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Tracking Issue for nested field access in offset_of #120140

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#128284
Closed
Tracking Issue for nested field access in offset_of#120140
#128284
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B-unstableBlocker: Implemented in the nightly compiler and unstable.C-tracking-issueCategory: An issue tracking the progress of sth. like the implementation of an RFCF-offset_of_nested`#![feature(offset_of_nested)]`T-langRelevant to the language teamT-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

@GKFX
GKFX
opened on Jan 19, 2024

Feature gate: #![feature(offset_of_nested)]

This is a tracking issue for the use of nested field access in offset_of. This is currently implemented with dot-separated fields, like in C. The original RFC for offset_of was rust-lang/rfcs#3308.

Public API

pub macro offset_of($Container:ty, $($fields:expr)+ $(,)?) { ... }

struct Struct {
    field: A,
}
struct A {
    a: ((u8, u8), bool),
}
const OFFSET: usize = offset_of!(Struct, field.a.0.1);

Steps / History

Unresolved Questions

  • None yet.

Footnotes

  1. https://std-dev-guide.rust-lang.org/feature-lifecycle/stabilization.html ↩

Activity

  1. GKFX
    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 Jan 19, 2024

  2. est31 commented on Jan 20, 2024

    @est31
    est31
    on Jan 20, 2024
    Member

    This has been split off from #106655 to get the non-nested offset_off macro stabilized sooner.

    How the syntax for nested access should look like is still subject to discussion, especially also how it relates to enums.

    Some links:

  3. fmease
    added
    F-offset_of_nested`#![feature(offset_of_nested)]`
    on Feb 17, 2024

  4. tgross35 commented on May 22, 2024

    @tgross35
    tgross35
    on May 22, 2024
    Member

    Could the top example be updated to show what Struct looks like?

  5. GKFX commented on Jun 2, 2024

    @GKFX
    GKFX
    on Jun 2, 2024
    ContributorAuthor

    How close is this to stabilization? It looks like this is in use by the Linux kernel (https://lore.kernel.org/rust-for-linux/CAH5fLgjP98pS1wsP=YXP5Yr79Y62VF7XPKjbj+G75B3SOFt80g@mail.gmail.com/), as well as the standard library (for an enum):

    rust/library/core/src/option.rs

    Line 787 in 13e2d72

    (self as *const Self).byte_add(core::mem::offset_of!(Self, Some.0)).cast(),

    Stabilizing this may restrict future changes to the offset_of parsing logic (compiler/rustc_parse/src/parser/expr.rs:1191, parse_floating_field_access). Currently this has enum variants treated the same as fields and (with parsing support only) array indexing using [expr] syntax.

  6. fmease
    added
    B-unstableBlocker: Implemented in the nightly compiler and unstable.
    on Jun 9, 2024

  7. dingxiangfei2009 commented on Jun 11, 2024

    @dingxiangfei2009
    dingxiangfei2009
    on Jun 11, 2024
    Contributor

    Cross-posting stabilization report and request because the two feature gates need to stabilize together for best utility.

    Report

    cc @Darksonn @ojeda

  8. Amanieu
    added
    T-langRelevant to the language team
    on Jun 17, 2024

  9. Amanieu commented on Jun 17, 2024

    @Amanieu
    Amanieu
    on Jun 17, 2024
    Member

    @rfcbot fcp merge

  10. rfcbot commented on Jun 17, 2024

    @rfcbot
    rfcbot
    on Jun 17, 2024

    Team member @Amanieu 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!

    cc @rust-lang/lang-advisors: FCP proposed for lang, please feel free to register concerns.
    See this document for info about what commands tagged team members can give me.

  11. 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.
    on Jun 17, 2024

  12. tmandry commented on Jun 20, 2024

    @tmandry
    tmandry
    on Jun 20, 2024
    Member

    I'm unclear on whether this needs an RFC. I would defer to libs-api on that question though, since this is more a question of the syntax the macro supports than the capability it provides, and I think the syntax is more in the libs-api realm than the lang one (though it may be a bit of both).

    Regarding the syntax, there is still an open question about supporting enum variants in the future. I'm personally okay with moving forward with this subset for now, especially since I can imagine a syntax I don't dislike: offset_of!(Struct, (field.a as MyEnum::Variant).0.1). But I'm just documenting my thinking here for future reference; we should leave the bikeshed out of this issue if at all possible.

    @rfcbot reviewed

  13. nikomatsakis commented on Jul 12, 2024

    @nikomatsakis
    nikomatsakis
    on Jul 12, 2024
    Contributor

    @rfcbot reviewed

    I don't feel this requires an RFC. I think it is a natural extension to the existing functionality.

  14. rfcbot
    added
    final-comment-periodIn the final comment period and will be merged soon unless new substantive objections are raised.
    on Jul 18, 2024

  15. rfcbot commented on Jul 18, 2024

    @rfcbot
    rfcbot
    on Jul 18, 2024

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

  16. rfcbot
    removed
    proposed-final-comment-periodProposed to merge/close by relevant subteam, see T-<team> label. Will enter FCP once signed off.
    on Jul 18, 2024

  17. pnkfelix commented on Jul 19, 2024

    @pnkfelix
    pnkfelix
    on Jul 19, 2024
    Contributor

    @rfcbot reviewed

  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 Jul 28, 2024

  19. rfcbot commented on Jul 28, 2024

    @rfcbot
    rfcbot
    on Jul 28, 2024

    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. matthiaskrgr
    added a commit that references this issue on Jul 29, 2024

    Rollup merge of rust-lang#128284 - GKFX:stabilize-offset-of-nested, r…

    a73a025
  21. bors
    closed this as completedin a73a025on Jul 30, 2024
  22. rust-timer
    added a commit that references this issue on Jul 30, 2024

    Unrolled build for rust-lang#128284

    3566948
  23. apiraino
    removed
    to-announceAnnounce this issue on triage meeting
    on Aug 1, 2024
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    B-unstableBlocker: Implemented in the nightly compiler and unstable.C-tracking-issueCategory: An issue tracking the progress of sth. like the implementation of an RFCF-offset_of_nested`#![feature(offset_of_nested)]`T-langRelevant to the language teamT-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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