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Make std::mem::needs_drop accept ?Sized #97675

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Jun 17, 2022
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@nvzqz nvzqz commented Jun 3, 2022

This change attempts to make needs_drop work with types like [u8] and str.

This enables code in types like Arc<T> that was not possible before, such as #97676.

@rustbot rustbot added T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Jun 3, 2022
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Some changes occured to rustc_codegen_cranelift

cc @bjorn3

Some changes occured to rustc_codegen_gcc

cc @antoyo

Hey! It looks like you've submitted a new PR for the library teams!

If this PR contains changes to any rust-lang/rust public library APIs then please comment with r? rust-lang/libs-api @rustbot label +T-libs-api -T-libs to request review from a libs-api team reviewer. If you're unsure where your change falls no worries, just leave it as is and the reviewer will take a look and make a decision to forward on if necessary.

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r? @oli-obk

(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 3, 2022
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oli-obk commented Jun 3, 2022

The compiler's implementation already supported this. This is solely a libs-api decision on whether to expose it to users, as the change is insta-stable

r? rust-lang/libs-api

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oli-obk commented Jun 3, 2022

@rustbot label +T-libs-api -T-libs

@rustbot rustbot added T-libs-api Relevant to the library API team, which will review and decide on the PR/issue. and removed T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Jun 3, 2022
@dtolnay dtolnay added needs-fcp This change is insta-stable, so needs a completed FCP to proceed. and removed T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Jun 3, 2022
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dtolnay commented Jun 3, 2022

@rust-lang/libs-api:
@rfcbot fcp merge

I don't foresee this causing problems. needs_drop is defined as being allowed to be implemented conservatively, so in the worst case making it return true for "weird" unsized types would be okay.

/// This is purely an optimization hint, and may be implemented conservatively:
/// it may return `true` for types that don't actually need to be dropped.
/// As such always returning `true` would be a valid implementation of
/// this function. However if this function actually returns `false`, then you
/// can be certain dropping `T` has no side effect.

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Team member @dtolnay 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 3, 2022
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bjorn3 commented Jun 3, 2022

I think this change can be feature gated by introducing a new unstable function with the relaxed bound and merging it back in again once it is stabilized.

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I think this change can be feature gated by introducing a new unstable function with the relaxed bound and merging it back in again once it is stabilized.

I believe this path would be changing it on the intrinsic without changing it on the mem export, then later also changing the mem export after confidence has built up.

@rfcbot rfcbot added final-comment-period In the final comment period and will be merged soon unless new substantive objections are raised. and removed proposed-final-comment-period Proposed to merge/close by relevant subteam, see T-<team> label. Will enter FCP once signed off. labels Jun 6, 2022
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🔔 This is now entering its final comment period, as per the review above. 🔔

@rfcbot rfcbot added finished-final-comment-period The final comment period is finished for this PR / Issue. to-announce Announce this issue on triage meeting and removed final-comment-period In the final comment period and will be merged soon unless new substantive objections are raised. labels Jun 16, 2022
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The final comment period, with a disposition to merge, as per the review above, is now complete.

As the automated representative of the governance process, I would like to thank the author for their work and everyone else who contributed.

This will be merged soon.

@apiraino apiraino removed the to-announce Announce this issue on triage meeting label Jun 16, 2022
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dtolnay commented Jun 16, 2022

@bors r+

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bors commented Jun 16, 2022

📌 Commit 24b1b7c has been approved by dtolnay

@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-review Status: Awaiting review from the assignee but also interested parties. labels Jun 16, 2022
bors added a commit to rust-lang-ci/rust that referenced this pull request Jun 16, 2022
Rollup of 5 pull requests

Successful merges:

 - rust-lang#97377 (Do not suggest adding semicolon/changing delimiters for macros in item position that originates in macros)
 - rust-lang#97675 (Make `std::mem::needs_drop` accept `?Sized`)
 - rust-lang#98118 (Test NLL fix of bad lifetime inference for reference captured in closure.)
 - rust-lang#98166 (Add rustdoc-json regression test for rust-lang#98009)
 - rust-lang#98169 (Keyword docs: Link to wikipedia article for dynamic dispatch)

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup
@bors bors merged commit cf68fd7 into rust-lang:master Jun 17, 2022
@rustbot rustbot added this to the 1.63.0 milestone Jun 17, 2022
@nvzqz nvzqz deleted the unsized-needs-drop branch June 17, 2022 02:33
JohnTitor added a commit to JohnTitor/rust that referenced this pull request Jul 26, 2022
Make `std::mem::needs_drop` accept `?Sized`

This change attempts to make `needs_drop` work with types like `[u8]` and `str`.

This enables code in types like `Arc<T>` that was not possible before, such as rust-lang#97676.
netbsd-srcmastr pushed a commit to NetBSD/pkgsrc that referenced this pull request Oct 10, 2022
Pkgsrc changes:
 * Adjust patches as needed & checksum updates.

Upstream changes:

Version 1.63.0 (2022-08-11)
==========================

Language
--------
- [Remove migrate borrowck mode for pre-NLL errors.][95565]
- [Modify MIR building to drop repeat expressions with length zero.][95953]
- [Remove label/lifetime shadowing warnings.][96296]
- [Allow explicit generic arguments in the presence of `impl Trait` args.]
  [96868]
- [Make `cenum_impl_drop_cast` warnings deny-by-default.][97652]
- [Prevent unwinding when `-C panic=abort` is used regardless of
  declared ABI.][96959]
- [lub: don't bail out due to empty binders.][97867]

Compiler
--------
- [Stabilize the `bundle` native library modifier,][95818] also removing the
  deprecated `static-nobundle` linking kind.
- [Add Apple WatchOS compile targets\*.][95243]
- [Add a Windows application manifest to rustc-main.][96737]

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

Libraries
---------
- [Implement `Copy`, `Clone`, `PartialEq` and `Eq` for
  `core::fmt::Alignment`.][94530]
- [Extend `ptr::null` and `null_mut` to all thin (including extern)
  types.][94954]
- [`impl Read and Write for VecDeque<u8>`.][95632]
- [STD support for the Nintendo 3DS.][95897]
- [Make write/print macros eagerly drop temporaries.][96455]
- [Implement internal traits that enable `[OsStr]::join`.][96881]
- [Implement `Hash` for `core::alloc::Layout`.][97034]
- [Add capacity documentation for `OsString`.][97202]
- [Put a bound on collection misbehavior.][97316]
- [Make `std::mem::needs_drop` accept `?Sized`.][97675]
- [`impl Termination for Infallible` and then make the `Result` impls
  of `Termination` more generic.][97803]
- [Document Rust's stance on `/proc/self/mem`.][97837]

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`][`NonNull::<slice>::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.][cargo/10755]
- [Expose rust-version in the environment as
  `CARGO_PKG_RUST_VERSION`.][cargo/10713]

Compatibility Notes
-------------------

- [`#[link]` attributes are now checked more strictly,][96885]
  which may introduce errors for invalid attribute arguments that
  were previously ignored.

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.][94214]

[94214]: rust-lang/rust#94214
[94530]: rust-lang/rust#94530
[94954]: rust-lang/rust#94954
[95243]: rust-lang/rust#95243
[95565]: rust-lang/rust#95565
[95632]: rust-lang/rust#95632
[95818]: rust-lang/rust#95818
[95897]: rust-lang/rust#95897
[95953]: rust-lang/rust#95953
[96296]: rust-lang/rust#96296
[96455]: rust-lang/rust#96455
[96737]: rust-lang/rust#96737
[96868]: rust-lang/rust#96868
[96881]: rust-lang/rust#96881
[96885]: rust-lang/rust#96885
[96959]: rust-lang/rust#96959
[97034]: rust-lang/rust#97034
[97202]: rust-lang/rust#97202
[97316]: rust-lang/rust#97316
[97652]: rust-lang/rust#97652
[97675]: rust-lang/rust#97675
[97803]: rust-lang/rust#97803
[97837]: rust-lang/rust#97837
[97867]: rust-lang/rust#97867
[cargo/10713]: rust-lang/cargo#10713
[cargo/10755]: rust-lang/cargo#10755

[`array::from_fn`]: https://doc.rust-lang.org/stable/std/array/fn.from_fn.html
[`Box::into_pin`]: https://doc.rust-lang.org/stable/std/boxed/struct.Box.html#method.into_pin
[`BinaryHeap::try_reserve_exact`]: https://doc.rust-lang.org/stable/alloc/collections/binary_heap/struct.BinaryHeap.html#method.try_reserve_exact
[`BinaryHeap::try_reserve`]: https://doc.rust-lang.org/stable/std/collections/struct.BinaryHeap.html#method.try_reserve
[`OsString::try_reserve`]: https://doc.rust-lang.org/stable/std/ffi/struct.OsString.html#method.try_reserve
[`OsString::try_reserve_exact`]: https://doc.rust-lang.org/stable/std/ffi/struct.OsString.html#method.try_reserve_exact
[`PathBuf::try_reserve`]: https://doc.rust-lang.org/stable/std/path/struct.PathBuf.html#method.try_reserve
[`PathBuf::try_reserve_exact`]: https://doc.rust-lang.org/stable/std/path/struct.PathBuf.html#method.try_reserve_exact
[`Path::try_exists`]: https://doc.rust-lang.org/stable/std/path/struct.Path.html#method.try_exists
[`Ref::filter_map`]: https://doc.rust-lang.org/stable/std/cell/struct.Ref.html#method.filter_map
[`RefMut::filter_map`]: https://doc.rust-lang.org/stable/std/cell/struct.RefMut.html#method.filter_map
[`NonNull::<slice>::len`]: https://doc.rust-lang.org/stable/std/ptr/struct.NonNull.html#method.len
[`ToOwned::clone_into`]: https://doc.rust-lang.org/stable/std/borrow/trait.ToOwned.html#method.clone_into
[`Ipv6Addr::to_ipv4_mapped`]: https://doc.rust-lang.org/stable/std/net/struct.Ipv6Addr.html#method.to_ipv4_mapped
[`unix::io::AsFd`]: https://doc.rust-lang.org/stable/std/os/unix/io/trait.AsFd.html
[`unix::io::BorrowedFd<'fd>`]: https://doc.rust-lang.org/stable/std/os/unix/io/struct.BorrowedFd.html
[`unix::io::OwnedFd`]: https://doc.rust-lang.org/stable/std/os/unix/io/struct.OwnedFd.html
[`windows::io::AsHandle`]: https://doc.rust-lang.org/stable/std/os/windows/io/trait.AsHandle.html
[`windows::io::BorrowedHandle<'handle>`]: https://doc.rust-lang.org/stable/std/os/windows/io/struct.BorrowedHandle.html
[`windows::io::OwnedHandle`]: https://doc.rust-lang.org/stable/std/os/windows/io/struct.OwnedHandle.html
[`windows::io::HandleOrInvalid`]: https://doc.rust-lang.org/stable/std/os/windows/io/struct.HandleOrInvalid.html
[`windows::io::HandleOrNull`]: https://doc.rust-lang.org/stable/std/os/windows/io/struct.HandleOrNull.html
[`windows::io::InvalidHandleError`]: https://doc.rust-lang.org/stable/std/os/windows/io/struct.InvalidHandleError.html
[`windows::io::NullHandleError`]: https://doc.rust-lang.org/stable/std/os/windows/io/struct.NullHandleError.html
[`windows::io::AsSocket`]: https://doc.rust-lang.org/stable/std/os/windows/io/trait.AsSocket.html
[`windows::io::BorrowedSocket<'handle>`]: https://doc.rust-lang.org/stable/std/os/windows/io/struct.BorrowedSocket.html
[`windows::io::OwnedSocket`]: https://doc.rust-lang.org/stable/std/os/windows/io/struct.OwnedSocket.html
[`thread::scope`]: https://doc.rust-lang.org/stable/std/thread/fn.scope.html
[`thread::Scope`]: https://doc.rust-lang.org/stable/std/thread/struct.Scope.html
[`thread::ScopedJoinHandle`]: https://doc.rust-lang.org/stable/std/thread/struct.ScopedJoinHandle.html

[`array::from_ref`]: https://doc.rust-lang.org/stable/std/array/fn.from_ref.html
[`slice::from_ref`]: https://doc.rust-lang.org/stable/std/slice/fn.from_ref.html
[`intrinsics::copy`]: https://doc.rust-lang.org/stable/std/intrinsics/fn.copy.html
[`intrinsics::copy_nonoverlapping`]: https://doc.rust-lang.org/stable/std/intrinsics/fn.copy_nonoverlapping.html
[`<*const T>::copy_to`]: https://doc.rust-lang.org/stable/std/primitive.pointer.html#method.copy_to
[`<*const T>::copy_to_nonoverlapping`]: https://doc.rust-lang.org/stable/std/primitive.pointer.html#method.copy_to_nonoverlapping
[`<*mut T>::copy_to`]: https://doc.rust-lang.org/stable/std/primitive.pointer.html#method.copy_to-1
[`<*mut T>::copy_to_nonoverlapping`]: https://doc.rust-lang.org/stable/std/primitive.pointer.html#method.copy_to_nonoverlapping-1
[`<*mut T>::copy_from`]: https://doc.rust-lang.org/stable/std/primitive.pointer.html#method.copy_from
[`<*mut T>::copy_from_nonoverlapping`]: https://doc.rust-lang.org/stable/std/primitive.pointer.html#method.copy_from_nonoverlapping
[`str::from_utf8`]: https://doc.rust-lang.org/stable/std/str/fn.from_utf8.html
[`Utf8Error::error_len`]: https://doc.rust-lang.org/stable/std/str/struct.Utf8Error.html#method.error_len
[`Utf8Error::valid_up_to`]: https://doc.rust-lang.org/stable/std/str/struct.Utf8Error.html#method.valid_up_to
[`Condvar::new`]: https://doc.rust-lang.org/stable/std/sync/struct.Condvar.html#method.new
[`Mutex::new`]: https://doc.rust-lang.org/stable/std/sync/struct.Mutex.html#method.new
[`RwLock::new`]: https://doc.rust-lang.org/stable/std/sync/struct.RwLock.html#method.new
matthiaskrgr pushed a commit to matthiaskrgr/rust that referenced this pull request Mar 7, 2023
Make `std::mem::needs_drop` accept `?Sized`

This change attempts to make `needs_drop` work with types like `[u8]` and `str`.

This enables code in types like `Arc<T>` that was not possible before, such as rust-lang#97676.
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