Replace OffsetOf by an actual sum of calls to intrinsic. #148151

cjgillot · 2025-10-26T20:55:55Z

This PR changes the way we compute the value of the offset_of! macro in MIR. The current implementation uses a dedicated MIR rvalue.

This PR proposes to replace it by an inline constant which sums calls to a new intrinsic offset_of(variant index, field index). The desugaring is done at THIR building time, easier that doing it on MIR.

The new intrinsic is only meant to be used by const-eval. LLVM codegen will refuse to generate code for it.

We replace:

a = offset_of!(T, Variant1.Field1.Variant2.Field2);

By:

a = const {constant#n};

{constant#n}: usize = {
    _1 = offset_of::<T>(index of Variant1, index of Field1);
    _2 = offset_of::<U>(index of Variant2, index of Field2); // Where T::Variant1::Field1 has type U
    _0 = _1 + _2
}

The second commit modifies intrinsic const checking to take allow_internal_unstable into account. The new intrinsic should only be called from stable offset_of! macro. The intrinsic itself is unstable, const-unstable, but rustc_intrinsic_const_stable_indirect.

Fixes #123959
Fixes #125680
Fixes #129425
Fixes #136175

r? @ghost

library/core/src/intrinsics/mod.rs

cjgillot · 2025-10-27T00:53:27Z

@bors try @rust-timer queue

Replace OffsetOf by an actual sum of calls to intrinsic.

rust-bors · 2025-10-27T03:07:33Z

☀️ Try build successful (CI)
Build commit: 7721497 (772149783c4c312fd88b7dfaa68a045b14db3280, parent: f37aa9955f03bb1bc6fe08670cb1ecae534b5815)

rust-timer · 2025-10-27T05:51:54Z

Finished benchmarking commit (7721497): comparison URL.

Overall result: ❌✅ regressions and improvements - no action needed

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

@bors rollup=never
@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.1%	[0.0%, 0.1%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.1%]	3
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (secondary -1.9%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.6%	[2.1%, 3.2%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.7%	[-4.9%, -1.5%]	5
All ❌✅ (primary)	-	-	0

Cycles

Results (secondary -0.8%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.7%	[1.6%, 5.0%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-6.1%	[-7.9%, -4.2%]	2
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 473.876s -> 473.729s (-0.03%)
Artifact size: 390.50 MiB -> 390.46 MiB (-0.01%)

bors · 2025-10-31T16:23:55Z

☔ The latest upstream changes (presumably #148324) made this pull request unmergeable. Please resolve the merge conflicts.

rustbot · 2025-11-01T23:47:25Z

Some changes occurred in match checking

cc @Nadrieril

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

⚠️ #[rustc_intrinsic_const_stable_indirect] controls whether intrinsics can be exposed to stable const
code; adding it needs t-lang approval.

cc @rust-lang/wg-const-eval

Some changes occurred to constck

cc @fee1-dead

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

This PR changes rustc_public

cc @oli-obk, @celinval, @ouz-a

Some changes occurred in rustc_ty_utils::consts.rs

cc @BoxyUwU

Some changes occurred to the intrinsics. Make sure the CTFE / Miri interpreter
gets adapted for the changes, if necessary.

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

This PR changes MIR

cc @oli-obk, @RalfJung, @JakobDegen, @vakaras

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

Some changes occurred in compiler/rustc_codegen_cranelift

cc @bjorn3

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri

Some changes occurred in compiler/rustc_codegen_ssa

cc @WaffleLapkin

This PR changes a file inside tests/crashes. If a crash was fixed, please move into the corresponding ui subdir and add 'Fixes #' to the PR description to autoclose the issue upon merge.

BoxyUwU · 2025-11-02T20:43:45Z

What's the justification/benefit for doing this, and what user facing impact does it have (i.e. why does it need a lang nomination)?

cjgillot · 2025-11-02T20:58:55Z

My justification is simplifying MIR. This feature was implemented as a specific MIR statement, but does not need to, an intrinsic is sufficient.

There should be no user-facing change because of the offset_of manipulation. Maybe some diagnostics, but not more.

However, there are 2 user-facing changes in this PR that t-lang may want to know:

const-stability of intrinsics now takes allow_internal_unstable into account;
we get an additional intrinsic with rustc_intrinsic_const_stable_indirect.

scottmcm · 2025-11-05T16:46:53Z

(With my lang hat) This is moving the current magic macro to a magic intrinsic, so a new never-directly-stable intrinic generated by the magic macro seems entirely reasonable.

@rfcbot merge

cc @RalfJung again explicitly as the current expert, AFAIK, for how const stability works to ensure that the changes here aren't violating the expectations from how that's working.

(With my compiler hat) Big fan of getting the &'tcx List out of NullOp. Expanding to multiple simpler things in MIR, while keeping the nice API in the public macro, sound good to me.

rust-rfcbot · 2025-11-05T16:46:55Z

Team member @scottmcm 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.

traviscross · 2025-11-05T16:47:26Z

@rfcbot reviewed

nikomatsakis · 2025-11-05T16:49:41Z

@rfcbot reviewed

rust-rfcbot · 2025-11-05T16:49:50Z

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

RalfJung

rustc_const_eval changes LGTM.

View changes since this review

RalfJung · 2025-11-06T22:24:31Z

compiler/rustc_const_eval/src/interpret/intrinsics.rs

+                let tp_ty = instance.args.type_at(0);
+
+                let u32_layout = self.layout_of(self.tcx.types.u32)?;
+                let variant = self.read_scalar(&args[0])?.to_bits(u32_layout.size)? as u32;


Suggested change

let variant = self.read_scalar(&args[0])?.to_bits(u32_layout.size)? as u32;

let variant = self.read_scalar(&args[0])?.to_u32()?;

RalfJung · 2025-11-06T22:24:54Z

compiler/rustc_const_eval/src/interpret/intrinsics.rs

+
+                let u32_layout = self.layout_of(self.tcx.types.u32)?;
+                let variant = self.read_scalar(&args[0])?.to_bits(u32_layout.size)? as u32;
+                let field = self.read_scalar(&args[1])?.to_bits(u32_layout.size)? as usize;


Suggested change

let field = self.read_scalar(&args[1])?.to_bits(u32_layout.size)? as usize;

let field = self.read_scalar(&args[1])?.to_u32()? as usize;

RalfJung · 2025-11-06T22:27:55Z

library/core/src/intrinsics/mod.rs

+/// Therefore, implementations must not require the user to uphold
+/// any safety invariants.
+///
+/// The stabilized version of this intrinsic is [`core::mem::offset_of`].


This can also only be called at compile time, right? Please add that note to the comment.

Also, would be good to explain why this is a lang item, since that is very unusual for intrinsics.

cjgillot force-pushed the no-offset-of branch from 0dc894f to e46804b Compare October 26, 2025 20:56

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cjgillot commented Oct 26, 2025

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library/core/src/intrinsics/mod.rs Outdated Show resolved Hide resolved

cjgillot force-pushed the no-offset-of branch from e46804b to 8f24a32 Compare October 27, 2025 00:50

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rust-bors bot added a commit that referenced this pull request Oct 27, 2025

Auto merge of #148151 - cjgillot:no-offset-of, r=<try>

7721497

Replace OffsetOf by an actual sum of calls to intrinsic.

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Oct 27, 2025

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Oct 27, 2025

cjgillot added 2 commits November 1, 2025 23:40

Replace OffsetOf by an actual sum.

0edcc26

Honor allow_internal_unstable for const intrinsics.

9fd16d6

cjgillot force-pushed the no-offset-of branch from 875c6f6 to 9fd16d6 Compare November 1, 2025 23:45

cjgillot marked this pull request as ready for review November 1, 2025 23:47

rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Nov 1, 2025

rustbot removed the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Nov 1, 2025

cjgillot added the I-lang-nominated Nominated for discussion during a lang team meeting. label Nov 2, 2025

oli-obk self-assigned this Nov 2, 2025

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traviscross added T-lang Relevant to the language team 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 labels Nov 5, 2025

scottmcm 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. T-clippy Relevant to the Clippy team. labels Nov 5, 2025

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 Nov 5, 2025

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traviscross removed I-lang-nominated Nominated for discussion during a lang team meeting. P-lang-drag-1 Lang team prioritization drag level 1. https://rust-lang.zulipchat.com/#narrow/channel/410516-t-lang labels Nov 5, 2025

rust-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 Nov 5, 2025

RalfJung reviewed Nov 6, 2025

View reviewed changes

cjgillot added 2 commits November 6, 2025 23:45

Simplify implementation.

35ea63b

Complete doc.

4699a81

	let variant = self.read_scalar(&args[0])?.to_bits(u32_layout.size)? as u32;
	let variant = self.read_scalar(&args[0])?.to_u32()?;

	let field = self.read_scalar(&args[1])?.to_bits(u32_layout.size)? as usize;
	let field = self.read_scalar(&args[1])?.to_u32()? as usize;

Replace OffsetOf by an actual sum of calls to intrinsic. #148151

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Replace OffsetOf by an actual sum of calls to intrinsic. #148151

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rust-timer commented Oct 27, 2025

Overall result: ❌✅ regressions and improvements - no action needed

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