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
• Make RwLockReadGuard covariant
• Implement FusedIterator for std::net::[Into]Incoming
• impl<T: AsRawFd> AsRawFd for {Arc,Box}<T>
• ptr::copy and ptr::swap are doing untyped copies
• Add cgroupv1 support to available_parallelism
• Mitigate many incorrect uses of mem::uninitialized

Stabilized APIs

• future::IntoFuture
• future::poll_fn
• task::ready!
• num::NonZero*::checked_mul
• num::NonZero*::checked_pow
• num::NonZero*::saturating_mul
• num::NonZero*::saturating_pow
• num::NonZeroI*::abs
• num::NonZeroI*::checked_abs
• num::NonZeroI*::overflowing_abs
• num::NonZeroI*::saturating_abs
• num::NonZeroI*::unsigned_abs
• num::NonZeroI*::wrapping_abs
• num::NonZeroU*::checked_add
• num::NonZeroU*::checked_next_power_of_two
• num::NonZeroU*::saturating_add
• os::unix::process::CommandExt::process_group
• os::windows::fs::FileTypeExt::is_symlink_dir
• os::windows::fs::FileTypeExt::is_symlink_file

These types were previously stable in std::ffi, but are now also available in core and alloc:

• core::ffi::CStr
• core::ffi::FromBytesWithNulError
• alloc::ffi::CString
• alloc::ffi::FromVecWithNulError
• alloc::ffi::IntoStringError
• alloc::ffi::NulError

These types were previously stable in std::os::raw, but are now also available in core::ffi and std::ffi:

• ffi::c_char
• ffi::c_double
• ffi::c_float
• ffi::c_int
• ffi::c_long
• ffi::c_longlong
• ffi::c_schar
• ffi::c_short
• ffi::c_uchar
• ffi::c_uint
• ffi::c_ulong
• ffi::c_ulonglong
• ffi::c_ushort

These APIs are now usable in const contexts:

• slice::from_raw_parts

Cargo

• Packages can now inherit settings from the workspace so that the settings can be centralized in one place. See workspace.package and workspace.dependencies for more details on how to define these common settings.
• Cargo commands can now accept multiple --target flags to build for multiple targets at once, and the build.target config option may now take an array of multiple targets.
• The --jobs argument can now take a negative number to count backwards from the max CPUs.
• cargo add will now update Cargo.lock.
• Added the --crate-type flag to cargo rustc to override the crate type.
• Significantly improved the performance fetching git dependencies from GitHub when using a hash in the rev field.

Misc

• The rust-analyzer rustup component is now available on the stable channel.

Compatibility Notes

• The minimum required versions for all -linux-gnu targets are now at least kernel 3.2 and glibc 2.17, for targets that previously supported older versions: Increase the minimum linux-gnu versions
• Network primitives are now implemented with the ideal Rust layout, not the C system layout. This can cause problems when transmuting the types.
• Add assertion that transmute_copy‘s U is not larger than T
• A soundness bug in BTreeMap was fixed that allowed data it was borrowing to be dropped before the container.
• The Drop behavior of C-like enums cast to ints has changed. These are already discouraged by a compiler warning.
• Relate late-bound closure lifetimes to parent fn in NLL
• Errors at const-eval time are now in future incompatibility reports
• On the thumbv6m-none-eabi target, some incorrect asm! statements were erroneously accepted if they used the high registers (r8 to r14) as an input/output operand. This is no longer accepted.
• impl Trait was accidentally accepted as the associated type value of return-position impl Trait, without fulfilling all the trait bounds of that associated type, as long as the hidden type satisfies said bounds. This has been fixed.

Internal Changes

These changes do not affect any public interfaces of Rust, but they represent significant improvements to the performance or internals of rustc and related tools.

• Windows builds now use profile-guided optimization, providing 10-20% improvements to compiler performance: Utilize PGO for windows x64 rustc dist builds
• Stop keeping metadata in memory before writing it to disk
• compiletest: strip debuginfo by default for mode=ui
• Many improvements to generated code for derives, including performance improvements:
  • Don’t use match-destructuring for derived ops on structs.
  • Many small deriving cleanups
  • More derive output improvements
  • Clarify deriving code
  • Final derive output improvements
  • Stop injecting #[allow(unused_qualifications)] in generated derive implementations
  • Improve derive(Debug)
• Bump to clap 3
• fully move dropck to mir
• Optimize Vec::insert for the case where index == len.
• Convert rust-analyzer to an in-tree tool

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.
• Extend ptr::null and null_mut to all thin (including extern) types.
• impl Read and Write for VecDeque<u8>.
• STD support for the Nintendo 3DS.
• Use rounding in float to Duration conversion methods.
• Make write/print macros eagerly drop temporaries.
• Implement internal traits that enable [OsStr]::join.
• Implement Hash for core::alloc::Layout.
• Add capacity documentation for OsString.
• Put a bound on collection misbehavior.
• Make std::mem::needs_drop accept ?Sized.
• impl Termination for Infallible and then make the Result impls of Termination more generic.
• Document Rust’s stance on /proc/self/mem.

Stabilized APIs

• array::from_fn
• Box::into_pin
• BinaryHeap::try_reserve
• BinaryHeap::try_reserve_exact
• OsString::try_reserve
• OsString::try_reserve_exact
• PathBuf::try_reserve
• PathBuf::try_reserve_exact
• Path::try_exists
• Ref::filter_map
• RefMut::filter_map
• NonNull::<[T]>::len
• ToOwned::clone_into
• Ipv6Addr::to_ipv4_mapped
• unix::io::AsFd
• unix::io::BorrowedFd<'fd>
• unix::io::OwnedFd
• windows::io::AsHandle
• windows::io::BorrowedHandle<'handle>
• windows::io::OwnedHandle
• windows::io::HandleOrInvalid
• windows::io::HandleOrNull
• windows::io::InvalidHandleError
• windows::io::NullHandleError
• windows::io::AsSocket
• windows::io::BorrowedSocket<'handle>
• windows::io::OwnedSocket
• thread::scope
• thread::Scope
• thread::ScopedJoinHandle

These APIs are now usable in const contexts:

• array::from_ref
• slice::from_ref
• intrinsics::copy
• intrinsics::copy_nonoverlapping
• <*const T>::copy_to
• <*const T>::copy_to_nonoverlapping
• <*mut T>::copy_to
• <*mut T>::copy_to_nonoverlapping
• <*mut T>::copy_from
• <*mut T>::copy_from_nonoverlapping
• str::from_utf8
• Utf8Error::error_len
• Utf8Error::valid_up_to
• Condvar::new
• Mutex::new
• RwLock::new

Cargo

• Stabilize the --config path command-line argument.
• Expose rust-version in the environment as CARGO_PKG_RUST_VERSION.

Compatibility Notes

• #[link] attributes are now checked more strictly, which may introduce errors for invalid attribute arguments that were previously ignored.
• Rounding is now used when converting a float to a Duration. The converted duration can differ slightly from what it was.

Internal Changes

These changes provide no direct user facing benefits, but represent significant improvements to the internals and overall performance of rustc and related tools.

• Prepare Rust for LLVM opaque pointers.

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*
• Implement a lint to warn about unused macro rules
• Promote x86_64-unknown-none target to Tier 2*

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

Libraries

• Windows: Use a pipe relay for chaining pipes
• Replace Linux Mutex and Condvar with futex based ones.
• Replace RwLock by a futex based one on Linux
• std: directly use pthread in UNIX parker implementation

Stabilized APIs

• bool::then_some
• f32::total_cmp
• f64::total_cmp
• Stdin::lines
• windows::CommandExt::raw_arg
• impl<T: Default> Default for AssertUnwindSafe<T>
• From<Rc<str>> for Rc<[u8]>
• From<Arc<str>> for Arc<[u8]>
• FusedIterator for EncodeWide
• RDM intrinsics on aarch64

Clippy

• Create clippy lint against unexpectedly late drop for temporaries in match scrutinee expressions

Cargo

• Added the cargo add command for adding dependencies to Cargo.toml from the command-line. docs
• Package ID specs now support name@version syntax in addition to the previous name:version to align with the behavior in cargo add and other tools. cargo install and cargo yank also now support this syntax so the version does not need to passed as a separate flag.
• The git and registry directories in Cargo’s home directory (usually ~/.cargo) are now marked as cache directories so that they are not included in backups or content indexing (on Windows).
• Added automatic @ argfile support, which will use “response files” if the command-line to rustc exceeds the operating system’s limit.

Compatibility Notes

• cargo test now passes --target to rustdoc if the specified target is the same as the host target. #10594
• rustdoc: doctests are now run on unexported macro_rules! macros, matching other private items
• rustdoc: Remove .woff font files
• Enforce Copy bounds for repeat elements while considering lifetimes
• Windows: Fix potential unsoundness by aborting if File reads or writes cannot complete synchronously.

Internal Changes

• Unify ReentrantMutex implementations across all platforms

These changes provide no direct user facing benefits, but represent significant improvements to the internals and overall performance of rustc and related tools.

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
• Consistently show absent stdio handles on Windows as NULL handles
• Make std::io::stdio::lock() return 'static handles. Previously, the creation of locked handles to stdin/stdout/stderr would borrow the handles being locked, which prevented writing let out = std::io::stdout().lock(); because out would outlive the return value of stdout(). Such code now works, eliminating a common pitfall that affected many Rust users.
• Vec::from_raw_parts is now less restrictive about its inputs
• std::thread::available_parallelism now takes cgroup quotas into account. Since available_parallelism is often used to create a thread pool for parallel computation, which may be CPU-bound for performance, available_parallelism will return a value consistent with the ability to use that many threads continuously, if possible. For instance, in a container with 8 virtual CPUs but quotas only allowing for 50% usage, available_parallelism will return 4.

Stabilized APIs

• Pin::static_mut
• Pin::static_ref
• Vec::retain_mut
• VecDeque::retain_mut
• Write for Cursor<[u8; N]>
• std::os::unix::net::SocketAddr::from_pathname
• std::process::ExitCode and std::process::Termination. The stabilization of these two APIs now makes it possible for programs to return errors from main with custom exit codes.
• std::thread::JoinHandle::is_finished

These APIs are now usable in const contexts:

• <*const T>::offset and <*mut T>::offset
• <*const T>::wrapping_offset and <*mut T>::wrapping_offset
• <*const T>::add and <*mut T>::add
• <*const T>::sub and <*mut T>::sub
• <*const T>::wrapping_add and <*mut T>::wrapping_add
• <*const T>::wrapping_sub and <*mut T>::wrapping_sub
• <[T]>::as_mut_ptr
• <[T]>::as_ptr_range
• <[T]>::as_mut_ptr_range

Cargo

No feature changes, but see compatibility notes.

Compatibility Notes

• Previously native static libraries were linked as whole-archive in some cases, but now rustc tries not to use whole-archive unless explicitly requested. This change may result in linking errors in some cases. To fix such errors, native libraries linked from the command line, build scripts, or #[link] attributes need to
  • (more common) either be reordered to respect dependencies between them (if a depends on b then a should go first and b second)
  • (less common) or be updated to use the +whole-archive modifier.
• Catching a second unwind from FFI code while cleaning up from a Rust panic now causes the process to abort
• Proc macros no longer see ident matchers wrapped in groups
• The number of # in r# raw string literals is now required to be less than 256
• When checking that a dyn type satisfies a trait bound, supertrait bounds are now enforced
• cargo vendor now only accepts one value for each --sync flag
• cfg predicates in all() and any() are always evaluated to detect errors, instead of short-circuiting. The compatibility considerations here arise in nightly-only code that used the short-circuiting behavior of all to write something like cfg(all(feature = "nightly", syntax-requiring-nightly)), which will now fail to compile. Instead, use either cfg_attr(feature = "nightly", ...) or nested uses of cfg.
• bootstrap: static-libstdcpp is now enabled by default, and can now be disabled when llvm-tools is enabled

Internal Changes

These changes provide no direct user facing benefits, but represent significant improvements to the internals and overall performance of rustc and related tools.

• debuginfo: Refactor debuginfo generation for types
• Remove the everybody loops pass

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

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

Libraries

• Guarantee call order for sort_by_cached_key
• Improve Duration::try_from_secs_f32/f64 accuracy by directly processing exponent and mantissa
• Make Instant::{duration_since, elapsed, sub} saturating
• Remove non-monotonic clocks workarounds in Instant::now
• Make BuildHasherDefault, iter::Empty and future::Pending covariant

Stabilized APIs

• Arc::new_cyclic
• Rc::new_cyclic
• slice::EscapeAscii
• <[u8]>::escape_ascii
• u8::escape_ascii
• Vec::spare_capacity_mut
• MaybeUninit::assume_init_drop
• MaybeUninit::assume_init_read
• i8::abs_diff
• i16::abs_diff
• i32::abs_diff
• i64::abs_diff
• i128::abs_diff
• isize::abs_diff
• u8::abs_diff
• u16::abs_diff
• u32::abs_diff
• u64::abs_diff
• u128::abs_diff
• usize::abs_diff
• Display for io::ErrorKind
• From<u8> for ExitCode
• Not for ! (the “never” type)
• OpAssign<$t> for Wrapping<$t>
• arch::is_aarch64_feature_detected!

Cargo

• Port cargo from toml-rs to toml_edit
• Stabilize -Ztimings as --timings
• Stabilize namespaced and weak dependency features.
• Accept more cargo:rustc-link-arg-* types from build script output.
• cargo-new should not add ignore rule on Cargo.lock inside subdirs

Misc

• Ship docs on Tier 2 platforms by reusing the closest Tier 1 platform docs
• Drop rustc-docs from complete profile
• bootstrap: tidy up flag handling for llvm build

Compatibility Notes

• Remove compiler-rt linking hack on Android
• Mitigations for platforms with non-monotonic clocks have been removed from Instant::now. On platforms that don’t provide monotonic clocks, an instant is not guaranteed to be greater than an earlier instant anymore.
• Instant::{duration_since, elapsed, sub} do not panic anymore on underflow, saturating to 0 instead. In the real world the panic happened mostly on platforms with buggy monotonic clock implementations rather than catching programming errors like reversing the start and end times. Such programming errors will now result in 0 rather than a panic.
• In a future release we’re planning to increase the baseline requirements for the Linux kernel to version 3.2, and for glibc to version 2.17. We’d love your feedback in PR #95026.

Internal Changes

These changes provide no direct user facing benefits, but represent significant improvements to the internals and overall performance of rustc and related tools.

• Switch all libraries to the 2021 edition

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

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 a known and relatively frequently occurring bug in incremental compilation, which causes builds to issue internal compiler errors. This particular bug is already fixed on nightly, but that fix has not yet rolled out to stable and is deemed too risky for a direct stable backport.

As always, we encourage users to test with nightly and report bugs so that we can track failures and fix issues earlier.

See 94124 for more details.

Libraries

• Remove unnecessary bounds for some Hash{Map,Set} methods

Stabilized APIs

• std::thread::available_parallelism
• Result::copied
• Result::cloned
• arch::asm!
• arch::global_asm!
• ops::ControlFlow::is_break
• ops::ControlFlow::is_continue
• TryFrom<char> for u8
• char::TryFromCharError implementing Clone, Debug, Display, PartialEq, Copy, Eq, Error
• iter::zip
• NonZeroU8::is_power_of_two
• NonZeroU16::is_power_of_two
• NonZeroU32::is_power_of_two
• NonZeroU64::is_power_of_two
• NonZeroU128::is_power_of_two
• NonZeroUsize::is_power_of_two
• DoubleEndedIterator for ToLowercase
• DoubleEndedIterator for ToUppercase
• TryFrom<&mut [T]> for [T; N]
• UnwindSafe for Once
• RefUnwindSafe for Once
• armv8 neon intrinsics for aarch64

Const-stable:

• mem::MaybeUninit::as_ptr
• mem::MaybeUninit::assume_init
• mem::MaybeUninit::assume_init_ref
• ffi::CStr::from_bytes_with_nul_unchecked

Cargo

• Stabilize the strip profile option
• Stabilize future-incompat-report
• Support abbreviating --release as -r
• Support term.quiet configuration
• Remove --host from cargo {publish,search,login}

Compatibility Notes

• Refactor weak symbols in std::sys::unix This may add new, versioned, symbols when building with a newer glibc, as the standard library uses weak linkage rather than dynamically attempting to load certain symbols at runtime.
• Deprecate crate_type and crate_name nested inside #![cfg_attr] This adds a future compatibility lint to supporting the use of cfg_attr wrapping either crate_type or crate_name specification within Rust files; it is recommended that users migrate to setting the equivalent command line flags.
• Remove effect of #[no_link] attribute on name resolution This may expose new names, leading to conflicts with preexisting names in a given namespace and a compilation failure.
• Cargo will document libraries before binaries.
• Respect doc=false in dependencies, not just the root crate
• Weaken guarantee around advancing underlying iterators in zip
• Make split_inclusive() on an empty slice yield an empty output
• Update std::env::temp_dir to use GetTempPath2 on Windows when available.
• unreachable! was updated to match other formatting macro behavior on Rust 2021

Internal Changes

These changes provide no direct user facing benefits, but represent significant improvements to the internals and overall performance of rustc and related tools.

• Fix many cases of normalization-related ICEs

• Replace dominators algorithm with simple Lengauer-Tarjan

• Store liveness in interval sets for region inference

• Remove in_band_lifetimes from the compiler and standard library, in preparation for removing this unstable feature.

• 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
• Update the minimum external LLVM to 12
• Add x86_64-unknown-none at Tier 3*
• Build musl dist artifacts with debuginfo enabled. When building release binaries using musl, you may want to use the newly stabilized strip option to remove these debug symbols, reducing the size of your binaries.
• Don’t abort compilation after giving a lint error
• Error messages point at the source of trait bound obligations in more places

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

Libraries

• All remaining functions in the standard library have #[must_use] annotations where appropriate, producing a warning when ignoring their return value. This helps catch mistakes such as expecting a function to mutate a value in place rather than return a new value.
• Paths are automatically canonicalized on Windows for operations that support it
• Re-enable debug checks for copy and copy_nonoverlapping
• Implement RefUnwindSafe for Rc<T>
• Make RSplit\<T, P>: Clone not require T: Clone
• Implement Termination for Result<Infallible, E>. This allows writing fn main() -> Result<Infallible, ErrorType>, for a program whose successful exits never involve returning from main (for instance, a program that calls exit, or that uses exec to run another program).

Stabilized APIs

• Metadata::is_symlink
• Path::is_symlink
• {integer}::saturating_div
• Option::unwrap_unchecked
• Result::unwrap_unchecked
• Result::unwrap_err_unchecked
• File::options

These APIs are now usable in const contexts:

• Duration::new
• Duration::checked_add
• Duration::saturating_add
• Duration::checked_sub
• Duration::saturating_sub
• Duration::checked_mul
• Duration::saturating_mul
• Duration::checked_div

Cargo

• Add –message-format for install command
• Warn when alias shadows external subcommand

Rustdoc

• Show all Deref implementations recursively in rustdoc
• Use computed visibility in rustdoc

Compatibility Notes

• Try all stable method candidates first before trying unstable ones. This change ensures that adding new nightly-only methods to the Rust standard library will not break code invoking methods of the same name from traits outside the standard library.
• Windows: std::process::Command will no longer search the current directory for executables.
• All proc-macro backward-compatibility lints are now deny-by-default.
• proc_macro: Append .0 to unsuffixed float if it would otherwise become int token
• Refactor weak symbols in std::sys::unix. This optimizes accesses to glibc functions, by avoiding the use of dlopen. This does not increase the minimum expected version of glibc. However, software distributions that use symbol versions to detect library dependencies, and which take weak symbols into account in that analysis, may detect rust binaries as requiring newer versions of glibc.
• rustdoc now rejects some unexpected semicolons in doctests

Internal Changes

These changes provide no direct user facing benefits, but represent significant improvements to the internals and overall performance of rustc and related tools.

• Implement coherence checks for negative trait impls
• Add rustc lint, warning when iterating over hashmaps
• Optimize live point computation
• Enable verification for 1/32nd of queries loaded from disk
• Implement version of normalize_erasing_regions that allows for normalization failure

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
• Optimize File::read_to_end and read_to_string
• Update to Unicode 14.0
• Many more functions are marked #[must_use], producing a warning when ignoring their return value. This helps catch mistakes such as expecting a function to mutate a value in place rather than return a new value.

Stabilised APIs

• [T; N]::as_mut_slice
• [T; N]::as_slice
• collections::TryReserveError
• HashMap::try_reserve
• HashSet::try_reserve
• String::try_reserve
• String::try_reserve_exact
• Vec::try_reserve
• Vec::try_reserve_exact
• VecDeque::try_reserve
• VecDeque::try_reserve_exact
• Iterator::map_while
• iter::MapWhile
• proc_macro::is_available
• Command::get_program
• Command::get_args
• Command::get_envs
• Command::get_current_dir
• CommandArgs
• CommandEnvs

These APIs are now usable in const contexts:

• hint::unreachable_unchecked

Cargo

• Stabilize custom profiles

Compatibility notes

• Ignore derived Clone and Debug implementations during dead code analysis. This will break some builds that set #![deny(dead_code)].

Internal changes

These changes provide no direct user facing benefits, but represent significant improvements to the internals and overall performance of rustc and related tools.

• Added an experimental backend for codegen with libgccjit.

• 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

• Allow writing of incomplete UTF-8 sequences via stdout/stderr on Windows. The Windows console still requires valid Unicode, but this change allows splitting a UTF-8 character across multiple write calls. This allows, for instance, programs that just read and write data buffers (e.g. copying a file to stdout) without regard for Unicode or character boundaries.
• Prefer AtomicU{64,128} over Mutex for Instant backsliding protection. For this use case, atomics scale much better under contention.
• Implement Extend<(A, B)> for (Extend<A>, Extend<B>)
• impl Default, Copy, Clone for std::io::Sink and std::io::Empty
• impl From<[(K, V); N]> for all collections.
• Remove P: Unpin bound on impl Future for Pin.
• Treat invalid environment variable names as nonexistent. Previously, the environment functions would panic if given a variable name with an internal null character or equal sign (=). Now, these functions will just treat such names as nonexistent variables, since the OS cannot represent the existence of a variable with such a name.

Stabilised APIs

• std::os::unix::fs::chroot
• UnsafeCell::raw_get
• BufWriter::into_parts
• core::panic::{UnwindSafe, RefUnwindSafe, AssertUnwindSafe} These APIs were previously stable in std, but are now also available in core.
• Vec::shrink_to
• String::shrink_to
• OsString::shrink_to
• PathBuf::shrink_to
• BinaryHeap::shrink_to
• VecDeque::shrink_to
• HashMap::shrink_to
• HashSet::shrink_to

These APIs are now usable in const contexts:

• std::mem::transmute
• [T]::first
• [T]::split_first
• [T]::last
• [T]::split_last

Cargo

• Cargo supports specifying a minimum supported Rust version in Cargo.toml. This has no effect at present on dependency version selection. We encourage crates to specify their minimum supported Rust version, and we encourage CI systems that support Rust code to include a crate’s specified minimum version in the test matrix for that crate by default.

Compatibility notes

• Update to new argument parsing rules on Windows. This adjusts Rust’s standard library to match the behavior of the standard libraries for C/C++. The rules have changed slightly over time, and this PR brings us to the latest set of rules (changed in 2008).
• Disallow the aapcs calling convention on aarch64 This was already not supported by LLVM; this change surfaces this lack of support with a better error message.
• Make SEMICOLON_IN_EXPRESSIONS_FROM_MACROS warn by default
• Warn when an escaped newline skips multiple lines.
• Calls to libc::getpid / std::process::id from Command::pre_exec may return different values on glibc \<= 2.24. Rust now invokes the clone3 system call directly, when available, to use new functionality available via that system call. Older versions of glibc cache the result of getpid, and only update that cache when calling glibc’s clone/fork functions, so a direct system call bypasses that cache update. glibc 2.25 and newer no longer cache getpid for exactly this reason.

Internal changes

These changes provide no direct user facing benefits, but represent significant improvements to the internals and overall performance of rustc and related tools.

• LLVM is compiled with PGO in published x86_64-unknown-linux-gnu artifacts. This improves the performance of most Rust builds.
• Unify representation of macros in internal data structures. This change fixes a host of bugs with the handling of macros by the compiler, as well as rustdoc.

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
• IntoInnerError::into_parts
• MaybeUninit::assume_init_mut
• MaybeUninit::assume_init_ref
• MaybeUninit::write
• array::map
• ops::ControlFlow
• x86::_bittest
• x86::_bittestandcomplement
• x86::_bittestandreset
• x86::_bittestandset
• x86_64::_bittest64
• x86_64::_bittestandcomplement64
• x86_64::_bittestandreset64
• x86_64::_bittestandset64

The following previously stable functions are now const.

• str::from_utf8_unchecked

Cargo

• Cargo will now deduplicate compiler diagnostics to the terminal when invoking rustc in parallel such as when using cargo test.
• The package definition in cargo metadata now includes the "default_run" field from the manifest.
• Added cargo d as an alias for cargo doc.
• Added {lib} as formatting option for cargo tree to print the "lib_name" of packages.

Rustdoc

• Added “Go to item on exact match” search option.
• The “Implementors” section on traits no longer shows redundant method definitions.
• Trait implementations are toggled open by default. This should make the implementations more searchable by tools like CTRL+F in your browser.
• Intra-doc links should now correctly resolve associated items (e.g. methods) through type aliases.
• Traits which are marked with #[doc(hidden)] will no longer appear in the “Trait Implementations” section.

Compatibility Notes

• std functions that return an io::Error will no longer use the ErrorKind::Other variant. This is to better reflect that these kinds of errors could be categorised into newer more specific ErrorKind variants, and that they do not represent a user error.
• Using environment variable names with process::Command on Windows now behaves as expected. Previously using environment variables with Command would cause them to be ASCII-uppercased.
• Rustdoc will now warn on using rustdoc lints that aren’t prefixed with rustdoc::
• RUSTFLAGS is no longer set for build scripts. Build scripts should use CARGO_ENCODED_RUSTFLAGS instead. See the documentation for more details.

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.
• Added tier 3* support for bpfel-unknown-none and bpfeb-unknown-none.
• -Zmutable-noalias=yes is enabled by default when using LLVM 12 or above.

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

Libraries

• panic::panic_any will now #[track_caller].
• Added OutOfMemory as a variant of io::ErrorKind.
• proc_macro::Literal now implements FromStr.
• The implementations of vendor intrinsics in core::arch have been significantly refactored. The main user-visible changes are a 50% reduction in the size of libcore.rlib and stricter validation of constant operands passed to intrinsics. The latter is technically a breaking change, but allows Rust to more closely match the C vendor intrinsics API.

Stabilized APIs

• BTreeMap::into_keys
• BTreeMap::into_values
• HashMap::into_keys
• HashMap::into_values
• arch::wasm32
• VecDeque::binary_search
• VecDeque::binary_search_by
• VecDeque::binary_search_by_key
• VecDeque::partition_point

Cargo

• Added the --prune <spec> option to cargo-tree to remove a package from the dependency graph.
• Added the --depth option to cargo-tree to print only to a certain depth in the tree
• Added the no-proc-macro value to cargo-tree --edges to hide procedural macro dependencies.
• A new environment variable named CARGO_TARGET_TMPDIR is available. This variable points to a directory that integration tests and benches can use as a “scratchpad” for testing filesystem operations.

Compatibility Notes

• Mixing Option and Result via ? is no longer permitted in closures for inferred types.
• Previously unsound code is no longer permitted where different constructors in branches could require different lifetimes.
• As previously mentioned the std::arch intrinsics now uses stricter const checking than before and may reject some previously accepted code.
• i128 multiplication on Cortex M0+ platforms currently unconditionally causes overflow when compiled with codegen-units = 1.

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.

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

Libraries

• Abort messages will now forward to android_set_abort_message on Android platforms when available.
• slice::IterMut<'_, T> now implements AsRef<[T]>
• Arrays of any length now implement IntoIterator. Currently calling .into_iter() as a method on an array will return impl Iterator<Item=&T>, but this may change in a future edition to change Item to T. Calling IntoIterator::into_iter directly on arrays will provide impl Iterator<Item=T> as expected.
• leading_zeros, and trailing_zeros are now available on all NonZero integer types.
• {f32, f64}::from_str now parse and print special values (NaN, -0) according to IEEE 754.
• You can now index into slices using (Bound<usize>, Bound<usize>).
• Add the BITS associated constant to all numeric types.

Stabilised APIs

• AtomicBool::fetch_update
• AtomicPtr::fetch_update
• BTreeMap::retain
• BTreeSet::retain
• BufReader::seek_relative
• DebugStruct::non_exhaustive
• Duration::MAX
• Duration::ZERO
• Duration::is_zero
• Duration::saturating_add
• Duration::saturating_mul
• Duration::saturating_sub
• ErrorKind::Unsupported
• Option::insert
• Ordering::is_eq
• Ordering::is_ge
• Ordering::is_gt
• Ordering::is_le
• Ordering::is_lt
• Ordering::is_ne
• OsStr::is_ascii
• OsStr::make_ascii_lowercase
• OsStr::make_ascii_uppercase
• OsStr::to_ascii_lowercase
• OsStr::to_ascii_uppercase
• Peekable::peek_mut
• Rc::decrement_strong_count
• Rc::increment_strong_count
• Vec::extend_from_within
• array::from_mut
• array::from_ref
• cmp::max_by_key
• cmp::max_by
• cmp::min_by_key
• cmp::min_by
• f32::is_subnormal
• f64::is_subnormal

Cargo

• Cargo now supports git repositories where the default HEAD branch is not “master”. This also includes a switch to the version 3 Cargo.lock format which can handle default branches correctly.
• macOS targets now default to unpacked split-debuginfo.
• The authors field is no longer included in Cargo.toml for new projects.

Rustdoc

• Added the rustdoc::bare_urls lint that warns when you have URLs without hyperlinks.

Compatibility Notes

• Implement token-based handling of attributes during expansion
• Ipv4::from_str will now reject octal format IP addresses in addition to rejecting hexadecimal IP addresses. The octal format can lead to confusion and potential security vulnerabilities and is no longer recommended.
• The added BITS constant may conflict with external definitions. In particular, this was known to be a problem in the lexical-core crate, but they have published fixes for semantic versions 0.4 through 0.7. To update this dependency alone, use cargo update -p lexical-core.
• Incremental compilation remains off by default, unless one uses the RUSTC_FORCE_INCREMENTAL=1 environment variable added in 1.52.1.

Internal Only

These changes provide no direct user facing benefits, but represent significant improvements to the internals and overall performance of rustc and related tools.

• Rework the std::sys::windows::alloc implementation.
• rustdoc: Don’t enter an infer_ctxt in get_blanket_impls for impls that aren’t blanket impls.
• rustdoc: Only look at blanket impls in get_blanket_impls
• Rework rustdoc const type

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

• Arguments::as_str
• char::MAX
• char::REPLACEMENT_CHARACTER
• char::UNICODE_VERSION
• char::decode_utf16
• char::from_digit
• char::from_u32_unchecked
• char::from_u32
• slice::partition_point
• str::rsplit_once
• str::split_once

The following previously stable APIs are now const.

• char::len_utf8
• char::len_utf16
• char::to_ascii_uppercase
• char::to_ascii_lowercase
• char::eq_ignore_ascii_case
• u8::to_ascii_uppercase
• u8::to_ascii_lowercase
• u8::eq_ignore_ascii_case

Rustdoc

• Rustdoc lints are now treated as a tool lint, meaning that lints are now prefixed with rustdoc:: (e.g. #[warn(rustdoc::broken_intra_doc_links)]). Using the old style is still allowed, and will become a warning in a future release.
• Rustdoc now supports argument files.
• Rustdoc now generates smart punctuation for documentation.
• You can now use “task lists” in Rustdoc Markdown. E.g.

  - [x] Complete
  - [ ] Todo

Misc

• You can now pass multiple filters to tests. E.g. cargo test -- foo bar will run all tests that match foo and bar.
• Rustup now distributes PDB symbols for the std library on Windows, allowing you to see std symbols when debugging.

Internal Only

These changes provide no direct user facing benefits, but represent significant improvements to the internals and overall performance of rustc and related tools.

• Check the result cache before the DepGraph when ensuring queries
• Try fast_reject::simplify_type in coherence before doing full check
• Only store a LocalDefId in some HIR nodes
• Store HIR attributes in a side table

Compatibility Notes

• Cargo build scripts are now forbidden from setting RUSTC_BOOTSTRAP.
• Removed support for the x86_64-rumprun-netbsd target.
• Deprecated the x86_64-sun-solaris target in favor of x86_64-pc-solaris.
• Rustdoc now only accepts ,, , and \t as delimiters for specifying languages in code blocks.
• Rustc now catches more cases of pub_use_of_private_extern_crate
• Changes in how proc macros handle whitespace may lead to panics when used with older proc-macro-hack versions. A cargo update should be sufficient to fix this in all cases.
• Turn #[derive] into a regular macro attribute

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.

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

Libraries

• Box::downcast is now also implemented for any dyn Any + Send + Sync object.
• str now implements AsMut<str>.
• u64 and u128 now implement From<char>.
• Error is now implemented for &T where T implements Error.
• Poll::{map_ok, map_err} are now implemented for Poll<Option<Result<T, E>>>.
• unsigned_abs is now implemented for all signed integer types.
• io::Empty now implements io::Seek.
• rc::Weak<T> and sync::Weak<T>‘s methods such as as_ptr are now implemented for T: ?Sized types.
• Div and Rem by their NonZero variant is now implemented for all unsigned integers.

Stabilized APIs

• Arc::decrement_strong_count
• Arc::increment_strong_count
• Once::call_once_force
• Peekable::next_if_eq
• Peekable::next_if
• Seek::stream_position
• array::IntoIter
• panic::panic_any
• ptr::addr_of!
• ptr::addr_of_mut!
• slice::fill_with
• slice::split_inclusive_mut
• slice::split_inclusive
• slice::strip_prefix
• slice::strip_suffix
• str::split_inclusive
• sync::OnceState
• task::Wake
• VecDeque::range
• VecDeque::range_mut

Cargo

• Added the split-debuginfo profile option to control the -Csplit-debuginfo codegen option.
• Added the resolver field to Cargo.toml to enable the new feature resolver and CLI option behavior. Version 2 of the feature resolver will try to avoid unifying features of dependencies where that unification could be unwanted. Such as using the same dependency with a std feature in a build scripts and proc-macros, while using the no-std feature in the final binary. See the Cargo book documentation for more information on the feature.

Rustdoc

• Rustdoc will now include documentation for methods available from nested Deref traits.
• You can now provide a --default-theme flag which sets the default theme to use for documentation.

Various improvements to intra-doc links:

• You can link to non-path primitives such as slice.
• You can link to associated items.
• You can now include generic parameters when linking to items, like Vec<T>.

Misc

• You can now pass --include-ignored to tests (e.g. with cargo test -- --include-ignored) to include testing tests marked #[ignore].

Compatibility Notes

• WASI platforms no longer use the wasm-bindgen ABI, and instead use the wasm32 ABI.
• rustc no longer promotes division, modulo and indexing operations to const that could fail.
• The minimum version of glibc for the following platforms has been bumped to version 2.31 for the distributed artifacts.
  • armv5te-unknown-linux-gnueabi
  • sparc64-unknown-linux-gnu
  • thumbv7neon-unknown-linux-gnueabihf
  • armv7-unknown-linux-gnueabi
  • x86_64-unknown-linux-gnux32
• atomic::spin_loop_hint has been deprecated. It’s recommended to use hint::spin_loop instead.

Internal Only

• Consistently avoid constructing optimized MIR when not doing codegen

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

• bool::then
• btree_map::Entry::or_insert_with_key
• f32::clamp
• f64::clamp
• hash_map::Entry::or_insert_with_key
• Ord::clamp
• RefCell::take
• slice::fill
• UnsafeCell::get_mut

The following previously stable methods are now const.

• IpAddr::is_ipv4
• IpAddr::is_ipv6
• IpAddr::is_unspecified
• IpAddr::is_loopback
• IpAddr::is_multicast
• Ipv4Addr::octets
• Ipv4Addr::is_loopback
• Ipv4Addr::is_private
• Ipv4Addr::is_link_local
• Ipv4Addr::is_multicast
• Ipv4Addr::is_broadcast
• Ipv4Addr::is_documentation
• Ipv4Addr::to_ipv6_compatible
• Ipv4Addr::to_ipv6_mapped
• Ipv6Addr::segments
• Ipv6Addr::is_unspecified
• Ipv6Addr::is_loopback
• Ipv6Addr::is_multicast
• Ipv6Addr::to_ipv4
• Layout::size
• Layout::align
• Layout::from_size_align
• pow for all integer types.
• checked_pow for all integer types.
• saturating_pow for all integer types.
• wrapping_pow for all integer types.
• next_power_of_two for all unsigned integer types.
• checked_next_power_of_two for all unsigned integer types.

Cargo

• Added the [build.rustc-workspace-wrapper] option. This option sets a wrapper to execute instead of rustc, for workspace members only.
• cargo:rerun-if-changed will now, if provided a directory, scan the entire contents of that directory for changes.
• Added the --workspace flag to the cargo update command.

Misc

• The search results tab and the help button are focusable with keyboard in rustdoc.
• Running tests will now print the total time taken to execute.

Compatibility Notes

• The compare_and_swap method on atomics has been deprecated. It’s recommended to use the compare_exchange and compare_exchange_weak methods instead.
• Changes in how TokenStreams are checked have fixed some cases where you could write unhygenic macro_rules! macros.
• #![test] as an inner attribute is now considered unstable like other inner macro attributes, and reports an error by default through the soft_unstable lint.
• Overriding a forbid lint at the same level that it was set is now a hard error.
• You can no longer intercept panic! calls by supplying your own macro. It’s recommended to use the #[panic_handler] attribute to provide your own implementation.
• Semi-colons after item statements (e.g. struct Foo {};) now produce a warning.

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

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

Libraries

• RangeInclusive now checks for exhaustion when calling contains and indexing.
• ToString::to_string now no longer shrinks the internal buffer in the default implementation.

Stabilized APIs

• slice::select_nth_unstable
• slice::select_nth_unstable_by
• slice::select_nth_unstable_by_key

The following previously stable methods are now const.

• Poll::is_ready
• Poll::is_pending

Cargo

• Building a crate with cargo-package should now be independently reproducible.
• cargo-tree now marks proc-macro crates.
• Added CARGO_PRIMARY_PACKAGE build-time environment variable. This variable will be set if the crate being built is one the user selected to build, either with -p or through defaults.
• You can now use glob patterns when specifying packages & targets.

Compatibility Notes

• Demoted i686-unknown-freebsd from host tier 2 to target tier 2 support.
• Macros that end with a semi-colon are now treated as statements even if they expand to nothing.
• Rustc will now check for the validity of some built-in attributes on enum variants. Previously such invalid or unused attributes could be ignored.
• Leading whitespace is stripped more uniformly in documentation comments, which may change behavior. You read this post about the changes for more details.
• Trait bounds are no longer inferred for associated types.

Internal Only

These changes provide no direct user facing benefits, but represent significant improvements to the internals and overall performance of rustc and related tools.

• rustc’s internal crates are now compiled using the initial-exec Thread Local Storage model.
• Calculate visibilities once in resolve.
• Added system to the llvm-libunwind bootstrap config option.
• Added --color for configuring terminal color support to bootstrap.

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>.
• The RefCell::{replace, replace_with, clone} methods now all use #[track_caller].

Stabilized APIs

• slice::as_ptr_range
• slice::as_mut_ptr_range
• VecDeque::make_contiguous
• future::pending
• future::ready

The following previously stable methods are now const fn‘s:

• Option::is_some
• Option::is_none
• Option::as_ref
• Result::is_ok
• Result::is_err
• Result::as_ref
• Ordering::reverse
• Ordering::then

Cargo

Rustdoc

• You can now link to items in rustdoc using the intra-doc link syntax. E.g. /// Uses [`std::future`] will automatically generate a link to std::future‘s documentation. See “Linking to items by name” for more information.
• You can now specify #[doc(alias = "<alias>")] on items to add search aliases when searching through rustdoc‘s UI.

Compatibility Notes

• Promotion of references to 'static lifetime inside const fn now follows the same rules as inside a fn body. In particular, &foo() will not be promoted to 'static lifetime any more inside const fns.
• Associated type bindings on trait objects are now verified to meet the bounds declared on the trait when checking that they implement the trait.
• When trait bounds on associated types or opaque types are ambiguous, the compiler no longer makes an arbitrary choice on which bound to use.
• Fixed recursive nonterminals not being expanded in macros during pretty-print/reparse check. This may cause errors if your macro wasn’t correctly handling recursive nonterminal tokens.
• &mut references to non zero-sized types are no longer promoted.
• rustc will now warn if you use attributes like #[link_name] or #[cold] in places where they have no effect.
• Updated _mm256_extract_epi8 and _mm256_extract_epi16 signatures in arch::{x86, x86_64} to return i32 to match the vendor signatures.
• mem::uninitialized will now panic if any inner types inside a struct or enum disallow zero-initialization.
• #[target_feature] will now error if used in a place where it has no effect.
• Foreign exceptions are now caught by catch_unwind and will cause an abort. Note: This behaviour is not guaranteed and is still considered undefined behaviour, see the catch_unwind documentation for further information.

Internal Only

These changes provide no direct user facing benefits, but represent significant improvements to the internals and overall performance of rustc and related tools.

• Building rustc from source now uses ninja by default over make. You can continue building with make by setting ninja=false in your bootstrap.toml.
• cg_llvm: fewer_names in uncached_llvm_type
• Made ensure_sufficient_stack() non-generic

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

• Ident::new_raw
• Range::is_empty
• RangeInclusive::is_empty
• Result::as_deref
• Result::as_deref_mut
• Vec::leak
• pointer::offset_from
• f32::TAU
• f64::TAU

The following previously stable APIs have now been made const.

• The new method for all NonZero integers.
• The checked_add,checked_sub,checked_mul,checked_neg, checked_shl, checked_shr, saturating_add, saturating_sub, and saturating_mul methods for all integers.
• The checked_abs, saturating_abs, saturating_neg, and signum for all signed integers.
• The is_ascii_alphabetic, is_ascii_uppercase, is_ascii_lowercase, is_ascii_alphanumeric, is_ascii_digit, is_ascii_hexdigit, is_ascii_punctuation, is_ascii_graphic, is_ascii_whitespace, and is_ascii_control methods for char and u8.

Cargo

• build-dependencies are now built with opt-level 0 by default. You can override this by setting the following in your Cargo.toml.

  [profile.release.build-override]
  opt-level = 3

• cargo-help will now display man pages for commands rather just the --help text.
• cargo-metadata now emits a test field indicating if a target has tests enabled.
• workspace.default-members now respects workspace.exclude.
• cargo-publish will now use an alternative registry by default if it’s the only registry specified in package.publish.

Misc

• Added a help button beside Rustdoc’s searchbar that explains rustdoc’s type based search.
• Added the Ayu theme to rustdoc.

Compatibility Notes

• Bumped the minimum supported Emscripten version to 1.39.20.
• Fixed a regression parsing {} && false in tail expressions.
• Added changes to how proc-macros are expanded in macro_rules! that should help to preserve more span information. These changes may cause compilation errors if your macro was unhygenic or didn’t correctly handle Delimiter::None.
• Moved support for the CloudABI target to tier 3.
• linux-gnu targets now require minimum kernel 2.6.32 and glibc 2.11.
• Added the rustc-docs component. This allows you to install and read the documentation for the compiler internal APIs. (Currently only available for x86_64-unknown-linux-gnu.)

Internal Only

• Improved default settings for bootstrapping in x.py. You can read details about this change in the “Changes to x.py defaults” post on the Inside Rust blog.

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.
• Removed support for the i686-unknown-dragonfly target.
• Added tier 3 support* for the riscv64gc-unknown-linux-gnu target.
• You can now pass an arguments file passing the @path syntax to rustc. Note that the format differs somewhat from what is found in other tooling; please see the documentation for more information.
• You can now provide --extern flag without a path, indicating that it is available from the search path or specified with an -L flag.

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

Libraries

• The core::panic module is now stable. It was already stable through std.
• NonZero* numerics now implement From<NonZero*> if it’s a smaller integer width. E.g. NonZeroU16 now implements From<NonZeroU8>.
• MaybeUninit<T> now implements fmt::Debug.

Stabilized APIs

• Result::map_or
• Result::map_or_else
• std::rc::Weak::weak_count
• std::rc::Weak::strong_count
• std::sync::Weak::weak_count
• std::sync::Weak::strong_count

Cargo

• Cargo will now document all the private items for binary crates by default.
• cargo-install will now reinstall the package if it detects that it is out of date.
• Cargo.lock now uses a more git friendly format that should help to reduce merge conflicts.
• You can now override specific dependencies’s build settings E.g. [profile.dev.package.image] opt-level = 2 sets the image crate’s optimisation level to 2 for debug builds. You can also use [profile.<profile>.build-override] to override build scripts and their dependencies.

Misc

• You can now specify edition in documentation code blocks to compile the block for that edition. E.g. edition2018 tells rustdoc that the code sample should be compiled the 2018 edition of Rust.
• You can now provide custom themes to rustdoc with --theme, and check the current theme with --check-theme.
• You can use #[cfg(doc)] to compile an item when building documentation.

Compatibility Notes

• As previously announced 1.41 will be the last tier 1 release for 32-bit Apple targets. This means that the source code is still available to build, but the targets are no longer being tested and release binaries for those platforms will no longer be distributed by the Rust project. Please refer to the linked blog post for more information.

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.
• Added tier 3 support* for the mipsel-sony-psp target.
• Added tier 3 support for the thumbv7a-uwp-windows-msvc target.
• Upgraded to LLVM 10.

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

Libraries

• net::{SocketAddr, SocketAddrV4, SocketAddrV6} now implements PartialOrd and Ord.
• proc_macro::TokenStream now implements Default.
• You can now use char with ops::{Range, RangeFrom, RangeFull, RangeInclusive, RangeTo} to iterate over a range of codepoints. E.g. you can now write the following;

  for ch in 'a'..='z' {
      print!("{}", ch);
  }
  println!();
  // Prints "abcdefghijklmnopqrstuvwxyz"

• OsString now implements FromStr.
• The saturating_neg method has been added to all signed integer primitive types, and the saturating_abs method has been added for all integer primitive types.
• Arc<T>, Rc<T> now implement From<Cow<'_, T>>, and Box now implements From<Cow> when T is [T: Copy], str, CStr, OsStr, or Path.
• Box<[T]> now implements From<[T; N]>.
• BitOr and BitOrAssign are implemented for all NonZero integer types.
• The fetch_min, and fetch_max methods have been added to all atomic integer types.
• The fetch_update method has been added to all atomic integer types.

Stabilized APIs

• Arc::as_ptr
• BTreeMap::remove_entry
• Rc::as_ptr
• rc::Weak::as_ptr
• rc::Weak::from_raw
• rc::Weak::into_raw
• str::strip_prefix
• str::strip_suffix
• sync::Weak::as_ptr
• sync::Weak::from_raw
• sync::Weak::into_raw
• char::UNICODE_VERSION
• Span::resolved_at
• Span::located_at
• Span::mixed_site
• unix::process::CommandExt::arg0

Cargo

• Cargo uses the embed-bitcode flag to optimize disk usage and build time.

Misc

• Rustdoc now supports strikethrough text in Markdown. E.g. ~~outdated information~~ becomes “outdated information“.
• Added an emoji to Rustdoc’s deprecated API message.

Compatibility Notes

• Trying to self initialize a static value (that is creating a value using itself) is unsound and now causes a compile error.
• {f32, f64}::powi now returns a slightly different value on Windows. This is due to changes in LLVM’s intrinsics which {f32, f64}::powi uses.
• Rustdoc’s CLI’s extra error exit codes have been removed. These were previously undocumented and not intended for public use. Rustdoc still provides a non-zero exit code on errors.
• Rustc’s lto flag is incompatible with the new embed-bitcode=no. This may cause issues if LTO is enabled through RUSTFLAGS or cargo rustc flags while cargo is adding embed-bitcode itself. The recommended way to control LTO is with Cargo profiles, either in Cargo.toml or .cargo/config, or by setting CARGO_PROFILE_<name>_LTO in the environment.

Internals Only

• Make clippy a git subtree instead of a git submodule
• Unify the undo log of all snapshot types

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.
• convert::Infallible now implements Hash.
• OsString now implements DerefMut and IndexMut returning a &mut OsStr.
• Unicode 13 is now supported.
• String now implements From<&mut str>.
• IoSlice now implements Copy.
• Vec<T> now implements From<[T; N]>. Where N is at most 32.
• proc_macro::LexError now implements fmt::Display and Error.
• from_le_bytes, to_le_bytes, from_be_bytes, to_be_bytes, from_ne_bytes, and to_ne_bytes methods are now const for all integer types.

Stabilized APIs

• PathBuf::with_capacity
• PathBuf::capacity
• PathBuf::clear
• PathBuf::reserve
• PathBuf::reserve_exact
• PathBuf::shrink_to_fit
• f32::to_int_unchecked
• f64::to_int_unchecked
• Layout::align_to
• Layout::pad_to_align
• Layout::array
• Layout::extend

Cargo

• Added the cargo tree command which will print a tree graph of your dependencies. E.g.

    mdbook v0.3.2 (/Users/src/rust/mdbook)
  ├── ammonia v3.0.0
  │   ├── html5ever v0.24.0
  │   │   ├── log v0.4.8
  │   │   │   └── cfg-if v0.1.9
  │   │   ├── mac v0.1.1
  │   │   └── markup5ever v0.9.0
  │   │       ├── log v0.4.8 (*)
  │   │       ├── phf v0.7.24
  │   │       │   └── phf_shared v0.7.24
  │   │       │       ├── siphasher v0.2.3
  │   │       │       └── unicase v1.4.2
  │   │       │           [build-dependencies]
  │   │       │           └── version_check v0.1.5
  ...

  You can also display dependencies on multiple versions of the same crate with cargo tree -d (short for cargo tree --duplicates).

Misc

• Rustdoc now allows you to specify --crate-version to have rustdoc include the version in the sidebar.

Compatibility Notes

• Rustc now correctly generates static libraries on Windows GNU targets with the .a extension, rather than the previous .lib.
• Removed the -C no_integrated_as flag from rustc.
• The file_name property in JSON output of macro errors now points the actual source file rather than the previous format of <NAME macros>. Note: this may not point to a file that actually exists on the user’s system.
• The minimum required external LLVM version has been bumped to LLVM 8.
• mem::{zeroed, uninitialised} will now panic when used with types that do not allow zero initialization such as NonZeroU8. This was previously a warning.
• In 1.45.0 (the next release) converting a f64 to u32 using the as operator has been defined as a saturating operation. 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.

Internal Only

These changes provide no direct user facing benefits, but represent significant improvements to the internals and overall performance of rustc and related tools.

• dep_graph Avoid allocating a set on when the number reads are small.
• Replace big JS dict with JSON parsing.

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
      Self: PartialOrd<Self> // can write `Self` instead of `List<T>`
  {
      Nil,
      Cons(T, Box<Self>) // likewise here
  }

• You can now mark traits with #[must_use]. This provides a warning if a impl Trait or dyn Trait is returned and unused in the program.

Compiler

• The default allocator has changed from jemalloc to the default allocator on your system. The compiler itself on Linux & macOS will still use jemalloc, but programs compiled with it will use the system allocator.
• Added the aarch64-pc-windows-msvc target.

Libraries

• PathBuf now implements FromStr.
• Box<[T]> now implements FromIterator<T>.
• The dbg! macro has been stabilized. This macro enables you to easily debug expressions in your rust program. E.g.

  let a = 2;
  let b = dbg!(a * 2) + 1;
  //      ^-- prints: [src/main.rs:4] a * 2 = 4
  assert_eq!(b, 5);

The following APIs are now const functions and can be used in a const context.

• Cell::as_ptr
• UnsafeCell::get
• char::is_ascii
• iter::empty
• ManuallyDrop::new
• ManuallyDrop::into_inner
• RangeInclusive::start
• RangeInclusive::end
• NonNull::as_ptr
• slice::as_ptr
• str::as_ptr
• Duration::as_secs
• Duration::subsec_millis
• Duration::subsec_micros
• Duration::subsec_nanos
• CStr::as_ptr
• Ipv4Addr::is_unspecified
• Ipv6Addr::new
• Ipv6Addr::octets

Stabilized APIs

• i8::to_be_bytes
• i8::to_le_bytes
• i8::to_ne_bytes
• i8::from_be_bytes
• i8::from_le_bytes
• i8::from_ne_bytes
• i16::to_be_bytes
• i16::to_le_bytes
• i16::to_ne_bytes
• i16::from_be_bytes
• i16::from_le_bytes
• i16::from_ne_bytes
• i32::to_be_bytes
• i32::to_le_bytes
• i32::to_ne_bytes
• i32::from_be_bytes
• i32::from_le_bytes
• i32::from_ne_bytes
• i64::to_be_bytes
• i64::to_le_bytes
• i64::to_ne_bytes
• i64::from_be_bytes
• i64::from_le_bytes
• i64::from_ne_bytes
• i128::to_be_bytes
• i128::to_le_bytes
• i128::to_ne_bytes
• i128::from_be_bytes
• i128::from_le_bytes
• i128::from_ne_bytes
• isize::to_be_bytes
• isize::to_le_bytes
• isize::to_ne_bytes
• isize::from_be_bytes
• isize::from_le_bytes
• isize::from_ne_bytes
• u8::to_be_bytes
• u8::to_le_bytes
• u8::to_ne_bytes
• u8::from_be_bytes
• u8::from_le_bytes
• u8::from_ne_bytes
• u16::to_be_bytes
• u16::to_le_bytes
• u16::to_ne_bytes
• u16::from_be_bytes
• u16::from_le_bytes
• u16::from_ne_bytes
• u32::to_be_bytes
• u32::to_le_bytes
• u32::to_ne_bytes
• u32::from_be_bytes
• u32::from_le_bytes
• u32::from_ne_bytes
• u64::to_be_bytes
• u64::to_le_bytes
• u64::to_ne_bytes
• u64::from_be_bytes
• u64::from_le_bytes
• u64::from_ne_bytes
• u128::to_be_bytes
• u128::to_le_bytes
• u128::to_ne_bytes
• u128::from_be_bytes
• u128::from_le_bytes
• u128::from_ne_bytes
• usize::to_be_bytes
• usize::to_le_bytes
• usize::to_ne_bytes
• usize::from_be_bytes
• usize::from_le_bytes
• usize::from_ne_bytes

Cargo

• You can now run cargo c as an alias for cargo check.
• Usernames are now allowed in alt registry URLs.

Misc

• libproc_macro has been added to the rust-src distribution.

Compatibility Notes

• The argument types for AVX’s _mm256_stream_si256, _mm256_stream_pd, _mm256_stream_ps have been changed from *const to *mut as the previous implementation was unsound.

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.
• Added tier 3 support for the sparc64-unknown-openbsd target.
• rustc will now trim code snippets in diagnostics to fit in your terminal. Note Cargo currently doesn’t use this feature. Refer to cargo#7315 to track this feature’s progress.
• You can now pass --show-output argument to test binaries to print the output of successful tests.

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

Libraries

• Vec::new and String::new are now const functions.
• LinkedList::new is now a const function.
• str::len, [T]::len and str::as_bytes are now const functions.
• The abs, wrapping_abs, and overflowing_abs numeric functions are now const.

Stabilized APIs

• Pin::into_inner
• Instant::checked_duration_since
• Instant::saturating_duration_since

Cargo

• You can now publish git dependencies if supplied with a version.
• The --all flag has been renamed to --workspace. Using --all is now deprecated.

Misc

• You can now pass -Clinker to rustdoc to control the linker used for compiling doctests.

Compatibility Notes

• Code that was previously accepted by the old borrow checker, but rejected by the NLL borrow checker is now a hard error in Rust 2018. This was previously a warning, and will also become a hard error in the Rust 2015 edition in the 1.40.0 release.
• rustdoc now requires rustc to be installed and in the same directory to run tests. This should improve performance when running a large amount of doctests.
• The try! macro will now issue a deprecation warning. It is recommended to use the ? operator instead.
• asinh(-0.0) now correctly returns -0.0. Previously this returned 0.0.

• 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

• You can now pass multiple lint flags to rustc to override the previous flags. For example; rustc -D unused -A unused-variables denies everything in the unused lint group except unused-variables which is explicitly allowed. However, passing rustc -A unused-variables -D unused denies everything in the unused lint group including unused-variables since the allow flag is specified before the deny flag (and therefore overridden).
• rustc will now prefer your system MinGW libraries over its bundled libraries if they are available on windows-gnu.
• rustc now buffers errors/warnings printed in JSON.

Libraries

• Arc<[T; N]>, Box<[T; N]>, and Rc<[T; N]>, now implement TryFrom<Arc<[T]>>,TryFrom<Box<[T]>>, and TryFrom<Rc<[T]>> respectively. Note These conversions are only available when N is 0..=32.
• You can now use associated constants on floats and integers directly, rather than having to import the module. e.g. You can now write u32::MAX or f32::NAN with no imports.
• u8::is_ascii is now const.
• String now implements AsMut<str>.
• Added the primitive module to std and core. This module reexports Rust’s primitive types. This is mainly useful in macros where you want avoid these types being shadowed.
• Relaxed some of the trait bounds on HashMap and HashSet.
• string::FromUtf8Error now implements Clone + Eq.

Stabilized APIs

• Once::is_completed
• f32::LOG10_2
• f32::LOG2_10
• f64::LOG10_2
• f64::LOG2_10
• iter::once_with

Cargo

• You can now set config [profile]s in your .cargo/config, or through your environment.
• Cargo will now set CARGO_BIN_EXE_<name> pointing to a binary’s executable path when running integration tests or benchmarks. <name> is the name of your binary as-is e.g. If you wanted the executable path for a binary named my-programyou would use env!("CARGO_BIN_EXE_my-program").

Misc

• Certain checks in the const_err lint were deemed unrelated to const evaluation, and have been moved to the unconditional_panic and arithmetic_overflow lints.

Compatibility Notes

• Having trailing syntax in the assert! macro is now a hard error. This has been a warning since 1.36.0.
• Fixed Self not having the correctly inferred type. This incorrectly led to some instances being accepted, and now correctly emits a hard error.

Internal Only

These changes provide no direct user facing benefits, but represent significant improvements to the internals and overall performance of rustc and related tools.

• All components are now built with opt-level=3 instead of 2.
• Improved how rustc generates drop code.
• Improved performance from #[inline]-ing certain hot functions.
• traits: preallocate 2 Vecs of known initial size
• Avoid exponential behaviour when relating types
• Skip Drop terminators for enum variants without drop glue
• Improve performance of coherence checks
• Deduplicate types in the generator witness
• Invert control in struct_lint_level.

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.

  const unsafe fn foo() -> i32 { 5 }
  const fn bar() -> i32 {
      unsafe { foo() }
  }

• You can now specify multiple attributes in a cfg_attr attribute. E.g. #[cfg_attr(all(), must_use, optimize)]
• You can now specify a specific alignment with the #[repr(packed)] attribute. E.g. #[repr(packed(2))] struct Foo(i16, i32); is a struct with an alignment of 2 bytes and a size of 6 bytes.
• You can now import an item from a module as an _. This allows you to import a trait’s impls, and not have the name in the namespace. E.g.

  use std::io::Read as _;
  
  // Allowed as there is only one `Read` in the module.
  pub trait Read {}

• You may now use Rc, Arc, and Pin as method receivers.

Compiler

• You can now set a linker flavor for rustc with the -Clinker-flavor command line argument.
• The minimum required LLVM version has been bumped to 6.0.
• Added support for the PowerPC64 architecture on FreeBSD.
• The x86_64-fortanix-unknown-sgx target support has been upgraded to tier 2 support. Visit the platform support page for information on Rust’s platform support.
• Added support for the thumbv7neon-linux-androideabi and thumbv7neon-unknown-linux-gnueabihf targets.
• Added support for the x86_64-unknown-uefi target.

Libraries

• The methods overflowing_{add, sub, mul, shl, shr} are now const functions for all numeric types.
• The methods rotate_left, rotate_right, and wrapping_{add, sub, mul, shl, shr} are now const functions for all numeric types.
• The methods is_positive and is_negative are now const functions for all signed numeric types.
• The get method for all NonZero types is now const.
• The methods count_ones, count_zeros, leading_zeros, trailing_zeros, swap_bytes, from_be, from_le, to_be, to_le are now const for all numeric types.
• Ipv4Addr::new is now a const function

Stabilized APIs

• unix::FileExt::read_exact_at
• unix::FileExt::write_all_at
• Option::transpose
• Result::transpose
• convert::identity
• pin::Pin
• marker::Unpin
• marker::PhantomPinned
• Vec::resize_with
• VecDeque::resize_with
• Duration::as_millis
• Duration::as_micros
• Duration::as_nanos

Cargo

• You can now publish crates that require a feature flag to compile with cargo publish --features or cargo publish --all-features.
• Cargo should now rebuild a crate if a file was modified during the initial build.

Compatibility Notes

• The methods str::{trim_left, trim_right, trim_left_matches, trim_right_matches} are now deprecated in the standard library, and their usage will now produce a warning. Please use the str::{trim_start, trim_end, trim_start_matches, trim_end_matches} methods instead.
• The Error::cause method has been deprecated in favor of Error::source which supports downcasting.
• Libtest no longer creates a new thread for each test when --test-threads=1. It also runs the tests in deterministic order

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.
• Added a lint warning when you have duplicated macro_exports.
• Reduced the number of allocations in the macro parser. This can improve compile times of macro heavy crates on average by 5%.

Libraries

• Implemented Default for &mut str.
• Implemented From<bool> for all integer and unsigned number types.
• Implemented Extend for ().
• The Debug implementation of time::Duration should now be more easily human readable. Previously a Duration of one second would printed as Duration { secs: 1, nanos: 0 } and will now be printed as 1s.
• Implemented From<&String> for Cow<str>, From<&Vec<T>> for Cow<[T]>, From<Cow<CStr>> for CString, From<CString>, From<CStr>, From<&CString> for Cow<CStr>, From<OsString>, From<OsStr>, From<&OsString> for Cow<OsStr>, From<&PathBuf> for Cow<Path>, and From<Cow<Path>> for PathBuf.
• Implemented Shl and Shr for Wrapping<u128> and Wrapping<i128>.
• DirEntry::metadata now uses fstatat instead of lstat when possible. This can provide up to a 40% speed increase.
• Improved error messages when using format!.

Stabilized APIs

• Iterator::step_by
• Path::ancestors
• SystemTime::UNIX_EPOCH
• alloc::GlobalAlloc
• alloc::Layout
• alloc::LayoutErr
• alloc::System
• alloc::alloc
• alloc::alloc_zeroed
• alloc::dealloc
• alloc::realloc
• alloc::handle_alloc_error
• btree_map::Entry::or_default
• fmt::Alignment
• hash_map::Entry::or_default
• iter::repeat_with
• num::NonZeroUsize
• num::NonZeroU128
• num::NonZeroU16
• num::NonZeroU32
• num::NonZeroU64
• num::NonZeroU8
• ops::RangeBounds
• slice::SliceIndex
• slice::from_mut
• slice::from_ref
• {Any + Send + Sync}::downcast_mut
• {Any + Send + Sync}::downcast_ref
• {Any + Send + Sync}::is

Cargo

• Cargo will now no longer allow you to publish crates with build scripts that modify the src directory. The src directory in a crate should be considered to be immutable.

Misc

• The suggestion_applicability field in rustc‘s json output is now stable. This will allow dev tools to check whether a code suggestion would apply to them.

Compatibility Notes

• Rust will consider trait objects with duplicated constraints to be the same type as without the duplicated constraint. For example the below code will now fail to compile.

  trait Trait {}
  
  impl Trait + Send {
      fn test(&self) { println!("one"); } //~ ERROR duplicate definitions with name `test`
  }
  
  impl Trait + Send + Send {
      fn test(&self) { println!("two"); }
  }

• 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.
• The rust-lldb wrapper script should now work again.

Libraries

• mem::MaybeUninit<T> is now ABI-compatible with T.

Stabilized APIs

• BufReader::buffer
• BufWriter::buffer
• Cell::from_mut
• Cell<[T]>::as_slice_of_cells
• DoubleEndedIterator::nth_back
• Option::xor
• Wrapping::reverse_bits
• i128::reverse_bits
• i16::reverse_bits
• i32::reverse_bits
• i64::reverse_bits
• i8::reverse_bits
• isize::reverse_bits
• slice::copy_within
• u128::reverse_bits
• u16::reverse_bits
• u32::reverse_bits
• u64::reverse_bits
• u8::reverse_bits
• usize::reverse_bits

Cargo

• Cargo.lock files are now included by default when publishing executable crates with executables.
• You can now specify default-run="foo" in [package] to specify the default executable to use for cargo run.

Misc

Compatibility Notes

• Using ... for inclusive range patterns will now warn by default. Please transition your code to using the ..= syntax for inclusive ranges instead.
• Using a trait object without the dyn will now warn by default. Please transition your code to use dyn Trait for trait objects instead.

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

• ascii::EscapeDefault now implements Clone and Display.
• Derive macros for prelude traits (e.g. Clone, Debug, Hash) are now available at the same path as the trait. (e.g. The Clone derive macro is available at std::clone::Clone). This also makes all built-in macros available in std/core root. e.g. std::include_bytes!.
• str::Chars now implements Debug.
• slice::{concat, connect, join} now accepts &[T] in addition to &T.
• *const T and *mut T now implement marker::Unpin.
• Arc<[T]> and Rc<[T]> now implement FromIterator<T>.
• Added euclidean remainder and division operations (div_euclid, rem_euclid) to all numeric primitives. Additionally checked, overflowing, and wrapping versions are available for all integer primitives.
• thread::AccessError now implements Clone, Copy, Eq, Error, and PartialEq.
• iter::{StepBy, Peekable, Take} now implement DoubleEndedIterator.

Stabilized APIs

• <*const T>::cast
• <*mut T>::cast
• Duration::as_secs_f32
• Duration::as_secs_f64
• Duration::div_f32
• Duration::div_f64
• Duration::from_secs_f32
• Duration::from_secs_f64
• Duration::mul_f32
• Duration::mul_f64
• any::type_name

Cargo

• Added pipelined compilation support to cargo.
• You can now pass the --features option multiple times to enable multiple features.

Rustdoc

• Documentation on pub use statements is prepended to the documentation of the re-exported item

Misc

• rustc will now warn about some incorrect uses of mem::{uninitialized, zeroed} that are known to cause undefined behaviour.

Compatibility Notes

• The x86_64-unknown-uefi platform can not be built with rustc 1.38.0.
• The armv7-unknown-linux-gnueabihf platform is known to have issues with certain crates such as libc.

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 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.

• 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

• You can now convert num::NonZero* types to their raw equivalents using the From trait. E.g. u8 now implements From<NonZeroU8>.
• You can now convert a &Option<T> into Option<&T> and &mut Option<T> into Option<&mut T> using the From trait.
• You can now multiply (*) a time::Duration by a u32.

Stabilized APIs

• slice::align_to
• slice::align_to_mut
• slice::chunks_exact
• slice::chunks_exact_mut
• slice::rchunks
• slice::rchunks_mut
• slice::rchunks_exact
• slice::rchunks_exact_mut
• Option::replace

Cargo

• Cargo will now download crates in parallel using HTTP/2.
• You can now rename packages in your Cargo.toml We have a guide on how to use the package key in your dependencies.

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.
• The methods overflowing_neg and wrapping_neg are now const functions for all numeric types.
• Indexing a str is now generic over all types that implement SliceIndex<str>.
• str::trim, str::trim_matches, str::trim_{start, end}, and str::trim_{start, end}_matches are now #[must_use] and will produce a warning if their returning type is unused.
• The methods checked_pow, saturating_pow, wrapping_pow, and overflowing_pow are now available for all numeric types. These are equivalent to methods such as wrapping_add for the pow operation.

Stabilized APIs

std & core

• Any::type_id
• Error::type_id
• atomic::AtomicI16
• atomic::AtomicI32
• atomic::AtomicI64
• atomic::AtomicI8
• atomic::AtomicU16
• atomic::AtomicU32
• atomic::AtomicU64
• atomic::AtomicU8
• convert::Infallible
• convert::TryFrom
• convert::TryInto
• iter::from_fn
• iter::successors
• num::NonZeroI128
• num::NonZeroI16
• num::NonZeroI32
• num::NonZeroI64
• num::NonZeroI8
• num::NonZeroIsize
• slice::sort_by_cached_key
• str::escape_debug
• str::escape_default
• str::escape_unicode
• str::split_ascii_whitespace

std

• Instant::checked_add
• Instant::checked_sub
• SystemTime::checked_add
• SystemTime::checked_sub

Cargo

• You can now use alternative registries to crates.io.

Misc

• You can now use the ? operator in your documentation tests without manually adding fn main() -> Result<(), _> {}.

Compatibility Notes

• Command::before_exec is being replaced by the unsafe method Command::pre_exec and will be deprecated with Rust 1.37.0.
• Use of ATOMIC_{BOOL, ISIZE, USIZE}_INIT is now deprecated as you can now use const functions in static variables.

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.
• Removed the Read trait bounds on the BufReader::{get_ref, get_mut, into_inner} methods.
• You can now call the dbg! macro without any parameters to print the file and line where it is called.
• In place ASCII case conversions are now up to 4× faster. e.g. str::make_ascii_lowercase
• hash_map::{OccupiedEntry, VacantEntry} now implement Sync and Send.

Stabilized APIs

• f32::copysign
• f64::copysign
• RefCell::replace_with
• RefCell::map_split
• ptr::hash
• Range::contains
• RangeFrom::contains
• RangeTo::contains
• RangeInclusive::contains
• RangeToInclusive::contains
• Option::copied

Cargo

• You can now set cargo:rustc-cdylib-link-arg at build time to pass custom linker arguments when building a cdylib. Its usage is highly platform specific.

Misc

• The Rust toolchain is now available natively for musl based distros.

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;
• You can now import and reexport macros from other crates with the use syntax. Macros exported with #[macro_export] are now placed into the root module of the crate. If your macro relies on calling other local macros, it is recommended to export with the #[macro_export(local_inner_macros)] attribute so users won’t have to import those macros.
• You can now catch visibility keywords (e.g. pub, pub(crate)) in macros using the vis specifier.
• Non-macro attributes now allow all forms of literals, not just strings. Previously, you would write #[attr("true")], and you can now write #[attr(true)].
• You can now specify a function to handle a panic in the Rust runtime with the #[panic_handler] attribute.

Compiler

• Added the riscv32imc-unknown-none-elf target.
• Added the aarch64-unknown-netbsd target
• Upgraded to LLVM 8.

Libraries

• ManuallyDrop now allows the inner type to be unsized.

Stabilized APIs

• Ipv4Addr::BROADCAST

• Ipv4Addr::LOCALHOST

• Ipv4Addr::UNSPECIFIED

• Ipv6Addr::LOCALHOST

• Ipv6Addr::UNSPECIFIED

• Iterator::find_map

  The following methods are replacement methods for trim_left, trim_right, trim_left_matches, and trim_right_matches, which will be deprecated in 1.33.0:

• str::trim_end_matches

• str::trim_end

• str::trim_start_matches

• str::trim_start

Cargo

• cargo run doesn’t require specifying a package in workspaces.
• cargo doc now supports --message-format=json. This is equivalent to calling rustdoc --error-format=json.
• Cargo will now provide a progress bar for builds.

Misc

• rustdoc allows you to specify what edition to treat your code as with the --edition option.
• rustdoc now has the --color (specify whether to output color) and --error-format (specify error format, e.g. json) options.
• We now distribute a rust-gdbgui script that invokes gdbgui with Rust debug symbols.
• Attributes from Rust tools such as rustfmt or clippy are now available, e.g. #[rustfmt::skip] will skip formatting the next item.

• 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>.
• You can now pass multiple arguments to the dbg! macro. dbg! will return a tuple of each argument when there is multiple arguments.
• Result::{is_err, is_ok} are now #[must_use] and will produce a warning if not used.

Stabilized APIs

• VecDeque::rotate_left
• VecDeque::rotate_right
• Iterator::copied
• io::IoSlice
• io::IoSliceMut
• Read::read_vectored
• Write::write_vectored
• str::as_mut_ptr
• mem::MaybeUninit
• pointer::align_offset
• future::Future
• task::Context
• task::RawWaker
• task::RawWakerVTable
• task::Waker
• task::Poll

Cargo

• Cargo will now produce an error if you attempt to use the name of a required dependency as a feature.
• You can now pass the --offline flag to run cargo without accessing the network.

You can find further change’s in Cargo’s 1.36.0 release notes.

Clippy

There have been numerous additions and fixes to clippy, see Clippy’s 1.36.0 release notes for more details.

Misc

Compatibility Notes

• With the stabilisation of mem::MaybeUninit, mem::uninitialized use is no longer recommended, and will be deprecated in 1.39.0.

• 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.
• Added tier 3 support for the aarch64-unknown-none-softfloat target.
• Added tier 3 support for the mips64-unknown-linux-muslabi64, and mips64el-unknown-linux-muslabi64 targets.

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

Libraries

• The is_power_of_two method on unsigned numeric types is now a const function.

Stabilized APIs

• BTreeMap::get_key_value
• HashMap::get_key_value
• Option::as_deref_mut
• Option::as_deref
• Option::flatten
• UdpSocket::peer_addr
• f32::to_be_bytes
• f32::to_le_bytes
• f32::to_ne_bytes
• f64::to_be_bytes
• f64::to_le_bytes
• f64::to_ne_bytes
• f32::from_be_bytes
• f32::from_le_bytes
• f32::from_ne_bytes
• f64::from_be_bytes
• f64::from_le_bytes
• f64::from_ne_bytes
• mem::take
• slice::repeat
• todo!

Cargo

• Cargo will now always display warnings, rather than only on fresh builds.
• Feature flags (except --all-features) passed to a virtual workspace will now produce an error. Previously these flags were ignored.
• You can now publish dev-dependencies without including a version.

Misc

• You can now specify the #[cfg(doctest)] attribute to include an item only when running documentation tests with rustdoc.

Compatibility Notes

• As previously announced, any previous NLL warnings in the 2015 edition are now hard errors.
• The include! macro will now warn if it failed to include the entire file. The include! macro unintentionally only includes the first expression in a file, and this can be unintuitive. This will become either a hard error in a future release, or the behavior may be fixed to include all expressions as expected.
• Using #[inline] on function prototypes and consts now emits a warning under unused_attribute lint. Using #[inline] anywhere else inside traits or extern blocks now correctly emits a hard error.

• 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.
• cargo-install will now allow you to cross compile an install using --target.
• Added the cargo-fix subcommand to automatically move project code from 2015 edition to 2018.
• cargo doc can now optionally document private types using the --document-private-items flag.

Misc

• rustdoc now has the --cap-lints option which demotes all lints above the specified level to that level. For example --cap-lints warn will demote deny and forbid lints to warn.
• rustc and rustdoc will now have the exit code of 1 if compilation fails and 101 if there is a panic.
• A preview of clippy has been made available through rustup. You can install the preview with rustup component add clippy-preview.

Compatibility Notes

• str::{slice_unchecked, slice_unchecked_mut} are now deprecated. Use str::get_unchecked(begin..end) instead.
• std::env::home_dir is now deprecated for its unintuitive behavior. Consider using the home_dir function from https://crates.io/crates/dirs instead.
• rustc will no longer silently ignore invalid data in target spec.
• cfg attributes and --cfg command line flags are now more strictly validated.