Replace `Unique` in `Box` with a `(NonNull, PhantomData)` wrapper by maxdexh · Pull Request #162849 · rust-lang/rust

maxdexh · 2026-09-16T10:54:41Z

Follow-up to #162804

This is only the first step in actually refactoring Box, and is essentially just a rename.
The layout of Box stays as-is for now, due to #162850.

Needs perf run

rustbot · 2026-09-16T17:22:14Z

miri is developed in its own repository. If the Miri part of this change can be broken out, consider making this change to rust-lang/miri instead. However, if Miri needs adjusting for rustc changes, just ignore this message.

cc @rust-lang/miri

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

rustbot · 2026-09-16T17:22:17Z

r? @nnethercote

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maxdexh · 2026-09-16T17:22:36Z

@hanna-kruppe do you want to take this one too?

r? hanna-kruppe

Reroll libs if not, please (sorry, should have asked before marking the PR as ready)

maxdexh · 2026-09-16T17:23:05Z

ignore rustbot, all changes outside libs were to test files ^^

rustbot · 2026-09-16T17:25:17Z

hanna-kruppe is currently at their maximum review capacity.
They may take a while to respond.

hanna-kruppe · 2026-09-18T20:14:23Z

@bors try @rust-timer queue

Replace `Unique` in `Box` with a `(NonNull, PhantomData)` wrapper

rust-bors · 2026-09-18T21:14:40Z

☀️ Try build successful (CI)
Build commit: 5b25bbf (5b25bbfa1be1e911397b56089384cce001a1a9e2)
Base parent: 420ed2a (420ed2a0c3d7225b1744266fd884d431b4d8cfe0)

rust-timer · 2026-09-18T21:58:10Z

Finished benchmarking commit (5b25bbf): comparison URL.

Overall result: ❌✅ regressions and improvements - please read:

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

@bors rollup=never rustc-perf
@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)	0.4%	[0.3%, 0.5%]	3
Regressions ❌ (secondary)	0.3%	[0.3%, 0.3%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.4%	[-0.4%, -0.3%]	2
All ❌✅ (primary)	0.4%	[0.3%, 0.5%]	3

Max RSS (memory usage)

Results (primary -2.9%, secondary 1.5%)

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)	3.1%	[2.6%, 3.6%]	2
Improvements ✅ (primary)	-2.9%	[-2.9%, -2.9%]	1
Improvements ✅ (secondary)	-1.9%	[-1.9%, -1.9%]	1
All ❌✅ (primary)	-2.9%	[-2.9%, -2.9%]	1

Cycles

Results (secondary 0.9%)

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)	4.3%	[4.2%, 4.3%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.5%	[-2.8%, -2.2%]	2
All ❌✅ (primary)	-	-	0

Binary size

Results (primary -0.3%, secondary -0.4%)

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)	-0.3%	[-1.5%, -0.1%]	44
Improvements ✅ (secondary)	-0.4%	[-0.8%, -0.2%]	14
All ❌✅ (primary)	-0.3%	[-1.5%, -0.1%]	44

Bootstrap: 498.333s -> 498.695s (0.07%)
Artifact size: 408.93 MiB -> 408.97 MiB (0.01%)

RalfJung · 2026-09-20T13:12:07Z

@bors delegate try

Now you can do it :)

rust-bors · 2026-09-20T13:13:41Z

✌️ @maxdexh, you can now perform try builds on this pull request!

You can now post @bors try to start a try build.

rust-bors · 2026-09-20T13:18:49Z

Unknown argument "test-aarch64-apple-1,test-x86_64-msvc-1". Did you mean to use @bors jobs=<jobs>|parent=<parent>? Run @bors help or go to https://bors.rust-lang.org/help to see available commands.

maxdexh · 2026-09-20T13:19:28Z

@bors try jobs=test-aarch64-apple-1,test-x86_64-msvc-1

Replace `Unique` in `Box` with a `(NonNull, PhantomData)` wrapper try-job: test-aarch64-apple-1 try-job: test-x86_64-msvc-1

maxdexh · 2026-09-20T13:58:11Z

I suspect the few compile time regressions are coming from my rewrite of Box::<str>::default. Hopefully this fixes that. Shouldn't affect the bors run.

maxdexh · 2026-09-20T13:58:37Z

Can I do another bors run for perf without cancelling the ongoing one?

maxdexh · 2026-09-20T14:00:50Z

yolo
@bors try @rust-timer queue

edit: rip, it cancelled them ^^

Replace `Unique` in `Box` with a `(NonNull, PhantomData)` wrapper

RalfJung · 2026-09-20T14:09:33Z

The answer is no, the 2nd try run cancels the first.

rust-bors · 2026-09-20T15:01:22Z

☀️ Try build successful (CI)
Build commit: ee81c60 (ee81c60a6e1754205bd38a27d0af4d502a4e1b42)
Base parent: fc7358c (fc7358c9223bbf6b30741438fc8588dad7e4671c)

rust-timer · 2026-09-20T15:45:10Z

Finished benchmarking commit (ee81c60): comparison URL.

Overall result: ❌✅ regressions and improvements - please read:

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

@bors rollup=never rustc-perf
@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)	0.4%	[0.2%, 0.6%]	4
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.3%	[-0.4%, -0.2%]	9
All ❌✅ (primary)	0.4%	[0.2%, 0.6%]	4

Max RSS (memory usage)

Results (primary 0.1%)

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

	mean	range	count
Regressions ❌ (primary)	2.9%	[2.6%, 3.1%]	3
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-2.8%	[-3.9%, -2.2%]	3
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.1%	[-3.9%, 3.1%]	6

Cycles

Results (secondary -2.4%)

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
Improvements ✅ (secondary)	-2.4%	[-2.7%, -2.1%]	2
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)	0.2%	[0.0%, 0.4%]	64
Regressions ❌ (secondary)	0.1%	[0.0%, 0.2%]	48
Improvements ✅ (primary)	-0.3%	[-1.3%, -0.0%]	23
Improvements ✅ (secondary)	-0.3%	[-0.6%, -0.0%]	9
All ❌✅ (primary)	0.0%	[-1.3%, 0.4%]	87

Bootstrap: 500.549s -> 509.726s (1.83%)
Artifact size: 408.70 MiB -> 408.81 MiB (0.03%)

maxdexh · 2026-09-20T16:02:55Z

I guess that's better? It's not really what I expected to happen but okay

maxdexh · 2026-09-20T16:03:17Z

@bors try jobs=test-aarch64-apple-1,test-x86_64-msvc-1

Replace `Unique` in `Box` with a `(NonNull, PhantomData)` wrapper try-job: test-aarch64-apple-1 try-job: test-x86_64-msvc-1

rust-bors · 2026-09-20T19:01:11Z

☀️ Try build successful (CI)
Build commit: 76a8f8a (76a8f8ab565a4f05ad35f3fb67dfca69022a93e6)
Base parent: fc7358c (fc7358c9223bbf6b30741438fc8588dad7e4671c)

maxdexh · 2026-09-20T19:14:15Z

Awesome, I think this is ready then ^^

rustbot · 2026-09-21T08:26:32Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.