Rust 1.56.0

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.

Rust 1.55.0

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.

Rust 1.54.0

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.

Rust 1.53.0

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

Rust 1.52.1

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.

Rust 1.52.0

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

Rust 1.51.0

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

Rust 1.50.0

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.

Rust 1.49.0

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.

Rust 1.48.0

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 config.toml.
• cg_llvm: fewer_names in uncached_llvm_type
• Made ensure_sufficient_stack() non-generic