Rust release · Mar 12, 2020

Rust 1.42: what's new, what was stabilized

Short answerRust 1.42 was released on Mar 12, 2020: 8 newly stable APIs (CondVar::wait_while, CondVar::wait_timeout_while, DebugMap::key, DebugMap::value, ...); 1 compatibility note that can affect existing code.

Mar 12, 2020release date

0language stabilizations

8APIs stabilized

-new edition

← Rust 1.41 All versions Rust 1.43 →

8 APIs stabilized

CondVar::wait_while, CondVar::wait_timeout_while, DebugMap::key, DebugMap::value, ManuallyDrop::take, matches!, ptr::slice_from_raw_parts_mut, ptr::slice_from_raw_parts

Can it break my build?

The release lists these compatibility notes. Most only affect rare code, but read them before you raise your toolchain in CI:

[Error::description has been deprecated, and its use will now produce a

Use it

rustup update stable        # get the newest release
rustup install 1.42          # or exactly this one
rustup override set 1.42     # pin a project to it

To require at least this version from users of your crate, set rust-version = "1.42" under [package] in Cargo.toml. Cargo then refuses to build it on older compilers with a clear message, and with resolver 3 (the default on the 2024 edition, Rust 1.85+) it also prefers dependency versions that support it.

Full release notes

Language

[You can now use the slice pattern syntax with subslices.][67712] e.g. ``rust fn foo(words: &[&str]) { match words { ["Hello", "World", "!", ..] => println!("Hello World!"), ["Foo", "Bar", ..] => println!("Baz"), rest => println!("{:?}", rest), } } ``
[You can now use #[repr(transparent)] on univariant enums.][68122] Meaning that you can create an enum that has the exact layout and ABI of the type it contains.
[You can now use outer attribute procedural macros on inline modules.][64273]
[There are some *syntax-only* changes:][67131]
default is syntactically allowed before items in trait definitions.
Items in impls (i.e. consts, types, and fns) may syntactically leave out their bodies in favor of ;.
Bounds on associated types in impls are now syntactically allowed (e.g. type Foo: Ord;).
... (the C-variadic type) may occur syntactically directly as the type of any function parameter.

These are still rejected *semantically*, so you will likely receive an error but these changes can be seen and parsed by procedural macros and conditional compilation.

Compiler

[Added tier 2\* support for armv7a-none-eabi.][68253]
[Added tier 2 support for riscv64gc-unknown-linux-gnu.][68339]
[Option::{expect,unwrap} and Result::{expect, expect_err, unwrap, unwrap_err} now produce panic messages pointing to the location where they were called, rather than core's internals. ][67887]

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

Libraries

[iter::Empty<T> now implements Send and Sync for any T.][68348]
[Pin::{map_unchecked, map_unchecked_mut} no longer require the return type to implement Sized.][67935]
[io::Cursor now derives PartialEq and Eq.][67233]
[Layout::new is now const.][66254]
[Added Standard Library support for riscv64gc-unknown-linux-gnu.][66899]

Stabilized APIs

[CondVar::wait_while]
[CondVar::wait_timeout_while]
[DebugMap::key]
[DebugMap::value]
[ManuallyDrop::take]
[matches!]
[ptr::slice_from_raw_parts_mut]
[ptr::slice_from_raw_parts]

Cargo

[You no longer need to include extern crate proc_macro; to be able to use proc_macro; in the 2018 edition.][cargo/7700]

Compatibility Notes

Error::description has been deprecated, and its use will now produce a warning. It's recommended to use Display/to_string instead.

Release notes from the Rust project's RELEASES.md (MIT or Apache-2.0); summaries above are ours.