1.42.0 #

• Released on: 12 March, 2020
• Branched from master on: 24 January, 2020

Language #

• You can now use the slice pattern syntax with subslices. e.g.

  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. 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.

• There are some syntax-only changes:

  • 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.
• Added tier 2 support for riscv64gc-unknown-linux-gnu.
• 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.

* 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.
• Pin::{map_unchecked, map_unchecked_mut} no longer require the return type to implement Sized.
• io::Cursor now derives PartialEq and Eq.
• Layout::new is now const.
• Added Standard Library support for riscv64gc-unknown-linux-gnu.

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.

Compatibility Notes #

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