Updatify / Rust | Release notes

Language

• Unions with mutable references or tuples of allowed types are now allowed
• It is now considered valid to deallocate memory pointed to by a shared reference &T if every byte in T is inside an UnsafeCell
• Unused tuple struct fields are now warned against in an allow-by-default lint, unused_tuple_struct_fields, similar to the existing warning for unused struct fields. This lint will become warn-by-default in the future.

Compiler

• Add Nintendo Switch as tier 3 target
  • Refer to Rust’s platform support page for more information on Rust’s tiered platform support.
• Only compile #[used] as llvm.compiler.used for ELF targets
• Add the --diagnostic-width compiler flag to define the terminal width.
• Add support for link-flavor rust-lld for iOS, tvOS and watchOS

Libraries

• Remove restrictions on compare-exchange memory ordering.
• You can now write! or writeln! into an OsString: Implement fmt::Write for OsString

Language

• Remove migrate borrowck mode for pre-NLL errors.
• Modify MIR building to drop repeat expressions with length zero.
• Remove label/lifetime shadowing warnings.
• Allow explicit generic arguments in the presence of impl Trait args.
• Make cenum_impl_drop_cast warnings deny-by-default.
• Prevent unwinding when -C panic=abort is used regardless of declared ABI.
• lub: don’t bail out due to empty binders.

Compiler

• Stabilize the bundle native library modifier, also removing the deprecated static-nobundle linking kind.
• Add Apple WatchOS compile targets*.
• Add a Windows application manifest to rustc-main.

* Refer to Rust’s platform support page for more information on Rust’s tiered platform support.

Libraries

• Implement Copy, Clone, PartialEq and Eq for core::fmt::Alignment.

Rust 1.62.1 addresses a few recent regressions in the compiler and standard library, and also mitigates a CPU vulnerability on Intel SGX.

• The compiler fixed unsound function coercions involving impl Trait return types.
• The compiler fixed an incremental compilation bug with async fn lifetimes.
• Windows added a fallback for overlapped I/O in synchronous reads and writes.
• The x86_64-fortanix-unknown-sgx target added a mitigation for the MMIO stale data vulnerability, advisory INTEL-SA-00615.

Language

• Stabilize #[derive(Default)] on enums with a #[default] variant
• Teach flow sensitive checks that visibly uninhabited call expressions never return
• Fix constants not getting dropped if part of a diverging expression
• Support unit struct/enum variant in destructuring assignment
• Remove mutable_borrow_reservation_conflict lint and allow the code pattern
• const functions may now specify extern "C" or extern "Rust"

Compiler

• linker: Stop using whole-archive on dependencies of dylibs
• Make unaligned_references lint deny-by-default This lint is also a future compatibility lint, and is expected to eventually become a hard error.
• Only add codegen backend to dep info if -Zbinary-dep-depinfo is used
• Reject #[thread_local] attribute on non-static items
• Add tier 3 aarch64-pc-windows-gnullvm and x86_64-pc-windows-gnullvm targets*

Language

• const fn signatures can now include generic trait bounds
• const fn signatures can now use impl Trait in argument and return position
• Function pointers can now be created, cast, and passed around in a const fn
• Recursive calls can now set the value of a function’s opaque impl Trait return type

Compiler

• Linking modifier syntax in #[link] attributes and on the command line, as well as the whole-archive modifier specifically, are now supported
• The char type is now described as UTF-32 in debuginfo
• The #[target_feature] attribute can now be used with aarch64 features
• X86 #[target_feature = "adx"] is now stable

Libraries

• ManuallyDrop<T> is now documented to have the same layout as T
• #[ignore = "…"] messages are printed when running tests

Language

• Stabilize #[cfg(panic = "...")] for either "unwind" or "abort".
• Stabilize #[cfg(target_has_atomic = "...")] for each integer size and "ptr".

Compiler

• Enable combining +crt-static and relocation-model=pic on x86_64-unknown-linux-gnu
• Fixes wrong unreachable_pub lints on nested and glob public reexport
• Stabilize -Z instrument-coverage as -C instrument-coverage
• Stabilize -Z print-link-args as --print link-args
• Add new Tier 3 target mips64-openwrt-linux-musl*
• Add new Tier 3 target armv7-unknown-linux-uclibceabi (softfloat)*
• Fix invalid removal of newlines from doc comments
• Add kernel target for RustyHermit
• Deny mixing bin crate type with lib crate types
• Make rustc use RUST_BACKTRACE=full by default
• Upgrade to LLVM 14

Language

• Stabilize default arguments for const parameters and remove the ordering restriction for type and const parameters
• Stabilize destructuring assignment
• Relax private in public lint on generic bounds and where clauses of trait impls
• Stabilize asm! and global_asm! for x86, x86_64, ARM, Aarch64, and RISC-V

Compiler

• Stabilize new symbol mangling format, leaving it opt-in (-Csymbol-mangling-version=v0)
• Emit LLVM optimization remarks when enabled with -Cremark
• Fix sparc64 ABI for aggregates with floating point members
• Warn when a #[test]-like built-in attribute macro is present multiple times.
• Add support for riscv64gc-unknown-freebsd
• Stabilize -Z emit-future-incompat as --json future-incompat
• Soft disable incremental compilation

• Fix race condition in std::fs::remove_dir_all (CVE-2022-21658)
• Handle captured arguments in the useless_format Clippy lint
• Move non_send_fields_in_send_ty Clippy lint to nursery
• Fix wrong error message displayed when some imports are missing
• Fix rustfmt not formatting generated files from stdin

Language

• Format strings can now capture arguments simply by writing {ident} in the string. This works in all macros accepting format strings. Support for this in panic! (panic!("{ident}")) requires the 2021 edition; panic invocations in previous editions that appear to be trying to use this will result in a warning lint about not having the intended effect.
• *const T pointers can now be dereferenced in const contexts.
• The rules for when a generic struct implements Unsize have been relaxed.

Compiler

• Add LLVM CFI support to the Rust compiler
• Stabilize -Z strip as -C strip. Note that while release builds already don’t add debug symbols for the code you compile, the compiled standard library that ships with Rust includes debug symbols, so you may want to use the strip option to remove these symbols to produce smaller release binaries. Note that this release only includes support in rustc, not directly in cargo.
• Add support for LLVM coverage mapping format versions 5 and 6
• Emit LLVM optimization remarks when enabled with -Cremark

Language

• Macro attributes may follow #[derive] and will see the original (pre-cfg) input.
• Accept curly-brace macros in expressions, like m!{ .. }.method() and m!{ .. }?.
• Allow panicking in constant evaluation.
• Ignore derived Clone and Debug implementations during dead code analysis.

Compiler

• Create more accurate debuginfo for vtables.
• Add armv6k-nintendo-3ds at Tier 3*.
• Add armv7-unknown-linux-uclibceabihf at Tier 3*.
• Add m68k-unknown-linux-gnu at Tier 3*.
• Add SOLID targets at Tier 3*: aarch64-kmc-solid_asp3, armv7a-kmc-solid_asp3-eabi, armv7a-kmc-solid_asp3-eabihf

* Refer to Rust’s platform support page for more information on Rust’s tiered platform support.

Libraries

• Avoid allocations and copying in Vec::leak
• Add #[repr(i8)] to Ordering

• New lints to detect the presence of bidirectional-override Unicode codepoints in the compiled source code (CVE-2021-42574)

Language

• The 2021 Edition is now stable. See the edition guide for more details.
• The pattern in binding @ pattern can now also introduce new bindings.
• Union field access is permitted in const fn.

Compiler

• Upgrade to LLVM 13.
• Support memory, address, and thread sanitizers on aarch64-unknown-freebsd.
• Allow specifying a deployment target version for all iOS targets
• Warnings can be forced on with --force-warn. This feature is primarily intended for usage by cargo fix, rather than end users.
• Promote aarch64-apple-ios-sim to Tier 2*.
• Add powerpc-unknown-freebsd at Tier 3*.
• Add riscv32imc-esp-espidf at Tier 3*.

* Refer to Rust’s platform support page for more information on Rust’s tiered platform support.

Libraries

Language

• You can now write open “from” range patterns (X..), which will start at X and will end at the maximum value of the integer.
• You can now explicitly import the prelude of different editions through std::prelude (e.g. use std::prelude::rust_2021::*;).

Compiler

• Added tier 3* support for powerpc64le-unknown-freebsd.

* Refer to Rust’s platform support page for more information on Rust’s tiered platform support.

Libraries

• Updated std’s float parsing to use the Eisel-Lemire algorithm. These improvements should in general provide faster string parsing of floats, no longer reject certain valid floating point values, and reduce the produced code size for non-stripped artifacts.
• string::Drain now implements AsRef<str> and AsRef<[u8]>.

Stabilised APIs

• Bound::cloned
• Drain::as_str
• IntoInnerError::into_error

Language

• You can now use macros for values in some built-in attributes. This primarily allows you to call macros within the #[doc] attribute. For example, to include external documentation in your crate, you can now write the following:

  #![doc = include_str!("README.md")]

• You can now cast between unsized slice types (and types which contain unsized slices) in const fn.

• You can now use multiple generic lifetimes with impl Trait where the lifetimes don’t explicitly outlive another. In code this means that you can now have impl Trait<'a, 'b> where as before you could only have impl Trait<'a, 'b> where 'b: 'a.

Compiler

• Rustc will now search for custom JSON targets in /lib/rustlib/<target-triple>/target.json where / is the “sysroot” directory. You can find your sysroot directory by running rustc --print sysroot.
• Added wasm as a target_family for WebAssembly platforms.
• You can now use #[target_feature] on safe functions when targeting WebAssembly platforms.
• Improved debugger output for enums on Windows MSVC platforms.

Language

• You can now use unicode for identifiers. This allows multilingual identifiers but still doesn’t allow glyphs that are not considered characters such as ◆ or 🦀. More specifically you can now use any identifier that matches the UAX #31 “Unicode Identifier and Pattern Syntax” standard. This is the same standard as languages like Python, however Rust uses NFC normalization which may be different from other languages.
• You can now specify “or patterns” inside pattern matches. Previously you could only use | (OR) on complete patterns. E.g.

  let x = Some(2u8);
  // Before
  matches!(x, Some(1) | Some(2));
  // Now
  matches!(x, Some(1 | 2));

• Added the :pat_param macro_rules! matcher. This matcher has the same semantics as the :pat matcher. This is to allow :pat to change semantics to being a pattern fragment in a future edition.

Compiler

• Updated the minimum external LLVM version to LLVM 10.
• Added Tier 3* support for the wasm64-unknown-unknown target.
• Improved debuginfo for closures and async functions on Windows MSVC.

This release disables incremental compilation, unless the user has explicitly opted in via the newly added RUSTC_FORCE_INCREMENTAL=1 environment variable.

This is due to the widespread, and frequently occurring, breakage encountered by Rust users due to newly enabled incremental verification in 1.52.0. Notably, Rust users should upgrade to 1.52.0 or 1.52.1: the bugs that are detected by newly added incremental verification are still present in past stable versions, and are not yet fixed on any channel. These bugs can lead to miscompilation of Rust binaries.

These problems only affect incremental builds, so release builds with Cargo should not be affected unless the user has explicitly opted into incremental. Debug and check builds are affected.

See 84970 for more details.

Language

• Added the unsafe_op_in_unsafe_fn lint, which checks whether the unsafe code in an unsafe fn is wrapped in a unsafe block. This lint is allowed by default, and may become a warning or hard error in a future edition.
• You can now cast mutable references to arrays to a pointer of the same type as the element.

Compiler

• Upgraded the default LLVM to LLVM 12.

Added tier 3* support for the following targets.

• s390x-unknown-linux-musl
• riscv32gc-unknown-linux-musl & riscv64gc-unknown-linux-musl
• powerpc-unknown-openbsd

* Refer to Rust’s platform support page for more information on Rust’s tiered platform support.

Libraries

• OsString now implements Extend and FromIterator.
• cmp::Reverse now has #[repr(transparent)] representation.
• Arc<impl Error> now implements error::Error.
• All integer division and remainder operations are now const.

Stabilised APIs

Language

• You can now parameterize items such as functions, traits, and structs by constant values in addition to by types and lifetimes. Also known as “const generics” E.g. you can now write the following. Note: Only values of primitive integers, bool, or char types are currently permitted.

  struct GenericArray<T, const LENGTH: usize> {
      inner: [T; LENGTH]
  }
  
  impl<T, const LENGTH: usize> GenericArray<T, LENGTH> {
      const fn last(&self) -> Option<&T> {
          if LENGTH == 0 {
              None
          } else {
              Some(&self.inner[LENGTH - 1])
          }
      }
  }

Compiler

• Added the -Csplit-debuginfo codegen option for macOS platforms. This option controls whether debug information is split across multiple files or packed into a single file. Note This option is unstable on other platforms.
• Added tier 3* support for aarch64_be-unknown-linux-gnu, aarch64-unknown-linux-gnu_ilp32, and aarch64_be-unknown-linux-gnu_ilp32 targets.
• Added tier 3 support for i386-unknown-linux-gnu and i486-unknown-linux-gnu targets.
• The target-cpu=native option will now detect individual features of CPUs.

Language

• You can now use const values for x in [x; N] array expressions. This has been technically possible since 1.38.0, as it was unintentionally stabilized.
• Assignments to ManuallyDrop<T> union fields are now considered safe.

Compiler

• Added tier 3* support for the armv5te-unknown-linux-uclibceabi target.
• Added tier 3 support for the aarch64-apple-ios-macabi target.
• The x86_64-unknown-freebsd is now built with the full toolset.
• Dropped support for all cloudabi targets.

* Refer to Rust’s platform support page for more information on Rust’s tiered platform support.

Libraries

• proc_macro::Punct now implements PartialEq<char>.
• ops::{Index, IndexMut} are now implemented for fixed sized arrays of any length.
• On Unix platforms, the std::fs::File type now has a “niche” of -1. This value cannot be a valid file descriptor, and now means Option<File> takes up the same amount of space as File.

Stabilized APIs

Language

• Unions can now implement Drop, and you can now have a field in a union with ManuallyDrop<T>.
• You can now cast uninhabited enums to integers.
• You can now bind by reference and by move in patterns. This allows you to selectively borrow individual components of a type. E.g.

  #[derive(Debug)]
  struct Person {
      name: String,
      age: u8,
  }
  
  let person = Person {
      name: String::from("Alice"),
      age: 20,
  };
  
  // `name` is moved out of person, but `age` is referenced.
  let Person { name, ref age } = person;
  println!("{} {}", name, age);

Compiler

• Added tier 1* support for aarch64-unknown-linux-gnu.
• Added tier 2 support for aarch64-apple-darwin.
• Added tier 2 support for aarch64-pc-windows-msvc.
• Added tier 3 support for mipsel-unknown-none.
• Raised the minimum supported LLVM version to LLVM 9.
• Output from threads spawned in tests is now captured.
• Change os and vendor values to “none” and “unknown” for some targets

Language

• The unsafe keyword is now syntactically permitted on modules. This is still rejected semantically, but can now be parsed by procedural macros.

Compiler

• Stabilised the -C link-self-contained=<yes|no> compiler flag. This tells rustc whether to link its own C runtime and libraries or to rely on a external linker to find them. (Supported only on windows-gnu, linux-musl, and wasi platforms.)
• You can now use -C target-feature=+crt-static on linux-gnu targets. Note: If you’re using cargo you must explicitly pass the --target flag.
• Added tier 2* support for aarch64-unknown-linux-musl.

* Refer to Rust’s platform support page for more information on Rust’s tiered platform support.

Libraries

• io::Write is now implemented for &ChildStdin &Sink, &Stdout, and &Stderr.
• All arrays of any length now implement TryFrom<Vec<T>>.
• The matches! macro now supports having a trailing comma.
• Vec<A> now implements PartialEq<[B]> where A: PartialEq<B>.

Language

• Closures will now warn when not used.

Compiler

• Stabilized the -C control-flow-guard codegen option, which enables Control Flow Guard for Windows platforms, and is ignored on other platforms.
• Upgraded to LLVM 11.
• Added tier 3* support for the thumbv4t-none-eabi target.
• Upgrade the FreeBSD toolchain to version 11.4
• RUST_BACKTRACE‘s output is now more compact.

* Refer to Rust’s platform support page for more information on Rust’s tiered platform support.

Libraries

• CStr now implements Index<RangeFrom<usize>>.
• Traits in std/core are now implemented for arrays of any length, not just those of length less than 33.
• ops::RangeFull and ops::Range now implement Default.
• panic::Location now implements Copy, Clone, Eq, Hash, Ord, PartialEq, and PartialOrd.

Stabilized APIs

Language

• You can now pass type parameters to foreign items when implementing traits. E.g. You can now write impl<T> From<Foo> for Vec<T> {}.
• You can now arbitrarily nest receiver types in the self position. E.g. you can now write fn foo(self: Box<Box<Self>>) {}. Previously only Self, &Self, &mut Self, Arc<Self>, Rc<Self>, and Box<Self> were allowed.
• You can now use any valid identifier in a format_args macro. Previously identifiers starting with an underscore were not allowed.
• Visibility modifiers (e.g. pub) are now syntactically allowed on trait items and enum variants. These are still rejected semantically, but can be seen and parsed by procedural macros and conditional compilation.
• You can now define a Rust extern "C" function with Box<T> and use T* as the corresponding type on the C side. Please see the documentation for more information, including the important caveat about preferring to avoid Box<T> in Rust signatures for functions defined in C.

Compiler

• Rustc will now warn if you have unused loop 'labels.

Language

• Out of range float to int conversions using as has been defined as a saturating conversion. This was previously undefined behaviour, but you can use the {f64, f32}::to_int_unchecked methods to continue using the current behaviour, which may be desirable in rare performance sensitive situations.
• mem::Discriminant<T> now uses T‘s discriminant type instead of always using u64.
• Function like procedural macros can now be used in expression, pattern, and statement positions. This means you can now use a function-like procedural macro anywhere you can use a declarative (macro_rules!) macro.

Compiler

• You can now override individual target features through the target-feature flag. E.g. -C target-feature=+avx2 -C target-feature=+fma is now equivalent to -C target-feature=+avx2,+fma.
• Added the force-unwind-tables flag. This option allows rustc to always generate unwind tables regardless of panic strategy.
• Added the embed-bitcode flag. This codegen flag allows rustc to include LLVM bitcode into generated rlibs (this is on by default).
• Added the tiny value to the code-model codegen flag.

Language

• You can now use async/.await with #[no_std] enabled.
• Added the unused_braces lint.

Syntax-only changes

• Expansion-driven outline module parsing

#[cfg(FALSE)]
mod foo {
    mod bar {
        mod baz; // `foo/bar/baz.rs` doesn't exist, but no error!
    }
}

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

Compiler

• Rustc now respects the -C codegen-units flag in incremental mode. Additionally when in incremental mode rustc defaults to 256 codegen units.
• Refactored catch_unwind to have zero-cost, unless unwinding is enabled and a panic is thrown.
• Added tier 3* support for the aarch64-unknown-none and aarch64-unknown-none-softfloat targets.
• Added tier 3 support for arm64-apple-tvos and x86_64-apple-tvos targets.

Libraries

• Special cased vec![] to map directly to Vec::new(). This allows vec![] to be able to be used in const contexts.

Language

2018 edition

• You can now use the ? operator in macro definitions. The ? operator allows you to specify zero or one repetitions similar to the * and + operators.
• Module paths with no leading keyword like super, self, or crate, will now always resolve to the item (enum, struct, etc.) available in the module if present, before resolving to a external crate or an item the prelude. E.g.

  enum Color { Red, Green, Blue }
  
  use Color::*;

All editions

• You can now match against PhantomData<T> types.
• You can now match against literals in macros with the literal specifier. This will match against a literal of any type. E.g. 1, 'A', "Hello World"
• Self can now be used as a constructor and pattern for unit and tuple structs. E.g.

  struct Point(i32, i32);
  
  impl Point {
      pub fn new(x: i32, y: i32) -> Self {
          Self(x, y)
      }
  
      pub fn is_origin(&self) -> bool {
          match self {
              Self(0, 0) => true,
              _ => false,
          }
      }
  }

• Self can also now be used in type definitions. E.g.

  enum List<T>
  where

Language

• You can now create async functions and blocks with async fn, async move {}, and async {} respectively, and you can now call .await on async expressions.
• You can now use certain attributes on function, closure, and function pointer parameters. These attributes include cfg, cfg_attr, allow, warn, deny, forbid as well as inert helper attributes used by procedural macro attributes applied to items. e.g.

  fn len(
      #[cfg(windows)] slice: &[u16],
      #[cfg(not(windows))] slice: &[u8],
  ) -> usize {
      slice.len()
  }

• You can now take shared references to bind-by-move patterns in the if guards of match arms. e.g.

  fn main() {
      let array: Box<[u8; 4]> = Box::new([1, 2, 3, 4]);
  
      match array {
          nums
  //      ---- `nums` is bound by move.
              if nums.iter().sum::<u8>() == 10
  //                 ^------ `.iter()` implicitly takes a reference to `nums`.
          => {
              drop(nums);
  //          ----------- Legal as `nums` was bound by move and so we have ownership.
          }
          _ => unreachable!(),
      }
  }

Compiler

• Added tier 3* support for the i686-unknown-uefi target.

• Fix Rust failing to build on powerpc-unknown-netbsd
• Fix broken go-to-definition in RLS
• Fix infinite loop on hover in RLS

Language

• Fixed using binary operations with &{number} (e.g. &1.0) not having the type inferred correctly.
• Attributes such as #[cfg()] can now be used on if expressions.

Syntax only changes

• Allow type Foo: Ord syntactically.
• Fuse associated and extern items up to defaultness.
• Syntactically allow self in all fn contexts.
• Merge fn syntax + cleanup item parsing.
• item macro fragments can be interpolated into traits, impls, and extern blocks. For example, you may now write:

  macro_rules! mac_trait {
      ($i:item) => {
          trait T { $i }
      }
  }
  mac_trait! {
      fn foo() {}
  }

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

Compiler

Language

• You can now use the cfg(target_vendor) attribute. E.g. #[cfg(target_vendor="apple")] fn main() { println!("Hello Apple!"); }
• Integer patterns such as in a match expression can now be exhaustive. E.g. You can have match statement on a u8 that covers 0..=255 and you would no longer be required to have a _ => unreachable!() case.
• You can now have multiple patterns in if let and while let expressions. You can do this with the same syntax as a match expression. E.g.

  enum Creature {
      Crab(String),
      Lobster(String),
      Person(String),
  }
  
  fn main() {
      let state = Creature::Crab("Ferris");
  
      if let Creature::Crab(name) | Creature::Person(name) = state {
          println!("This creature's name is: {}", name);
      }
  }

• You can now have irrefutable if let and while let patterns. Using this feature will by default produce a warning as this behaviour can be unintuitive. E.g. if let _ = 5 {}
• You can now use let bindings, assignments, expression statements, and irrefutable pattern destructuring in const functions.
• You can now call unsafe const functions. E.g.

Language

• The #[repr(transparent)] attribute is now stable. This attribute allows a Rust newtype wrapper (struct NewType<T>(T);) to be represented as the inner type across Foreign Function Interface (FFI) boundaries.
• The keywords pure, sizeof, alignof, and offsetof have been unreserved and can now be used as identifiers.
• The GlobalAlloc trait and #[global_allocator] attribute are now stable. This will allow users to specify a global allocator for their program.
• Unit test functions marked with the #[test] attribute can now return Result<(), E: Debug> in addition to ().
• The lifetime specifier for macro_rules! is now stable. This allows macros to easily target lifetimes.

Compiler

• The s and z optimisation levels are now stable. These optimisations prioritise making smaller binary sizes. z is the same as s with the exception that it does not vectorise loops, which typically results in an even smaller binary.
• The short error format is now stable. Specified with --error-format=short this option will provide a more compressed output of rust error messages.

• Workaround for an aliasing-related LLVM bug, which caused miscompilation.
• The rls-preview component on the windows-gnu targets has been restored.

• Always check types of static items
• Always check lifetime bounds of Copy impls
• Fix miscompilation in callers of Layout::repeat
• Rust 1.41.0 was announced as the last Rust release with tier 1 or tier 2 support for 32-bit Apple targets. That announcement did not expect a patch release. 1.41.1 also includes release binaries for these targets.

Language

• #[must_use] will now warn if the type is contained in a tuple, Box, or an array and unused.
• You can now use the cfg and cfg_attr attributes on generic parameters.
• You can now use enum variants through type alias. e.g. You can write the following:

  type MyOption = Option<u8>;
  
  fn increment_or_zero(x: MyOption) -> u8 {
      match x {
          MyOption::Some(y) => y + 1,
          MyOption::None => 0,
      }
  }

• You can now use _ as an identifier for consts. e.g. You can write const _: u32 = 5;.
• You can now use #[repr(align(X)] on enums.
• The ? Kleene macro operator is now available in the 2015 edition.

Compiler

• You can now enable Profile-Guided Optimization with the -C profile-generate and -C profile-use flags. For more information on how to use profile guided optimization, please refer to the rustc book.

Security Notes

• The standard library’s str::repeat function contained an out of bounds write caused by an integer overflow. This has been fixed by deterministically panicking when an overflow happens.

  Thank you to Scott McMurray for responsibly disclosing this vulnerability to us.

• Fix false positives for the redundant_closure Clippy lint
• Fix false positives for the missing_const_for_fn Clippy lint
• Fix Clippy panic when checking some macros

• Updated openssl-src to 1.1.1g for CVE-2020-1967.
• Fixed the stabilization of AVX-512 features.
• Fixed cargo package --list not working with unpublished dependencies.

Language

• The #[global_allocator] attribute can now be used in submodules.
• The #[deprecated] attribute can now be used on macros.

Compiler

• Added pipelined compilation support to rustc. This will improve compilation times in some cases. For further information please refer to the “Evaluating pipelined rustc compilation” thread.
• Added tier 3* support for the aarch64-uwp-windows-msvc, i686-uwp-windows-gnu, i686-uwp-windows-msvc, x86_64-uwp-windows-gnu, and x86_64-uwp-windows-msvc targets.
• Added tier 3 support for the armv7-unknown-linux-gnueabi and armv7-unknown-linux-musleabi targets.
• Added tier 3 support for the hexagon-unknown-linux-musl target.
• Added tier 3 support for the riscv32i-unknown-none-elf target.
• Upgraded to LLVM 9.

* Refer to Rust’s platform support page for more information on Rust’s tiered platform support.

Libraries

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.

• Fix const propagation with references.
• rustfmt accepts rustfmt_skip in cfg_attr again.
• Avoid spurious implicit region bound.
• Install clippy on x.py install

Language

• 🎉 This version marks the release of the 2018 edition of Rust. 🎉
• New lifetime elision rules now allow for eliding lifetimes in functions and impl headers. E.g. impl<'a> Reader for BufReader<'a> {} can now be impl Reader for BufReader<'_> {}. Lifetimes are still required to be defined in structs.
• You can now define and use const functions. These are currently a strict minimal subset of the const fn RFC. Refer to the language reference for what exactly is available.
• You can now use tool lints, which allow you to scope lints from external tools using attributes. E.g. #[allow(clippy::filter_map)].
• #[no_mangle] and #[export_name] attributes can now be located anywhere in a crate, not just in exported functions.
• You can now use parentheses in pattern matches.

Compiler

• Updated musl to 1.1.20

Libraries

Language

• You can now use #[deprecated = "reason"] as a shorthand for #[deprecated(note = "reason")]. This was previously allowed by mistake but had no effect.
• You can now accept token streams in #[attr()],#[attr[]], and #[attr{}] procedural macros.
• You can now write extern crate self as foo; to import your crate’s root into the extern prelude.

Compiler

• You can now target riscv64imac-unknown-none-elf and riscv64gc-unknown-none-elf.
• You can now enable linker plugin LTO optimisations with -C linker-plugin-lto. This allows rustc to compile your Rust code into LLVM bitcode allowing LLVM to perform LTO optimisations across C/C++ FFI boundaries.
• You can now target powerpc64-unknown-freebsd.

Libraries

• The trait bounds have been removed on some of HashMap<K, V, S>‘s and HashSet<T, S>‘s basic methods. Most notably you no longer require the Hash trait to create an iterator.
• The Ord trait bounds have been removed on some of BinaryHeap<T>‘s basic methods. Most notably you no longer require the Ord trait to create an iterator.

Language

• FnOnce, FnMut, and the Fn traits are now implemented for Box<FnOnce>, Box<FnMut>, and Box<Fn> respectively.
• You can now coerce closures into unsafe function pointers. e.g.

  unsafe fn call_unsafe(func: unsafe fn()) {
      func()
  }
  
  pub fn main() {
      unsafe { call_unsafe(|| {}); }
  }

Compiler

• Added the armv6-unknown-freebsd-gnueabihf and armv7-unknown-freebsd-gnueabihf targets.
• Added the wasm32-unknown-wasi target.

Libraries

• Thread will now show its ID in Debug output.
• StdinLock, StdoutLock, and StderrLock now implement AsRawFd.
• alloc::System now implements Default.
• Expanded Debug output ({:#?}) for structs now has a trailing comma on the last field.
• char::{ToLowercase, ToUppercase} now implement ExactSizeIterator.
• All NonZero numeric types now implement FromStr.

Language

• Procedural macros are now available. These kinds of macros allow for more powerful code generation. There is a new chapter available in the Rust Programming Language book that goes further in depth.
• You can now use keywords as identifiers using the raw identifiers syntax (r#), e.g. let r#for = true;
• Using anonymous parameters in traits is now deprecated with a warning and will be a hard error in the 2018 edition.
• You can now use crate in paths. This allows you to refer to the crate root in the path, e.g. use crate::foo; refers to foo in src/lib.rs.
• Using a external crate no longer requires being prefixed with ::. Previously, using a external crate in a module without a use statement required let json = ::serde_json::from_str(foo); but can now be written as let json = serde_json::from_str(foo);.
• You can now apply the #[used] attribute to static items to prevent the compiler from optimising them away, even if they appear to be unused, e.g. #[used] static FOO: u32 = 1;

• Destabilize the Error::type_id function due to a security vulnerability (CVE-2019-12083)

Language

• Non-Lexical Lifetimes are now enabled on the 2015 edition.
• The order of traits in trait objects no longer affects the semantics of that object. e.g. dyn Send + fmt::Debug is now equivalent to dyn fmt::Debug + Send, where this was previously not the case.

Libraries

• HashMap‘s implementation has been replaced with hashbrown::HashMap implementation.
• TryFromSliceError now implements From<Infallible>.
• mem::needs_drop is now available as a const fn.
• alloc::Layout::from_size_align_unchecked is now available as a const fn.
• String now implements BorrowMut<str>.
• io::Cursor now implements Default.
• Both NonNull::{dangling, cast} are now const fns.
• The alloc crate is now stable. alloc allows you to use a subset of std (e.g. Vec, Box, Arc) in #![no_std] environments if the environment has access to heap memory allocation.
• String now implements From<&String>.

• Fixed overflow ICE in rustdoc
• Cap Cargo progress bar width at 60 in MSYS terminals

Language

• You can now use tuple structs and tuple enum variant’s constructors in const contexts. e.g.

  pub struct Point(i32, i32);
  
  const ORIGIN: Point = {
      let constructor = Point;
  
      constructor(0, 0)
  };

• You can now mark structs, enums, and enum variants with the #[non_exhaustive] attribute to indicate that there may be variants or fields added in the future. For example this requires adding a wild-card branch (_ => {}) to any match statements on a non-exhaustive enum. (RFC 2008)

• You can now use function-like procedural macros in extern blocks and in type positions. e.g. type Generated = macro!();

• Function-like and attribute procedural macros can now emit macro_rules! items, so you can now have your macros generate macros.

• The meta pattern matcher in macro_rules! now correctly matches the modern attribute syntax. For example (#[$m:meta]) now matches #[attr], #[attr{tokens}], #[attr[tokens]], and #[attr(tokens)].

Compiler

• Added tier 3 support* for the thumbv7neon-unknown-linux-musleabihf target.

• Fix bindings in tuple struct patterns
• Fix track_caller integration with trait objects

Compiler

• Bumped minimum LLVM version to 5.0.
• Added powerpc64le-unknown-linux-musl target.
• Added aarch64-unknown-hermit and x86_64-unknown-hermit targets.
• Upgraded to LLVM 7.

Libraries

• Once::call_once no longer requires Once to be 'static.
• BuildHasherDefault now implements PartialEq and Eq.
• Box<CStr>, Box<OsStr>, and Box<Path> now implement Clone.
• Implemented PartialEq<&str> for OsString and PartialEq<OsString> for &str.
• Cell<T> now allows T to be unsized.
• SocketAddr is now stable on Redox.

Stabilized APIs

• Arc::downcast
• Iterator::flatten
• Rc::downcast

Cargo

• Cargo can silently fix some bad lockfiles. You can use --locked to disable this behavior.