Implement `MaybeDangling` compiler support by WaffleLapkin · Pull Request #150447 · rust-lang/rust

WaffleLapkin · 2025-12-27T21:16:15Z

Tracking issue: #118166

tests/codegen-llvm/manually_drop_refs.rs

compiler/rustc_middle/src/ty/layout.rs

Implement `MaybeDangling` compiler support

rust-bors · 2026-03-03T13:08:40Z

☀️ Try build successful (CI)
Build commit: e147e93 (e147e934a1e67cc70979480d1f951518bb5f7dbe, parent: 1b7d722f429f09c87b08b757d89c689c6cf7f6e7)

rust-timer · 2026-03-03T13:48:39Z

Finished benchmarking commit (e147e93): 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)	0.0%	[0.0%, 0.0%]	1
Improvements ✅ (primary)	-0.3%	[-0.3%, -0.3%]	1
Improvements ✅ (secondary)	-0.2%	[-0.3%, -0.1%]	2
All ❌✅ (primary)	-0.3%	[-0.3%, -0.3%]	1

Max RSS (memory usage)

Results (primary -0.8%, secondary -1.3%)

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)	1.3%	[1.3%, 1.3%]	1
Improvements ✅ (primary)	-0.8%	[-0.8%, -0.8%]	1
Improvements ✅ (secondary)	-3.8%	[-3.8%, -3.8%]	1
All ❌✅ (primary)	-0.8%	[-0.8%, -0.8%]	1

Cycles

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)	6.9%	[6.9%, 6.9%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.1%	[-3.1%, -3.1%]	1
All ❌✅ (primary)	-	-	0

Binary size

Results (primary -0.0%, secondary -0.0%)

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)	-	-	0
Improvements ✅ (primary)	-0.0%	[-0.0%, -0.0%]	1
Improvements ✅ (secondary)	-0.0%	[-0.0%, -0.0%]	2
All ❌✅ (primary)	-0.0%	[-0.0%, -0.0%]	1

Bootstrap: 479.356s -> 478.805s (-0.11%)
Artifact size: 396.94 MiB -> 394.94 MiB (-0.50%)

RalfJung

I like the new test, thanks. :)

Minor nit on the doc comment. IIUC the scalar is only treated like &mut if it's a ptr scalar; if it is an int, drop_target_pointee gets ignored.

r=me with that fixed. Thanks so much for finally pushing this forward ❤️

View changes since this review

compiler/rustc_ty_utils/src/abi.rs

Make `size`/`align` always correct rather than conditionally on the `safe` field. This makes it less error prone and easier to work with for `MaybeDangling` / potential future pointer kinds like `Aligned<_>`.

Instead of defaulting to `None` it now defaults to `Align::ONE` i.e. no alignment restriction. Codegen test changes are due to us now skipping `align 1` annotations (they are useless; not skipping them makes all the raw pointers gain an `align 1` annotation which doesn't seem any good)

WaffleLapkin · 2026-03-05T10:54:29Z

@bors r=RalfJung

rust-bors · 2026-03-05T10:54:32Z

📌 Commit d6ca5c3 has been approved by RalfJung

It is now in the queue for this repository.

JonathanBrouwer · 2026-03-05T12:17:34Z

@bors p=6
Scheduling this before the next rollup

rust-bors · 2026-03-05T16:03:01Z

☀️ Test successful - CI
Approved by: RalfJung
Duration: 3h 41m 22s
Pushing 64b72a1 to main...

github-actions · 2026-03-05T16:06:03Z

What is this?

This is an experimental post-merge analysis report that shows differences in test outcomes between the merged PR and its parent PR.

Comparing 70d86e3 (parent) -> 64b72a1 (this PR)

Test differences

Show 21 test diffs

Stage 1

[codegen] tests/codegen-llvm/maybe_dangling_refs.rs: [missing] -> pass (J0)

Stage 2

[codegen] tests/codegen-llvm/maybe_dangling_refs.rs: [missing] -> pass (J1)

Additionally, 19 doctest diffs were found. These are ignored, as they are noisy.

Job group index

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard 64b72a1fa5449d928d5f553b01a596b78ee255d2 --output-dir test-dashboard

And then open test-dashboard/index.html in your browser to see an overview of all executed tests.

Job duration changes

dist-x86_64-illumos: 2h 16m -> 1h 45m (-22.6%)
dist-aarch64-llvm-mingw: 1h 55m -> 1h 33m (-18.8%)
x86_64-gnu-aux: 2h 1m -> 2h 21m (+16.5%)
test-various: 1h 51m -> 2h 7m (+14.9%)
dist-apple-various: 2h 7m -> 1h 49m (-13.9%)
dist-aarch64-apple: 2h 34m -> 2h 13m (-13.7%)
dist-aarch64-msvc: 1h 48m -> 1h 37m (-10.4%)
aarch64-gnu: 2h 4m -> 2h 16m (+9.6%)
aarch64-apple: 3h 18m -> 3h 34m (+8.1%)
dist-x86_64-apple: 2h 34m -> 2h 22m (-7.6%)

How to interpret the job duration changes?

Job durations can vary a lot, based on the actual runner instance
that executed the job, system noise, invalidated caches, etc. The table above is provided
mostly for t-infra members, for simpler debugging of potential CI slow-downs.

rust-timer · 2026-03-05T16:59:09Z

Finished benchmarking commit (64b72a1): comparison URL.

Overall result: ✅ improvements - no action needed

@rustbot label: -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
Improvements ✅ (primary)	-0.5%	[-0.5%, -0.5%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.5%	[-0.5%, -0.5%]	1

Max RSS (memory usage)

Results (primary 0.3%, secondary 3.5%)

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

	mean	range	count
Regressions ❌ (primary)	1.7%	[0.8%, 2.7%]	2
Regressions ❌ (secondary)	3.5%	[3.4%, 3.6%]	2
Improvements ✅ (primary)	-2.5%	[-2.5%, -2.5%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.3%	[-2.5%, 2.7%]	3

Cycles

Results (primary -1.3%, secondary -4.0%)

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)	-	-	0
Improvements ✅ (primary)	-1.3%	[-1.3%, -1.3%]	1
Improvements ✅ (secondary)	-4.0%	[-4.0%, -4.0%]	1
All ❌✅ (primary)	-1.3%	[-1.3%, -1.3%]	1

Binary size

Results (primary -0.0%, secondary -0.0%)

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)	-	-	0
Improvements ✅ (primary)	-0.0%	[-0.0%, -0.0%]	1
Improvements ✅ (secondary)	-0.0%	[-0.0%, -0.0%]	2
All ❌✅ (primary)	-0.0%	[-0.0%, -0.0%]	1

Bootstrap: 480.787s -> 479.735s (-0.22%)
Artifact size: 395.02 MiB -> 395.00 MiB (-0.00%)

WaffleLapkin force-pushed the maybe-dangling-semantics branch 2 times, most recently from aebfd56 to 63e07cf Compare December 27, 2025 21:28

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RalfJung reviewed Dec 28, 2025

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tests/codegen-llvm/manually_drop_refs.rs Outdated Show resolved Hide resolved

RalfJung reviewed Dec 28, 2025

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RalfJung mentioned this pull request Jan 4, 2026

miri/const eval: support MaybeDangling #150446

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WaffleLapkin force-pushed the maybe-dangling-semantics branch from 63e07cf to c40ca7d Compare February 19, 2026 14:35

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rustbot added the A-LLVM Area: Code generation parts specific to LLVM. Both correctness bugs and optimization-related issues. label Feb 19, 2026

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WaffleLapkin force-pushed the maybe-dangling-semantics branch from c135ac1 to f399322 Compare February 25, 2026 23:07

rustbot added the PG-exploit-mitigations Project group: Exploit mitigations label Feb 25, 2026

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rust-bors bot pushed a commit that referenced this pull request Mar 3, 2026

Auto merge of #150447 - WaffleLapkin:maybe-dangling-semantics, r=<try>

e147e93

Implement `MaybeDangling` compiler support

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Mar 3, 2026

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WaffleLapkin force-pushed the maybe-dangling-semantics branch from 632b33d to d5e1b0e Compare March 3, 2026 13:30

rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Mar 3, 2026

RalfJung approved these changes Mar 3, 2026

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compiler/rustc_ty_utils/src/abi.rs Outdated Show resolved Hide resolved

WaffleLapkin added 5 commits March 5, 2026 11:53

refactor PointeeInfo

312055f

Make `size`/`align` always correct rather than conditionally on the `safe` field. This makes it less error prone and easier to work with for `MaybeDangling` / potential future pointer kinds like `Aligned<_>`.

add a codegen-llvm test for ManuallyDrop

2ce2e04

add semantics to MaybeDangling

8aafa53

strip readonly/captures from MaybeDangling<&T>

d6ca5c3

WaffleLapkin force-pushed the maybe-dangling-semantics branch from d5e1b0e to d6ca5c3 Compare March 5, 2026 10:53

rust-bors bot 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-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Mar 5, 2026

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rust-bors bot added merged-by-bors This PR was explicitly merged by bors. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Mar 5, 2026

rust-bors bot merged commit 64b72a1 into rust-lang:main Mar 5, 2026
12 checks passed

rustbot added this to the 1.96.0 milestone Mar 5, 2026

WaffleLapkin deleted the maybe-dangling-semantics branch March 5, 2026 16:34

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rust-timer commented Mar 3, 2026

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

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github-actions bot commented Mar 5, 2026

Test differences

Stage 1

Stage 2

Job duration changes

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rust-timer commented Mar 5, 2026

Overall result: ✅ improvements - no action needed

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