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allow unions with mutable references and tuples of allowed types #97995

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merged 6 commits into from
Jul 14, 2022

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RalfJung
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@RalfJung RalfJung commented Jun 11, 2022

We currently allow shared references in unions, but not mutable references. That seems somewhat inconsistent. So let's allow all references, and while we are at it, let's make sure the set of allowed types is closed under tuples.

This will need T-lang FCP (at least).

Then remove the tagged_unions feature, since we do not plan to stabilize any more of it.
Closes #55149

@rustbot rustbot added the T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. label Jun 11, 2022
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r? @petrochenkov

(rust-highfive has picked a reviewer for you, use r? to override)

@rust-highfive rust-highfive added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Jun 11, 2022
@RalfJung RalfJung added T-lang Relevant to the language team, which will review and decide on the PR/issue. I-lang-nominated Nominated for discussion during a lang team meeting. and removed T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Jun 11, 2022
) -> bool {
// We don't just accept all !needs_drop fields, due to semver concerns.
match ty.kind() {
ty::TyKind::Ref(..) => true, // references never drop (even mutable refs, which are non-Copy and hence fail the later check)
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I noticed that is_trivially_pure_clone_copy is, for some reason, careful around references and raw pointers:

ty::Ref(_, _, hir::Mutability::Not) | ty::RawPtr(..) => false,

Not sure if that should be a concern here.

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at least mutable references can't be cloned or copied, so it makes sense.

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Hm, the comment on that calls out custom metadata ("Thin pointers & thin shared references are pure-clone-copy, but for anything with custom metadata it might be more complicated."), which I think makes sense for Copy/Clone -- we might eventually support having metadata be !Copy, I guess. But here we really care about !Drop, not !Copy, and I can't ever see &T/&mut T/*const/*mut gaining any kind of drop impl, so I think we're OK here.

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That check is just an optimization though. It could be adjusted once we do have custom metadata.

We already guarantee on stable that all raw ptrs can be duplicated.

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we might eventually support having metadata be !Copy, I guess

Not to my understanding, at least not for simply ?Sized (you would need even more opt-outs).

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Soveu commented Jun 11, 2022

Could we allow also to place unions inside unions? For example this code doesn't compile, because U1 is not Copy.

union U1 {
    a: (),
}

union U2 {
    b: U1,
}

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RalfJung commented Jun 11, 2022 via email

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Soveu commented Jun 11, 2022

Good point, and what about arrays? They are a special case of a tuple ([T; 3] == (T, T, T)).

We could also allow slices, so that we don't print E0658 (!Copy types in unions are unstable),
just E0277 (unsized values not allowed in unions).

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Arrays are a bit tricky because of the size 0 case. So I figured I'd start with what is simple an obvious.

@petrochenkov petrochenkov added S-waiting-on-team Status: Awaiting decision from the relevant subteam (see the T-<team> label). and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jun 12, 2022
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@rfcbot merge

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rfcbot commented Jun 13, 2022

Team member @joshtriplett has proposed to merge this. The next step is review by the rest of the tagged team members:

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

@rfcbot rfcbot added proposed-final-comment-period Proposed to merge/close by relevant subteam, see T-<team> label. Will enter FCP once signed off. disposition-merge This issue / PR is in PFCP or FCP with a disposition to merge it. labels Jun 13, 2022
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Easy +1 on &mut, given that it supports & already.

It's not clear to me that tuples are quite so obvious -- especially if zero-length arrays are non-obvious (#97995 (comment)) then wouldn't zero-length tuples also be non-obvious?

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@rfcbot reviewed

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Hey @RalfJung -- we were discussing in the @rust-lang/lang meeting today and we had a question.

It seems like we could trivially permit [T; N] so long as T is permitted. (We might later want to add a rule to permit [T; 0] in any situation.) Do you foresee potential complications with that, and -- if so -- what are they? (Or is it just annoying to implement?)

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It seems like we could trivially permit [T; N] so long as T is permitted. (We might later want to add a rule to permit [T; 0] in any situation.) Do you foresee potential complications with that, and -- if so -- what are they? (Or is it just annoying to implement?)

My concern was that I am not sure whether [T; 0] is always Copy, so we need to be very careful here to avoid regressing existing code. But it looks like empty arrays are not always Copy, in which case ignoring the length and just recursing to the field type is I think always correct.

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It seems like we could trivially permit [T; N] so long as T is permitted.

I have now implemented this.

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@rfcbot reviewed

@rfcbot rfcbot added the final-comment-period In the final comment period and will be merged soon unless new substantive objections are raised. label Jun 19, 2022
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rfcbot commented Jun 19, 2022

🔔 This is now entering its final comment period, as per the review above. 🔔

@rfcbot rfcbot removed the proposed-final-comment-period Proposed to merge/close by relevant subteam, see T-<team> label. Will enter FCP once signed off. label Jun 19, 2022
@joshtriplett joshtriplett removed the I-lang-nominated Nominated for discussion during a lang team meeting. label Jun 21, 2022
@rfcbot rfcbot added the finished-final-comment-period The final comment period is finished for this PR / Issue. label Jun 30, 2022
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rustbot commented Jul 13, 2022

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

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@bors r=Mark-Simulacrum rollup=never

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bors commented Jul 13, 2022

📌 Commit e43a0de62a6dbe0fab34b59baa0da0453dc29ac3 has been approved by Mark-Simulacrum

It is now in the queue for this repository.

@bors bors added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Jul 13, 2022
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@bors r=Mark-Simulacrum

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bors commented Jul 13, 2022

📌 Commit 07fe988 has been approved by Mark-Simulacrum

It is now in the queue for this repository.

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bors commented Jul 13, 2022

⌛ Testing commit 07fe988 with merge cbb07c2...

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bors commented Jul 14, 2022

☀️ Test successful - checks-actions
Approved by: Mark-Simulacrum
Pushing cbb07c2 to master...

@bors bors added the merged-by-bors This PR was explicitly merged by bors. label Jul 14, 2022
@bors bors merged commit cbb07c2 into rust-lang:master Jul 14, 2022
@rustbot rustbot added this to the 1.64.0 milestone Jul 14, 2022
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📣 Toolstate changed by #97995!

Tested on commit cbb07c2.
Direct link to PR: #97995

💔 miri on windows: test-pass → test-fail (cc @RalfJung @oli-obk).
💔 miri on linux: test-pass → test-fail (cc @RalfJung @oli-obk).

rust-highfive added a commit to rust-lang-nursery/rust-toolstate that referenced this pull request Jul 14, 2022
Tested on commit rust-lang/rust@cbb07c2.
Direct link to PR: <rust-lang/rust#97995>

💔 miri on windows: test-pass → test-fail (cc @RalfJung @oli-obk).
💔 miri on linux: test-pass → test-fail (cc @RalfJung @oli-obk).
@RalfJung RalfJung deleted the union-more-nodrop branch July 14, 2022 02:43
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Finished benchmarking commit (cbb07c2): comparison url.

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

Results
  • Primary benchmarks: no relevant changes found
  • Secondary benchmarks: mixed results
mean1 max count2
Regressions 😿
(primary)
N/A N/A 0
Regressions 😿
(secondary)
9.1% 9.1% 1
Improvements 🎉
(primary)
N/A N/A 0
Improvements 🎉
(secondary)
-3.1% -3.1% 1
All 😿🎉 (primary) N/A N/A 0

Cycles

Results
  • Primary benchmarks: 😿 relevant regression found
  • Secondary benchmarks: 🎉 relevant improvement found
mean1 max count2
Regressions 😿
(primary)
2.0% 2.0% 1
Regressions 😿
(secondary)
N/A N/A 0
Improvements 🎉
(primary)
N/A N/A 0
Improvements 🎉
(secondary)
-3.4% -3.4% 1
All 😿🎉 (primary) 2.0% 2.0% 1

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

@rustbot label: -perf-regression

Footnotes

  1. the arithmetic mean of the percent change 2

  2. number of relevant changes 2

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ehuss commented Jul 16, 2022

@RalfJung Would you mind posting a PR to the reference to update it for this change? IIUC, this expands the list of allowed fields to Copy, ManuallyDrop, &mut, and tuples containing any combination of those? I'm not sure if there is a term to succinctly refer to what those types are, but there are severally places that could use updating I think:

yvt added a commit to r3-os/r3 that referenced this pull request Aug 13, 2022
Unions with `impl Copy` and `ManuallyDrop` fields have been stabilized
by [rust-lang/rust#77547][1] back in 2020, making enabling the feature
already unnecessary for us. A few more field types have been added to
the allowlist later by [rust-lang/rust#97995][2], which also removed the
feature as there was no intent to stabilize more field types.

[1]: rust-lang/rust#77547
[2]: rust-lang/rust#97995
@apiraino apiraino removed the to-announce Announce this issue on triage meeting label Sep 8, 2022
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Thank you

wip-sync pushed a commit to NetBSD/pkgsrc-wip that referenced this pull request Oct 11, 2022
Pkgsrc changes:
 * Add patch to fix vendor/kqueue issue (on 32-bit hosts)
 * Adjust other patches & line numbers
 * Version bumps & checksum changes.

Upstream changes:

Version 1.64.0 (2022-09-22)
===========================

Language
--------
- [Unions with mutable references or tuples of allowed types are
  now allowed](rust-lang/rust#97995)

- It is now considered valid to deallocate memory pointed to by a
  shared reference `&T` [if every byte in `T` is inside an
  `UnsafeCell`](rust-lang/rust#98017)

- Unused tuple struct fields are now warned against in an
  allow-by-default lint, [`unused_tuple_struct_fields`]
  (rust-lang/rust#95977), 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]
  (rust-lang/rust#88991)
  - Refer to Rust's [platform support page][platform-support-doc] for more
    information on Rust's tiered platform support.
- [Only compile `#[used]` as llvm.compiler.used for ELF targets]
  (rust-lang/rust#93718)
- [Add the `--diagnostic-width` compiler flag to define the terminal width.]
  (rust-lang/rust#95635)
- [Add support for link-flavor `rust-lld` for iOS, tvOS and watchOS]
  (rust-lang/rust#98771)

Libraries
---------
- [Remove restrictions on compare-exchange memory ordering.]
  (rust-lang/rust#98383)
- You can now `write!` or `writeln!` into an `OsString`: [Implement
  `fmt::Write` for `OsString`](rust-lang/rust#97915)
- [Make RwLockReadGuard covariant]
  (rust-lang/rust#96820)
- [Implement `FusedIterator` for `std::net::[Into]Incoming`]
  (rust-lang/rust#97300)
- [`impl<T: AsRawFd> AsRawFd for {Arc,Box}<T>`]
  (rust-lang/rust#97437)
- [`ptr::copy` and `ptr::swap` are doing untyped copies]
  (rust-lang/rust#97712)
- [Add cgroupv1 support to `available_parallelism`]
  (rust-lang/rust#97925)
- [Mitigate many incorrect uses of `mem::uninitialized`]
  (rust-lang/rust#99182)

Stabilized APIs
---------------
- [`future::IntoFuture`]
  (https://doc.rust-lang.org/stable/std/future/trait.IntoFuture.html)
- [`future::poll_fn`]
  (https://doc.rust-lang.org/stable/std/future/fn.poll_fn.html)
- [`task::ready!`]
  (https://doc.rust-lang.org/stable/std/task/macro.ready.html)
- [`num::NonZero*::checked_mul`]
  (https://doc.rust-lang.org/stable/std/num/struct.NonZeroUsize.html#method.checked_mul)
- [`num::NonZero*::checked_pow`]
  (https://doc.rust-lang.org/stable/std/num/struct.NonZeroUsize.html#method.checked_pow)
- [`num::NonZero*::saturating_mul`]
  (https://doc.rust-lang.org/stable/std/num/struct.NonZeroUsize.html#method.saturating_mul)
- [`num::NonZero*::saturating_pow`]
  (https://doc.rust-lang.org/stable/std/num/struct.NonZeroUsize.html#method.saturating_pow)
- [`num::NonZeroI*::abs`]
  (https://doc.rust-lang.org/stable/std/num/struct.NonZeroIsize.html#method.abs)
- [`num::NonZeroI*::checked_abs`]
  (https://doc.rust-lang.org/stable/std/num/struct.NonZeroIsize.html#method.checked_abs)
- [`num::NonZeroI*::overflowing_abs`]
  (https://doc.rust-lang.org/stable/std/num/struct.NonZeroIsize.html#method.overflowing_abs)
- [`num::NonZeroI*::saturating_abs`]
  (https://doc.rust-lang.org/stable/std/num/struct.NonZeroIsize.html#method.saturating_abs)
- [`num::NonZeroI*::unsigned_abs`]
  (https://doc.rust-lang.org/stable/std/num/struct.NonZeroIsize.html#method.unsigned_abs)
- [`num::NonZeroI*::wrapping_abs`]
  (https://doc.rust-lang.org/stable/std/num/struct.NonZeroIsize.html#method.wrapping_abs)
- [`num::NonZeroU*::checked_add`]
  (https://doc.rust-lang.org/stable/std/num/struct.NonZeroUsize.html#method.checked_add)
- [`num::NonZeroU*::checked_next_power_of_two`]
  (https://doc.rust-lang.org/stable/std/num/struct.NonZeroUsize.html#method.checked_next_power_of_two)
- [`num::NonZeroU*::saturating_add`]
  (https://doc.rust-lang.org/stable/std/num/struct.NonZeroUsize.html#method.saturating_add)
- [`os::unix::process::CommandExt::process_group`]
  (https://doc.rust-lang.org/stable/std/os/unix/process/trait.CommandExt.html#tymethod.process_group)
- [`os::windows::fs::FileTypeExt::is_symlink_dir`]
  (https://doc.rust-lang.org/stable/std/os/windows/fs/trait.FileTypeExt.html#tymethod.is_symlink_dir)
- [`os::windows::fs::FileTypeExt::is_symlink_file`]
  (https://doc.rust-lang.org/stable/std/os/windows/fs/trait.FileTypeExt.html#tymethod.is_symlink_file)

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

- [`core::ffi::CStr`]
  (https://doc.rust-lang.org/stable/core/ffi/struct.CStr.html)
- [`core::ffi::FromBytesWithNulError`]
  (https://doc.rust-lang.org/stable/core/ffi/struct.FromBytesWithNulError.html)
- [`alloc::ffi::CString`]
  (https://doc.rust-lang.org/stable/alloc/ffi/struct.CString.html)
- [`alloc::ffi::FromVecWithNulError`]
  (https://doc.rust-lang.org/stable/alloc/ffi/struct.FromVecWithNulError.html)
- [`alloc::ffi::IntoStringError`]
  (https://doc.rust-lang.org/stable/alloc/ffi/struct.IntoStringError.html)
- [`alloc::ffi::NulError`]
  (https://doc.rust-lang.org/stable/alloc/ffi/struct.NulError.html)

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

- [`ffi::c_char`]
  (https://doc.rust-lang.org/stable/std/ffi/type.c_char.html)
- [`ffi::c_double`]
  (https://doc.rust-lang.org/stable/std/ffi/type.c_double.html)
- [`ffi::c_float`]
  (https://doc.rust-lang.org/stable/std/ffi/type.c_float.html)
- [`ffi::c_int`]
  (https://doc.rust-lang.org/stable/std/ffi/type.c_int.html)
- [`ffi::c_long`]
  (https://doc.rust-lang.org/stable/std/ffi/type.c_long.html)
- [`ffi::c_longlong`]
  (https://doc.rust-lang.org/stable/std/ffi/type.c_longlong.html)
- [`ffi::c_schar`]
  (https://doc.rust-lang.org/stable/std/ffi/type.c_schar.html)
- [`ffi::c_short`]
  (https://doc.rust-lang.org/stable/std/ffi/type.c_short.html)
- [`ffi::c_uchar`]
  (https://doc.rust-lang.org/stable/std/ffi/type.c_uchar.html)
- [`ffi::c_uint`]
  (https://doc.rust-lang.org/stable/std/ffi/type.c_uint.html)
- [`ffi::c_ulong`]
  (https://doc.rust-lang.org/stable/std/ffi/type.c_ulong.html)
- [`ffi::c_ulonglong`]
  (https://doc.rust-lang.org/stable/std/ffi/type.c_ulonglong.html)
- [`ffi::c_ushort`]
  (https://doc.rust-lang.org/stable/std/ffi/type.c_ushort.html)

These APIs are now usable in const contexts:

- [`slice::from_raw_parts`]
  (https://doc.rust-lang.org/stable/core/slice/fn.from_raw_parts.html)

Cargo
-----
- [Packages can now inherit settings from the workspace so that
  the settings can be centralized in one place.]
  (rust-lang/cargo#10859) See
  [`workspace.package`](https://doc.rust-lang.org/nightly/cargo/reference/workspaces.html#the-workspacepackage-table)
  and
  [`workspace.dependencies`](https://doc.rust-lang.org/nightly/cargo/reference/workspaces.html#the-workspacedependencies-table)
  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]
  (rust-lang/cargo#10766), and the
  [`build.target`](https://doc.rust-lang.org/nightly/cargo/reference/config.html#buildtarget)
  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.]
  (rust-lang/cargo#10844)
- [`cargo add` will now update `Cargo.lock`.]
  (rust-lang/cargo#10902)
- [Added](rust-lang/cargo#10838) the
  [`--crate-type`](https://doc.rust-lang.org/nightly/cargo/commands/cargo-rustc.html#option-cargo-rustc---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.]
  (rust-lang/cargo#10079)

Misc
----
- [The `rust-analyzer` rustup component is now available on the stable channel.]
  (rust-lang/rust#98640)

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](rust-lang/rust#95026)
- [Network primitives are now implemented with the ideal Rust
  layout, not the C system layout]
  (rust-lang/rust#78802). This can
  cause problems when transmuting the types.
- [Add assertion that `transmute_copy`'s `U` is not larger than `T`]
  (rust-lang/rust#98839)
- [A soundness bug in `BTreeMap` was fixed]
  (rust-lang/rust#99413) that allowed data
  it was borrowing to be dropped before the container.
- [The Drop behavior of C-like enums cast to ints has changed]
  (rust-lang/rust#96862). These are already
  discouraged by a compiler warning.
- [Relate late-bound closure lifetimes to parent fn in NLL]
  (rust-lang/rust#98835)
- [Errors at const-eval time are now in future incompatibility reports]
  (rust-lang/rust#97743)
- 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]
  (rust-lang/rust#99155).
- [`impl Trait` was accidentally accepted as the associated type
  value of return-position `impl Trait`]
  (rust-lang/rust#97346), 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]
  (rust-lang/rust#96978)
- [Stop keeping metadata in memory before writing it to disk]
  (rust-lang/rust#96544)
- [compiletest: strip debuginfo by default for mode=ui]
  (rust-lang/rust#98140)
- Many improvements to generated code for derives, including
  performance improvements:
  - [Don't use match-destructuring for derived ops on structs.]
    (rust-lang/rust#98446)
  - [Many small deriving cleanups]
    (rust-lang/rust#98741)
  - [More derive output improvements]
    (rust-lang/rust#98758)
  - [Clarify deriving code](rust-lang/rust#98915)
  - [Final derive output improvements]
    (rust-lang/rust#99046)
  - [Stop injecting `#[allow(unused_qualifications)]` in generated
    `derive` implementations](rust-lang/rust#99485)
  - [Improve `derive(Debug)`](rust-lang/rust#98190)
- [Bump to clap 3](rust-lang/rust#98213)
- [fully move dropck to mir](rust-lang/rust#98641)
- [Optimize `Vec::insert` for the case where `index == len`.]
  (rust-lang/rust#98755)
- [Convert rust-analyzer to an in-tree tool]
  (rust-lang/rust#99603)
netbsd-srcmastr pushed a commit to NetBSD/pkgsrc that referenced this pull request Nov 16, 2022
Pkgsrc changes:
 * This package now contains rust-analyzer, so implicitly
   conflicts with that pkgsrc package.  The same goes for
   the rust-src package.
 * Add NetBSD/arm6 port
 * Add unfinished NetBSD/mipsel port
 * Revert the use of the internal LLVM,
   should now build with the new pkgsrc LLVM (15).
 * Add depndence on compat80 for sparc64 to fix the build
 * Adapt patches
 * Add CHECK_INTERPRETER_SKIP for a few (mostly unused) files.
   (A proper fix may come later.)

Upstream changes:

Version 1.64.0 (2022-09-22)
===========================

Language
--------
- [Unions with mutable references or tuples of allowed types are
  now allowed](rust-lang/rust#97995)

- It is now considered valid to deallocate memory pointed to by a
  shared reference `&T` [if every byte in `T` is inside an
  `UnsafeCell`](rust-lang/rust#98017)

- Unused tuple struct fields are now warned against in an
  allow-by-default lint, [`unused_tuple_struct_fields`]
  (rust-lang/rust#95977), 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]
  (rust-lang/rust#88991)
  - Refer to Rust's [platform support page][platform-support-doc] for more
    information on Rust's tiered platform support.
- [Only compile `#[used]` as llvm.compiler.used for ELF targets]
  (rust-lang/rust#93718)
- [Add the `--diagnostic-width` compiler flag to define the terminal width.]
  (rust-lang/rust#95635)
- [Add support for link-flavor `rust-lld` for iOS, tvOS and watchOS]
  (rust-lang/rust#98771)

Libraries
---------
- [Remove restrictions on compare-exchange memory ordering.]
  (rust-lang/rust#98383)
- You can now `write!` or `writeln!` into an `OsString`: [Implement
  `fmt::Write` for `OsString`](rust-lang/rust#97915)
- [Make RwLockReadGuard covariant]
  (rust-lang/rust#96820)
- [Implement `FusedIterator` for `std::net::[Into]Incoming`]
  (rust-lang/rust#97300)
- [`impl<T: AsRawFd> AsRawFd for {Arc,Box}<T>`]
  (rust-lang/rust#97437)
- [`ptr::copy` and `ptr::swap` are doing untyped copies]
  (rust-lang/rust#97712)
- [Add cgroupv1 support to `available_parallelism`]
  (rust-lang/rust#97925)
- [Mitigate many incorrect uses of `mem::uninitialized`]
  (rust-lang/rust#99182)

Stabilized APIs
---------------
- [`future::IntoFuture`]
  (https://doc.rust-lang.org/stable/std/future/trait.IntoFuture.html)
- [`future::poll_fn`]
  (https://doc.rust-lang.org/stable/std/future/fn.poll_fn.html)
- [`task::ready!`]
  (https://doc.rust-lang.org/stable/std/task/macro.ready.html)
- [`num::NonZero*::checked_mul`]
  (https://doc.rust-lang.org/stable/std/num/struct.NonZeroUsize.html#method.checked_mul)
- [`num::NonZero*::checked_pow`]
  (https://doc.rust-lang.org/stable/std/num/struct.NonZeroUsize.html#method.checked_pow)
- [`num::NonZero*::saturating_mul`]
  (https://doc.rust-lang.org/stable/std/num/struct.NonZeroUsize.html#method.saturating_mul)
- [`num::NonZero*::saturating_pow`]
  (https://doc.rust-lang.org/stable/std/num/struct.NonZeroUsize.html#method.saturating_pow)
- [`num::NonZeroI*::abs`]
  (https://doc.rust-lang.org/stable/std/num/struct.NonZeroIsize.html#method.abs)
- [`num::NonZeroI*::checked_abs`]
  (https://doc.rust-lang.org/stable/std/num/struct.NonZeroIsize.html#method.checked_abs)
- [`num::NonZeroI*::overflowing_abs`]
  (https://doc.rust-lang.org/stable/std/num/struct.NonZeroIsize.html#method.overflowing_abs)
- [`num::NonZeroI*::saturating_abs`]
  (https://doc.rust-lang.org/stable/std/num/struct.NonZeroIsize.html#method.saturating_abs)
- [`num::NonZeroI*::unsigned_abs`]
  (https://doc.rust-lang.org/stable/std/num/struct.NonZeroIsize.html#method.unsigned_abs)
- [`num::NonZeroI*::wrapping_abs`]
  (https://doc.rust-lang.org/stable/std/num/struct.NonZeroIsize.html#method.wrapping_abs)
- [`num::NonZeroU*::checked_add`]
  (https://doc.rust-lang.org/stable/std/num/struct.NonZeroUsize.html#method.checked_add)
- [`num::NonZeroU*::checked_next_power_of_two`]
  (https://doc.rust-lang.org/stable/std/num/struct.NonZeroUsize.html#method.checked_next_power_of_two)
- [`num::NonZeroU*::saturating_add`]
  (https://doc.rust-lang.org/stable/std/num/struct.NonZeroUsize.html#method.saturating_add)
- [`os::unix::process::CommandExt::process_group`]
  (https://doc.rust-lang.org/stable/std/os/unix/process/trait.CommandExt.html#tymethod.process_group)
- [`os::windows::fs::FileTypeExt::is_symlink_dir`]
  (https://doc.rust-lang.org/stable/std/os/windows/fs/trait.FileTypeExt.html#tymethod.is_symlink_dir)
- [`os::windows::fs::FileTypeExt::is_symlink_file`]
  (https://doc.rust-lang.org/stable/std/os/windows/fs/trait.FileTypeExt.html#tymethod.is_symlink_file)

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

- [`core::ffi::CStr`]
  (https://doc.rust-lang.org/stable/core/ffi/struct.CStr.html)
- [`core::ffi::FromBytesWithNulError`]
  (https://doc.rust-lang.org/stable/core/ffi/struct.FromBytesWithNulError.html)
- [`alloc::ffi::CString`]
  (https://doc.rust-lang.org/stable/alloc/ffi/struct.CString.html)
- [`alloc::ffi::FromVecWithNulError`]
  (https://doc.rust-lang.org/stable/alloc/ffi/struct.FromVecWithNulError.html)
- [`alloc::ffi::IntoStringError`]
  (https://doc.rust-lang.org/stable/alloc/ffi/struct.IntoStringError.html)
- [`alloc::ffi::NulError`]
  (https://doc.rust-lang.org/stable/alloc/ffi/struct.NulError.html)

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

- [`ffi::c_char`]
  (https://doc.rust-lang.org/stable/std/ffi/type.c_char.html)
- [`ffi::c_double`]
  (https://doc.rust-lang.org/stable/std/ffi/type.c_double.html)
- [`ffi::c_float`]
  (https://doc.rust-lang.org/stable/std/ffi/type.c_float.html)
- [`ffi::c_int`]
  (https://doc.rust-lang.org/stable/std/ffi/type.c_int.html)
- [`ffi::c_long`]
  (https://doc.rust-lang.org/stable/std/ffi/type.c_long.html)
- [`ffi::c_longlong`]
  (https://doc.rust-lang.org/stable/std/ffi/type.c_longlong.html)
- [`ffi::c_schar`]
  (https://doc.rust-lang.org/stable/std/ffi/type.c_schar.html)
- [`ffi::c_short`]
  (https://doc.rust-lang.org/stable/std/ffi/type.c_short.html)
- [`ffi::c_uchar`]
  (https://doc.rust-lang.org/stable/std/ffi/type.c_uchar.html)
- [`ffi::c_uint`]
  (https://doc.rust-lang.org/stable/std/ffi/type.c_uint.html)
- [`ffi::c_ulong`]
  (https://doc.rust-lang.org/stable/std/ffi/type.c_ulong.html)
- [`ffi::c_ulonglong`]
  (https://doc.rust-lang.org/stable/std/ffi/type.c_ulonglong.html)
- [`ffi::c_ushort`]
  (https://doc.rust-lang.org/stable/std/ffi/type.c_ushort.html)

These APIs are now usable in const contexts:

- [`slice::from_raw_parts`]
  (https://doc.rust-lang.org/stable/core/slice/fn.from_raw_parts.html)

Cargo
-----
- [Packages can now inherit settings from the workspace so that
  the settings can be centralized in one place.]
  (rust-lang/cargo#10859) See
  [`workspace.package`](https://doc.rust-lang.org/nightly/cargo/reference/workspaces.html#the-workspacepackage-table)
  and
  [`workspace.dependencies`](https://doc.rust-lang.org/nightly/cargo/reference/workspaces.html#the-workspacedependencies-table)
  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]
  (rust-lang/cargo#10766), and the
  [`build.target`](https://doc.rust-lang.org/nightly/cargo/reference/config.html#buildtarget)
  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.]
  (rust-lang/cargo#10844)
- [`cargo add` will now update `Cargo.lock`.]
  (rust-lang/cargo#10902)
- [Added](rust-lang/cargo#10838) the
  [`--crate-type`](https://doc.rust-lang.org/nightly/cargo/commands/cargo-rustc.html#option-cargo-rustc---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.]
  (rust-lang/cargo#10079)

Misc
----
- [The `rust-analyzer` rustup component is now available on the stable channel.]
  (rust-lang/rust#98640)

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](rust-lang/rust#95026)
- [Network primitives are now implemented with the ideal Rust
  layout, not the C system layout]
  (rust-lang/rust#78802). This can
  cause problems when transmuting the types.
- [Add assertion that `transmute_copy`'s `U` is not larger than `T`]
  (rust-lang/rust#98839)
- [A soundness bug in `BTreeMap` was fixed]
  (rust-lang/rust#99413) that allowed data
  it was borrowing to be dropped before the container.
- [The Drop behavior of C-like enums cast to ints has changed]
  (rust-lang/rust#96862). These are already
  discouraged by a compiler warning.
- [Relate late-bound closure lifetimes to parent fn in NLL]
  (rust-lang/rust#98835)
- [Errors at const-eval time are now in future incompatibility reports]
  (rust-lang/rust#97743)
- 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]
  (rust-lang/rust#99155).
- [`impl Trait` was accidentally accepted as the associated type
  value of return-position `impl Trait`]
  (rust-lang/rust#97346), 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]
  (rust-lang/rust#96978)
- [Stop keeping metadata in memory before writing it to disk]
  (rust-lang/rust#96544)
- [compiletest: strip debuginfo by default for mode=ui]
  (rust-lang/rust#98140)
- Many improvements to generated code for derives, including
  performance improvements:
  - [Don't use match-destructuring for derived ops on structs.]
    (rust-lang/rust#98446)
  - [Many small deriving cleanups]
    (rust-lang/rust#98741)
  - [More derive output improvements]
    (rust-lang/rust#98758)
  - [Clarify deriving code](rust-lang/rust#98915)
  - [Final derive output improvements]
    (rust-lang/rust#99046)
  - [Stop injecting `#[allow(unused_qualifications)]` in generated
    `derive` implementations](rust-lang/rust#99485)
  - [Improve `derive(Debug)`](rust-lang/rust#98190)
- [Bump to clap 3](rust-lang/rust#98213)
- [fully move dropck to mir](rust-lang/rust#98641)
- [Optimize `Vec::insert` for the case where `index == len`.]
  (rust-lang/rust#98755)
- [Convert rust-analyzer to an in-tree tool]
  (rust-lang/rust#99603)
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Tracking issue for RFC 2514, "Union initialization and Drop"