Vec<T>`,
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are now available in the `alloc` crate. The `std` crate then re-exports these parts.
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While `#![no_std]` binaries using `alloc` still require nightly Rust,
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`#![no_std]` library crates can use the `alloc` crate in stable Rust.
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Meanwhile, normal binaries, without `#![no_std]`, can depend on such library crates.
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We hope this will facilitate the development of a `#![no_std]` compatible ecosystem of libraries
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prior to stabilizing support for `#![no_std]` binaries using `alloc`.
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If you are the maintainer of a library that only relies on some allocation primitives to function,
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consider making your library `#[no_std]` compatible by using the following at the top of your `lib.rs` file:
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```rust
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#![no_std]
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extern crate alloc;
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use alloc::vec::Vec;
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```
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### [`MaybeUninit<T>`] instead of [`mem::uninitialized`]
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[`MaybeUninit<T>`]: https://doc.rust-lang.org/std/mem/union.MaybeUninit.html
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[`mem::uninitialized`]: https://doc.rust-lang.org/std/mem/fn.uninitialized.html
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[gankro-blog]: https://gankro.github.io/blah/initialize-me-maybe/
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[pr-60445]: https://github.com/rust-lang/rust/pull/60445
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In previous releases of Rust, the [`mem::uninitialized`] function has allowed you to bypass Rust's
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initialization checks by pretending that you've initialized a value at type `T` without doing anything.
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One of the main uses of this function has been to lazily allocate arrays.
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However, [`mem::uninitialized`] is an incredibly dangerous operation that essentially
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cannot be used correctly as the Rust compiler assumes that values are properly initialized.
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For example, calling `mem::uninitialized::<bool>()` causes *instantaneous __undefined behavior__*
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as, from Rust's point of view, the uninitialized bits are neither `0` (for `false`)
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nor `1` (for `true`) -- the only two allowed bit patterns for `bool`.
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To remedy this situation, in Rust 1.36.0, the type [`MaybeUninit<T>`] has been [stabilized][pr-60445].
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The Rust compiler will understand that it should not assume that a [`MaybeUninit<T>`] is a properly initialized `T`.
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Therefore, you can do gradual initialization more safely and eventually use `.assume_init()`
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once you are certain that `maybe_t: MaybeUninit<T>` contains an initialized `T`.
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As [`MaybeUninit<T>`] is the safer alternative, starting with Rust 1.38,
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the function [`mem::uninitialized`] will be deprecated.
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To find out more about uninitialized memory, [`mem::uninitialized`],
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and [`MaybeUninit<T>`], read [Alexis Beingessner's blog post][gankro-blog].
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The standard library also contains extensive documentation about [`MaybeUninit<T>`].
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### NLL for Rust 2015
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[nll-2018]: https://blog.rust-lang.org/2018/12/06/Rust-1.31-and-rust-2018.html#non-lexical-lifetimes
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[soundness]: https://en.wikipedia.org/wiki/Soundness
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[felix-blog]: http://blog.pnkfx.org/blog/2019/06/26/breaking-news-non-lexical-lifetimes-arrives-for-everyone/
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[crater-nll]: https://github.com/rust-lang/rust/issues/60680#issuecomment-495089654
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[In the announcement for Rust 1.31.0][nll-2018], we told you about NLL (Non-Lexical Lifetimes),
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an improvement to the language that makes the borrow checker smarter and more user friendly.
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For example, you may now write:
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```rust
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fn main() {
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let mut x = 5;
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let y = &x;
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let z = &mut x; // This was not allowed before 1.31.0.
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}
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```
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In 1.31.0 NLL was stabilized only for Rust 2018,
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with a promise that we would backport it to Rust 2015 as well.
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With Rust 1.36.0, we are happy to announce that we have done so! NLL is now available for Rust 2015.
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With NLL on both editions, we are closer to removing the old borrow checker.
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However, the old borrow checker unfortunately accepted some [unsound][soundness] code it should not have.
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As a result, NLL is currently in a "migration mode" wherein we will emit warnings instead
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of errors if the NLL borrow checker rejects code the old AST borrow checker would accept.
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Please see [this list][crater-nll] of public crates that are affected.
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To find out more about NLL, MIR, the story around fixing soundness holes,
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and what you can do about the warnings if you have them, read [Felix Klock's blog post][felix-blog].
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### A new [`HashMap<K, V>`] implementation
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[`hashbrown`]: https://crates.io/crates/hashbrown
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[`HashMap<K, V>`]: https://doc.rust-lang.org/std/collections/struct.HashMap.html
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[pr-hashbrown]: https://github.com/rust-lang/rust/pull/58623
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[SwissTable]: https://abseil.io/blog/20180927-swisstables
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[pr-hashbrown-perf]: https://perf.rust-lang.org/compare.html?start=b57fe74a27590289fd657614b8ad1f3eac8a7ad2&end=abade53a649583e40ed07c26ee10652703f09b58&stat=wall-time
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In Rust 1.36.0, the `HashMap<K, V>` implementation has been [replaced][pr-hashbrown]
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with the one in the [`hashbrown`] crate which is based on the [SwissTable] design.
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While the interface is the same, the `HashMap<K, V>` implementation is now
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[faster on average][pr-hashbrown-perf] and has lower memory overhead.
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Note that unlike the `hashbrown` crate,
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the implementation in `std` still defaults to the SipHash 1-3 hashing algorithm.
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### [`--offline`] support in Cargo
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[`--offline`]: https://doc.rust-lang.org/cargo/commands/cargo-build.html#cargo_build_manifest_options
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[`cargo fetch`]: https://doc.rust-lang.org/cargo/commands/cargo-fetch.html
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[nrc-blog]: https://www.ncameron.org/blog/cargo-offline/
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[relnotes-cargo]: https://github.com/rust-lang/cargo/blob/master/CHANGELOG.md#cargo-136-2019-07-04
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During most builds, Cargo doesn't interact with the network.
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Sometimes, however, Cargo has to.
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Such is the case when a dependency is added and the latest compatible version needs to be downloaded.
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At times, network access is not an option though, for example on an airplane or in isolated build environments.
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In Rust 1.36, a new Cargo flag has been stabilized: [`--offline`].
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The flag alters Cargo's dependency resolution algorithm to only use locally cached dependencies.
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When the required crates are not available offline, and a network access would be required,
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Cargo will exit with an error.
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To prepopulate the local cache in preparation for going offline,
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use the [`cargo fetch`] command, which downloads all the required dependencies for a project.
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To find out more about [`--offline`] and [`cargo fetch`], read [Nick Cameron's blog post][nrc-blog].
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For information on other changes to Cargo, see the [detailed release notes][relnotes-cargo].
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### Library changes
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[`dbg!`]: https://doc.rust-lang.org/std/macro.dbg.html
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The [`dbg!`] macro now supports multiple arguments.
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Additionally, a number of APIs have been made `const`:
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[`Layout::from_size_align_unchecked`]: https://doc.rust-lang.org/core/alloc/struct.Layout.html#method.from_size_align_unchecked
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[`mem::needs_drop`]: https://doc.rust-lang.org/std/mem/fn.needs_drop.html
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[`NonNull::dangling`]: https://doc.rust-lang.org/std/ptr/struct.NonNull.html#method.dangling
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[`NonNull::cast`]: https://doc.rust-lang.org/std/ptr/struct.NonNull.html#method.cast
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- [`Layout::from_size_align_unchecked`]
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- [`mem::needs_drop`]
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- [`NonNull::dangling`]
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- [`NonNull::cast`]
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New APIs have become stable, including:
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[`Iterator::copied`]: https://doc.rust-lang.org/std/iter/trait.Iterator.html#method.copied
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[`VecDeque::rotate_left`]: https://doc.rust-lang.org/std/collections/struct.VecDeque.html#method.rotate_left
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[`VecDeque::rotate_right`]: https://doc.rust-lang.org/std/collections/struct.VecDeque.html#method.rotate_right
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[`BorrowMut<str> for String`]: https://github.com/rust-lang/rust/pull/60404
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[`str::as_mut_ptr`]: https://doc.rust-lang.org/std/primitive.str.html#method.as_mut_ptr
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[`pointer::align_offset`]: https://doc.rust-lang.org/std/primitive.pointer.html#method.align_offset
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[`Read::read_vectored`]: https://doc.rust-lang.org/std/io/trait.Read.html#method.read_vectored
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[`Write::write_vectored`]: https://doc.rust-lang.org/std/io/trait.Write.html#method.write_vectored
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[`task::Waker`]: https://doc.rust-lang.org/std/task/struct.Waker.html
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[`task::Poll`]: https://doc.rust-lang.org/std/task/enum.Poll.html
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- [`task::Waker`] and [`task::Poll`]
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- [`VecDeque::rotate_left`] and [`VecDeque::rotate_right`]
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- [`Read::read_vectored`] and [`Write::write_vectored`]
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- [`Iterator::copied`]
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- [`BorrowMut<str> for String`]
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- [`str::as_mut_ptr`]
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Other library changes are available in the [detailed release notes][notes].
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### Other changes
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[relnotes-clippy]: https://github.com/rust-lang/rust-clippy/blob/master/CHANGELOG.md#rust-136
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Detailed 1.36.0 release notes are available for [Rust][notes],
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[Cargo][relnotes-cargo], and [Clippy][relnotes-clippy].
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## Contributors to 1.36.0
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Many people came together to create Rust 1.36.0. We couldn't have done it
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without all of you. [Thanks!](https://thanks.rust-lang.org/rust/1.36.0/)

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