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const-eval: fix and re-enable pointer fragment support #148259

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const-eval: fix and re-enable pointer fragment support #148259

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@RalfJung RalfJung commented Oct 29, 2025 •
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The pointer fragment support from #144081 got disabled due to #146291. This brings it back. To fix the issue, the per-byte provenance fragment tracking tracks both the provenance and raw address of the full pointer, so we can ensure that only fragments that are truly part of the same pointer are being merged.

r? @oli-obk
Cc @theemathas
Fixes rust-lang/const-eval#72 again.
Also fixes #147959.

@traviscross I assume this won't need another t-lang FCP since it already got FCP'd in #144081?

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rustbot commented Oct 29, 2025

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri, @RalfJung, @oli-obk, @lcnr

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

The Miri subtree was changed

cc @rust-lang/miri

@rustbot rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. 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 Oct 29, 2025
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oli-obk is not on the review rotation at the moment.
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traviscross commented Oct 29, 2025 •
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I assume this won't need another t-lang FCP since it already got FCP'd in #144081?

As I commented a moment ago on an unrelated matter in #145954 (comment):

As a procedural matter, though the FCP in ... did originally cover what's here, the fact that it was merged without that puts the ball back in our court, in the same way that if one of our stabilizations is reverted, we then need to re-FCP the restabilization. In this way, our FCPs represent a "point-in-time" decision to stabilize something.

We discussed that procedural analysis in the meeting and it seemed right to people.

In that light, we would FCP the restabilization here. It should be an easy call for us given the earlier decision.

@RalfJung RalfJung added I-lang-nominated Nominated for discussion during a lang team meeting. I-lang-easy-decision Issue: The decision needed by the team is conjectured to be easy; this does not imply nomination labels Oct 29, 2025
@traviscross traviscross added T-lang Relevant to the language team needs-fcp This change is insta-stable, or significant enough to need a team FCP to proceed. I-lang-radar Items that are on lang's radar and will need eventual work or consideration. P-lang-drag-1 Lang team prioritization drag level 1. https://rust-lang.zulipchat.com/#narrow/channel/410516-t-lang and removed 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 Oct 29, 2025
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This seems right to me. I propose we restabilize this.

@rfcbot fcp merge

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rust-rfcbot commented Oct 29, 2025 •
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Team member @traviscross 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.

@rust-rfcbot rust-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 Oct 29, 2025
@traviscross traviscross added the T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. label Oct 29, 2025
RalfJung
RalfJung commented Oct 29, 2025
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@RalfJung RalfJung force-pushed the const-ptr-fragment branch 2 times, most recently from 4e023d0 to 739d035 Compare October 29, 2025 19:38
RalfJung
RalfJung commented Oct 29, 2025
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compiler/rustc_middle/src/mir/interpret/pointer.rs
fn get_alloc_id(self) -> Option<AllocId>;

/// Defines the 'join' of provenance: what happens when doing a pointer load and different bytes have different provenance.
fn join(left: Self, right: Self) -> Option<Self>;
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I'm getting rid of the join function because the code that used it anyway had to special-case wildcards, and then it became easier to just have the entire wildcard logic there rather than having some of it inside join.

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theemathas reviewed Oct 30, 2025
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Do we need tests for #147959?

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Do we need tests for #147959?

Good point, I added that.

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oli-obk
oli-obk reviewed Nov 3, 2025
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compiler/rustc_middle/src/mir/interpret/allocation/provenance_map.rs
pub struct PointerFrag<Prov> {
/// The position of this fragment inside the pointer (in `0..8`).
pub idx: u8,
/// The pointer this is a fragment of: provenance and the "real" raw bytes.
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the real raw bytes are in the next field, not in the prov field.

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True. I wasn't sure how to best write a comment that spans both fields.

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disposition-merge This issue / PR is in PFCP or FCP with a disposition to merge it. I-lang-easy-decision Issue: The decision needed by the team is conjectured to be easy; this does not imply nomination I-lang-nominated Nominated for discussion during a lang team meeting. I-lang-radar Items that are on lang's radar and will need eventual work or consideration. needs-fcp This change is insta-stable, or significant enough to need a team FCP to proceed. P-lang-drag-1 Lang team prioritization drag level 1. https://rust-lang.zulipchat.com/#narrow/channel/410516-t-lang proposed-final-comment-period Proposed to merge/close by relevant subteam, see T-<team> label. Will enter FCP once signed off. S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. T-lang Relevant to the language team

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Consteval pointer fragment support was not properly disabled. Full support for pointer fragments

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