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Add 1.56.0 release notes #88893

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185 changes: 185 additions & 0 deletions RELEASES.md
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Rust 1.56.0 (2021-10-21)
========================

Language
--------

- [The 2021 Edition is now stable.][rust#88100]
See [the edition guide][rust-2021-edition-guide] for more details.
- [You can now specify explicit discriminant values on any Rust enum.][rust#86860]
- [The pattern in `binding @ pattern` can now also introduce new bindings.][rust#85305]
- [Union field access is permitted in `const fn`.][rust#85769]

[rust-2021-edition-guide]: https://doc.rust-lang.org/nightly/edition-guide/rust-2021/index.html

Compiler
--------

- [Upgrade to LLVM 13.][rust#87570]
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- [Support memory, address, and thread sanitizers on aarch64-unknown-freebsd.][rust#88023]
- [Allow specifying an deployment target version for all iOS targets][rust#87699]
- [Warnings can be forced on with `--force-warn`.][rust#87472]
This feature is primarily intended for usage by `cargo fix`, rather than end users.
- [Promote `aarch64-apple-ios-sim` to Tier 2\*.][rust#87760]
- [Add `powerpc-unknown-freebsd` at Tier 3\*.][rust#87370]

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

Libraries
---------

- [Allow writing of incomplete UTF-8 sequences via stdout/stderr on Windows.][rust#83342]
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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.][rust#83093]
For this use case, atomics scale much better under contention.
- [Implement `Extend<(A, B)>` for `(Extend<A>, Extend<B>)`][rust#85835]
- [impl Default, Copy, Clone for std::io::Sink and std::io::Empty][rust#86744]
- [`impl From<[(K, V); N]>` for all collections.][rust#84111]
- [Remove `P: Unpin` bound on impl Future for Pin.][rust#81363]
- [Treat invalid environment variable names as non-existent.][rust#86183]
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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 non-existent variables, since the OS cannot represent
the existence of a variable with such a name.

Stabilised APIs
---------------

- [`std::os::unix::fs::chroot`]
- [`Iterator::intersperse`]
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- [`Iterator::intersperse_with`]
- [`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`.
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- [`Vec::shrink_to`]
- [`String::shrink_to`]
- [`OsString::shrink_to`]
- [`PathBuf::shrink_to`]
- [`BinaryHeap::shrink_to`]
- [`VecDeque::shrink_to`]
- [`HashMap::shrink_to`]
- [`HashSet::shrink_to`]
- [`task::ready!`]

These APIs are now usable in const contexts:

- [`std::mem::transmute`]
- [`[T]::first`][`slice::first`]
- [`[T]::split_first`][`slice::split_first`]
- [`[T]::last`][`slice::last`]
- [`[T]::split_last`][`slice::split_last`]

Cargo
-----

- [Cargo supports specifying a minimum supported Rust version in Cargo.toml.][`rust-version`]
This has no effect at present on dependency version selection.
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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 text matrix for that
crate by default.

Compatibility notes
-------------------

- [Update to new argument parsing rules on Windows.][rust#87580]
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][rust#88399]
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][rust#87385]
- [Warn when an escaped newline skips multiple lines.][rust#87671]
- [Calls to `libc::getpid` / `std::process::id` from `Command::pre_exec`
may return different values on glibc <= 2.24.][rust#81825]
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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.][rust#88069]
This improves the performance of most Rust builds.
- [Unify representation of macros in internal data structures.][rust#88019]
This change fixes a host of bugs with the handling of macros by the compiler,
as well as rustdoc.

[`std::os::unix::fs::chroot`]: https://doc.rust-lang.org/stable/std/os/unix/fs/fn.chroot.html
[`Iterator::intersperse`]: https://doc.rust-lang.org/stable/std/iter/trait.Iterator.html#method.intersperse
[`Iterator::intersperse_with`]: https://doc.rust-lang.org/stable/std/iter/trait.Iterator.html#method.intersperse
[`UnsafeCell::raw_get`]: https://doc.rust-lang.org/stable/std/cell/struct.UnsafeCell.html#method.raw_get
[`BufWriter::into_parts`]: https://doc.rust-lang.org/stable/std/io/struct.BufWriter.html#method.into_parts
[`core::panic::{UnwindSafe, RefUnwindSafe, AssertUnwindSafe}`]: https://github.com/rust-lang/rust/pull/84662
[`Vec::shrink_to`]: https://doc.rust-lang.org/stable/std/vec/struct.Vec.html#method.shrink_to
[`String::shrink_to`]: https://doc.rust-lang.org/stable/std/string/struct.String.html#method.shrink_to
[`OsString::shrink_to`]: https://doc.rust-lang.org/stable/std/ffi/struct.OsString.html#method.shrink_to
[`PathBuf::shrink_to`]: https://doc.rust-lang.org/stable/std/path/struct.PathBuf.html#method.shrink_to
[`BinaryHeap::shrink_to`]: https://doc.rust-lang.org/stable/std/collections/struct.BinaryHeap.html#method.shrink_to
[`VecDeque::shrink_to`]: https://doc.rust-lang.org/stable/std/collections/struct.VecDeque.html#method.shrink_to
[`HashMap::shrink_to`]: https://doc.rust-lang.org/stable/std/collections/hash_map/struct.HashMap.html#method.shrink_to
[`HashSet::shrink_to`]: https://doc.rust-lang.org/stable/std/collections/hash_set/struct.HashSet.html#method.shrink_to
[`task::ready!`]: https://doc.rust-lang.org/stable/std/task/macro.ready.html
[`std::mem::transmute`]: https://doc.rust-lang.org/stable/std/mem/fn.transmute.html
[`slice::first`]: https://doc.rust-lang.org/stable/std/primitive.slice.html#method.first
[`slice::split_first`]: https://doc.rust-lang.org/stable/std/primitive.slice.html#method.split_first
[`slice::last`]: https://doc.rust-lang.org/stable/std/primitive.slice.html#method.last
[`slice::split_last`]: https://doc.rust-lang.org/stable/std/primitive.slice.html#method.split_last
[`rust-version`]: https://doc.rust-lang.org/nightly/cargo/reference/manifest.html#the-rust-version-field
[rust#87671]: https://github.com/rust-lang/rust/pull/87671
[rust#86183]: https://github.com/rust-lang/rust/pull/86183
[rust#87385]: https://github.com/rust-lang/rust/pull/87385
[rust#88100]: https://github.com/rust-lang/rust/pull/88100
[rust#86860]: https://github.com/rust-lang/rust/pull/86860
[rust#84039]: https://github.com/rust-lang/rust/pull/84039
[rust#86492]: https://github.com/rust-lang/rust/pull/86492
[rust#88363]: https://github.com/rust-lang/rust/pull/88363
[rust#85305]: https://github.com/rust-lang/rust/pull/85305
[rust#87832]: https://github.com/rust-lang/rust/pull/87832
[rust#88069]: https://github.com/rust-lang/rust/pull/88069
[rust#87472]: https://github.com/rust-lang/rust/pull/87472
[rust#87699]: https://github.com/rust-lang/rust/pull/87699
[rust#87570]: https://github.com/rust-lang/rust/pull/87570
[rust#88023]: https://github.com/rust-lang/rust/pull/88023
[rust#87760]: https://github.com/rust-lang/rust/pull/87760
[rust#87370]: https://github.com/rust-lang/rust/pull/87370
[rust#87580]: https://github.com/rust-lang/rust/pull/87580
[rust#83342]: https://github.com/rust-lang/rust/pull/83342
[rust#83093]: https://github.com/rust-lang/rust/pull/83093
[rust#88177]: https://github.com/rust-lang/rust/pull/88177
[rust#88548]: https://github.com/rust-lang/rust/pull/88548
[rust#88551]: https://github.com/rust-lang/rust/pull/88551
[rust#88299]: https://github.com/rust-lang/rust/pull/88299
[rust#88220]: https://github.com/rust-lang/rust/pull/88220
[rust#85835]: https://github.com/rust-lang/rust/pull/85835
[rust#86879]: https://github.com/rust-lang/rust/pull/86879
[rust#86744]: https://github.com/rust-lang/rust/pull/86744
[rust#84662]: https://github.com/rust-lang/rust/pull/84662
[rust#86593]: https://github.com/rust-lang/rust/pull/86593
[rust#81050]: https://github.com/rust-lang/rust/pull/81050
[rust#81363]: https://github.com/rust-lang/rust/pull/81363
[rust#84111]: https://github.com/rust-lang/rust/pull/84111
[rust#85769]: https://github.com/rust-lang/rust/pull/85769#issuecomment-854363720
[rust#88490]: https://github.com/rust-lang/rust/pull/88490
[rust#88269]: https://github.com/rust-lang/rust/pull/88269
[rust#84176]: https://github.com/rust-lang/rust/pull/84176
[rust#88399]: https://github.com/rust-lang/rust/pull/88399
[rust#88227]: https://github.com/rust-lang/rust/pull/88227
[rust#88200]: https://github.com/rust-lang/rust/pull/88200
[rust#82776]: https://github.com/rust-lang/rust/pull/82776
[rust#88077]: https://github.com/rust-lang/rust/pull/88077
[rust#87728]: https://github.com/rust-lang/rust/pull/87728
[rust#87050]: https://github.com/rust-lang/rust/pull/87050
[rust#87619]: https://github.com/rust-lang/rust/pull/87619
[rust#81825]: https://github.com/rust-lang/rust/pull/81825#issuecomment-808406918
[rust#88019]: https://github.com/rust-lang/rust/pull/88019

Version 1.55.0 (2021-09-09)
============================

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