Generalise (most) impls on `Box` by nia-e · Pull Request #161946 · rust-lang/rust

nia-e · 2026-08-28T18:16:14Z

Resolves the trait impl concern for #156882; Default wasn't touched since changing that would be breaking, but hopefully all of this should be fine - I'll do a crater run to be sure. Certain impls that were for one reason or another Annoying to generalise (e.g. Clone for Box<Path>) were also untouched since I believe we can add those in the future, as they're on concrete types.

r? clarfonthey

nia-e · 2026-08-28T18:16:37Z

also should do a perf run since monomorphisation.

@bors try @rust-timer queue

Generalise (most) impls on `Box`

clarfonthey · 2026-08-28T18:22:12Z

r=me pending crater + perf

rust-bors · 2026-08-28T19:17:56Z

☀️ Try build successful (CI)
Build commit: 0c3b2c2 (0c3b2c226322b623e493b7611e4909ee6bc6a48d)
Base parent: 344f790 (344f7902949345394fa40a5d7dda31f012ccbc0d)

nia-e · 2026-08-28T19:19:21Z

@craterbot check

craterbot · 2026-08-28T19:20:04Z

👌 Experiment pr-161946 created and queued.
🤖 Automatically detected try build 0c3b2c2
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

rust-timer · 2026-08-28T20:03:44Z

Finished benchmarking commit (0c3b2c2): comparison URL.

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

Benchmarking means the PR may be perf-sensitive. Consider adding rollup=never if this change is not fit for rolling up.

@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)	0.4%	[0.4%, 0.4%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.5%	[-0.5%, -0.5%]	1
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary 3.2%, secondary -0.3%)

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

	mean	range	count
Regressions ❌ (primary)	9.0%	[9.0%, 9.0%]	1
Regressions ❌ (secondary)	4.1%	[1.6%, 8.7%]	5
Improvements ✅ (primary)	-2.6%	[-2.6%, -2.6%]	1
Improvements ✅ (secondary)	-4.6%	[-5.7%, -2.4%]	5
All ❌✅ (primary)	3.2%	[-2.6%, 9.0%]	2

Cycles

Results (primary 2.7%, secondary -11.8%)

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

	mean	range	count
Regressions ❌ (primary)	2.7%	[2.7%, 2.7%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-11.8%	[-20.6%, -3.9%]	7
All ❌✅ (primary)	2.7%	[2.7%, 2.7%]	1

Binary size

This perf run didn't have relevant results for this metric.

Bootstrap: 475.673s -> 475.454s (-0.05%)
Artifact size: 403.60 MiB -> 402.86 MiB (-0.18%)

nia-e · 2026-08-28T20:05:34Z

that's a nothingburger, nice. time to just wait a zillion years for crater lol

theemathas · 2026-09-02T04:48:35Z

Is there a reason impl<T> From<T> for Box<T> wasn't generalized?

And how is changing the Default impl breaking?

zachs18 · 2026-09-02T05:32:34Z

I'm think this is the issue Nia's referring to: the Default impl would break type inference in some cases (playground). The From impl could also have the same issue (playground). So, those impls would probably need at least a crater run.

cc https://faultlore.com/blah/defaults-affect-inference/ which discusses the same issue for why Default for Vec<T, A> wasn't generalized (yet ™️ ).

clarfonthey · 2026-09-02T11:50:25Z

Is there a reason impl<T> From<T> for Box<T> wasn't generalized?

And how is changing the Default impl breaking?

Inference basically. It becomes ambiguous in allocator what a default box might mean, or a box converted from another type.

theemathas · 2026-09-03T14:00:28Z

@craterbot cancel

See #162233

craterbot · 2026-09-03T14:01:00Z

🗑️ Experiment pr-161946 deleted!

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

theemathas · 2026-09-06T02:19:04Z

@craterbot check crates=https://crater-reports.s3.amazonaws.com/pr-162233-crater-rollup/retry-regressed-list.txt p=1

craterbot · 2026-09-06T02:19:08Z

👌 Experiment pr-161946 created and queued.
🤖 Automatically detected try build 0c3b2c2
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

nia-e · 2026-09-06T05:01:52Z

looked at the regression list on the rollup and i think this pr was entirely harmless? if so yay

craterbot · 2026-09-08T19:43:01Z

🚧 Experiment pr-161946 is now running

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

craterbot · 2026-09-08T22:36:03Z

🎉 Experiment pr-161946 is completed!
📊 0 regressed and 0 fixed (7794 total)
📊 773 spurious results on the retry-regressed-list.txt, consider a retry¹ if this is a significant amount.
📰 Open the summary report.

⚠️ If you notice any spurious failure please add them to the denylist!
ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

re-run the experiment with crates=https://crater-reports.s3.amazonaws.com/pr-161946/retry-regressed-list.txt ↩

clarfonthey · 2026-09-09T01:46:05Z

Needs rebase for stable API timestamp (and thus has to be rollup=never) but otherwise good

clarfonthey · 2026-09-09T12:45:17Z

Side note: confused myself into thinking this needed FCP but that's included as part of the allocator FCP

nia-e · 2026-09-09T17:35:05Z

yeah this is not gonna need rebase dw. per the above,

@bors r=clarfonthey

rust-bors · 2026-09-09T17:35:08Z

📌 Commit f01082b has been approved by clarfonthey

It is now in the queue for this repository.

clarfonthey · 2026-09-09T17:57:47Z

Right, I guess this doesn't add new stable API.

Rollup of 8 pull requests Successful merges: - #162412 (Fix msvc-specific differences in debuginfo tests) - #160651 (mir: validate `Move` call arguments are locals or box derefs) - #161946 (Generalise (most) impls on `Box`) - #162192 (normalize CoerceShared field types before relating) - #160518 (fix: fix no resolution for self import in namespaced crates) - #162509 (Avoid suggesting gated generic arguments for Fn-family traits) - #162521 (Don't explicitly specify `OnDuplicate::Error` as it is the default) - #162526 (yeet VisitorExt)

Rollup merge of #161946 - nia-e:more-box-impls, r=clarfonthey Generalise (most) impls on `Box` Resolves the trait impl concern for #156882; `Default` wasn't touched since changing that would be breaking, but hopefully all of this should be fine - I'll do a crater run to be sure. Certain impls that were for one reason or another Annoying to generalise (e.g. `Clone for Box<Path>`) were also untouched since I believe we can add those in the future, as they're on concrete types. r? clarfonthey

generalise impls on box

f01082b

rustbot assigned clarfonthey Aug 28, 2026

rustbot added O-solid Operating System: SOLID O-windows Operating system: Windows S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Aug 28, 2026

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rust-bors Bot pushed a commit that referenced this pull request Aug 28, 2026

Auto merge of #161946 - nia-e:more-box-impls, r=<try>

0c3b2c2

Generalise (most) impls on `Box`

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Aug 28, 2026

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craterbot added S-waiting-on-crater Status: Waiting on a crater run to be completed. and removed S-waiting-on-perf Status: Waiting on a perf run to be completed. labels Aug 28, 2026

theemathas mentioned this pull request Sep 3, 2026

Crater rollup #162233

Draft

craterbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-crater Status: Waiting on a crater run to be completed. labels Sep 3, 2026

craterbot added S-waiting-on-crater Status: Waiting on a crater run to be completed. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Sep 6, 2026

craterbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-crater Status: Waiting on a crater run to be completed. labels Sep 8, 2026

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-review Status: Awaiting review from the assignee but also interested parties. labels Sep 9, 2026

jhpratt mentioned this pull request Sep 9, 2026

Rollup of 8 pull requests #162550

Merged

rust-bors Bot merged commit 5df5032 into rust-lang:main Sep 9, 2026
14 checks passed

rustbot added this to the 1.100.0 milestone Sep 9, 2026

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rust-timer commented Aug 28, 2026

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

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nia-e commented Aug 28, 2026 •

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