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Tracking Issue for const_ptr_read #80377

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Tracking Issue for const_ptr_read#80377
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A-const-evalArea: Constant evaluation, covers all const contexts (static, const fn, ...)C-tracking-issueCategory: An issue tracking the progress of sth. like the implementation of an RFCI-libs-radarLibs issues that are tracked on the team's radar.T-libs-api[DEPRECATED; DO NOT USE]disposition-mergeThis issue / PR is in PFCP or FCP with a disposition to merge it.finished-final-comment-periodThe final comment period is finished for this PR / Issue.

Description

@usbalbin
usbalbin
opened on Dec 26, 2020

Feature gate: #![feature(const_ptr_read)]

This is a tracking issue for making the functions ptr::read and ptr::read_unaligned, and the same methods on *const T and *mut T, const fn. This unlocks things like moving values out of arrays in const context.

Public API

mod ptr {
    pub const unsafe fn read<T>(src: *const T) -> T;
    pub const unsafe fn read_unaligned<T>(src: *const T) -> T;
}

impl<T> *const T {
    pub const unsafe fn read(self) -> T;
    pub const unsafe fn read_unaligned(self) -> T;
}

impl<T> *mut T {
    pub const unsafe fn read(self) -> T;
    pub const unsafe fn read_unaligned(self) -> T;
}

Steps / History

Related

Unresolved Questions

Inorder to make intrinsics::copy and intrinsics::copy_nonoverlapping compile as const fn, some checks were removed.
See comment for some more info

For this PR, I see two options:

  • Leave it as "something we can do once we have a story for const-dependent dispatch".
  • Comment out the debug assertions for now. Their usefulness is anyway limited since the libstd everyone uses is compiled without debug assertions.

I guess the question is one of evaluating the relative usefulness of these new const operations vs the assertions.

(#79684 did the Comment out the debug assertions for now.-thing).

So the question is, how do we bring them back?

Activity

  1. usbalbin
    added
    C-tracking-issueCategory: An issue tracking the progress of sth. like the implementation of an RFC
    T-libs-api[DEPRECATED; DO NOT USE]
    on Dec 26, 2020
  2. usbalbin
    changed the title [-]Tracking Issue for XXX[/-] [+]Tracking Issue for const_ptr_read[/+] on Dec 26, 2020
  3. usbalbin
    added a commit that references this issue on Dec 26, 2020

    Added reference to tracking issue rust-lang#80377

    0cea1c9

  4. mbartlett21 commented on Dec 30, 2020

    @mbartlett21
    mbartlett21
    on Dec 30, 2020
    Contributor

    Should this be mentioned in #57563?

  5. usbalbin commented on Jan 4, 2021

    @usbalbin
    usbalbin
    on Jan 4, 2021
    ContributorAuthor

    Perhaps #80697 is a better place to discuss the Comment out the debug assertions for now-question? Since #80697 is specifically for copy[_nonoverlapping]

  6. m-ou-se
    added
    I-libs-radarLibs issues that are tracked on the team's radar.
    on Jan 6, 2021

  7. c410-f3r commented on Jan 10, 2021

    @c410-f3r
    c410-f3r
    on Jan 10, 2021
    Contributor

    Are write variants also on the track for "constification"?

    I guess it can be checked now

  8. RalfJung commented on Jan 18, 2021

    @RalfJung
    RalfJung
    on Jan 18, 2021
    Member

    Are write variants also on the track for "constification"?

    Let's say they are on the wish list. ;) With #80290, making them const should actually be pretty easy now.

  9. usbalbin commented on Jan 18, 2021

    @usbalbin
    usbalbin
    on Jan 18, 2021
    ContributorAuthor

    Do we need to wait for #80418 (for allowing borrowing) to hit beta before write can be implemented without multiple implementations behind #[cfg(bootstrap)] or is there any other (not too ugly) way? :)

    Also intrinsics::forget seems to be not const while mem::forget is const but internally calls ManuallyDrop::new. So how would one best solve that without undoing the "inlining" in #80290?

  10. RalfJung commented on Jan 18, 2021

    @RalfJung
    RalfJung
    on Jan 18, 2021
    Member

    Do we need to wait for #80418 (for allowing borrowing) to hit beta before write can be implemented without multiple implementations behind #[cfg(bootstrap)] or is there any other (not too ugly) way? :)

    Well... you could use &mut. 😆
    Probably it's better to wait, though.

    mem::forget is const but internally calls ManuallyDrop::new

    This is supposed to be changed in #79989.

  11. camelid
    added
    A-const-fn
    on Jan 18, 2021

  12. c410-f3r commented on Jan 19, 2021

    @c410-f3r
    c410-f3r
    on Jan 19, 2021
    Contributor

    Thank you, @RalfJung and @usbalbin !

  13. slightlyoutofphase commented on Mar 13, 2021

    @slightlyoutofphase
    slightlyoutofphase
    on Mar 13, 2021
    Contributor

    Just popping in to say that I just found out that this and const_ptr_write exist and it's like Christmas TBH. The next version I release of my crate staticvec is going to be absolutely nuts thanks to these, with const push, const pop, const insert, and various other things.

    One thing I'd ask though is if there are plans to make some_ptr.copy_to(), some_ptr.copy_from() and so on also const-compatible? It seems like there's nothing blocking it, as all of those functions just directly call these intrinsic ones IIRC, and it would be a nice addition on top of this from an ergonomics standpoint.

  14. RalfJung commented on Mar 13, 2021

    @RalfJung
    RalfJung
    on Mar 13, 2021
    Member

    One thing I'd ask though is if there are plans to make some_ptr.copy_to(), some_ptr.copy_from() and so on also const-compatible?

    Yeah, those could all easily be made const fn as well, I think.

  15. 11 remaining items

  16. rfcbot
    added
    proposed-final-comment-periodProposed to merge/close by relevant subteam, see T-<team> label. Will enter FCP once signed off.
    disposition-mergeThis issue / PR is in PFCP or FCP with a disposition to merge it.
    final-comment-periodIn the final comment period and will be merged soon unless new substantive objections are raised.
    and removed
    proposed-final-comment-periodProposed to merge/close by relevant subteam, see T-<team> label. Will enter FCP once signed off.
    on Apr 5, 2023

  17. rfcbot commented on Apr 5, 2023

    @rfcbot
    rfcbot
    on Apr 5, 2023

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

  18. rfcbot
    added
    finished-final-comment-periodThe final comment period is finished for this PR / Issue.
    to-announceAnnounce this issue on triage meeting
    and removed
    final-comment-periodIn the final comment period and will be merged soon unless new substantive objections are raised.
    on Apr 15, 2023

  19. rfcbot commented on Apr 15, 2023

    @rfcbot
    rfcbot
    on Apr 15, 2023

    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.

  20. apiraino
    removed
    to-announceAnnounce this issue on triage meeting
    on Apr 20, 2023
  21. matthiaskrgr
    added 2 commits that reference this issue on May 9, 2023

    Rollup merge of rust-lang#97320 - usbalbin:stabilize_const_ptr_read, …

    8488f24

    Rollup merge of rust-lang#97320 - usbalbin:stabilize_const_ptr_read, …

    88fbfaf

  22. RalfJung commented on Aug 28, 2023

    @RalfJung
    RalfJung
    on Aug 28, 2023
    Member

    Is there anything left here? #97320 should have closed this issue, no?

  23. RalfJung
    closed this as completedon Nov 10, 2023
  24. RalfJung
    added
    A-const-evalArea: Constant evaluation, covers all const contexts (static, const fn, ...)
    on Dec 1, 2024
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    A-const-evalArea: Constant evaluation, covers all const contexts (static, const fn, ...)C-tracking-issueCategory: An issue tracking the progress of sth. like the implementation of an RFCI-libs-radarLibs issues that are tracked on the team's radar.T-libs-api[DEPRECATED; DO NOT USE]disposition-mergeThis issue / PR is in PFCP or FCP with a disposition to merge it.finished-final-comment-periodThe final comment period is finished for this PR / Issue.

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