Forbid ShallowInitBox after box deref elaboration. #147687

cjgillot · 2025-10-14T16:43:32Z

MIR currently contains a ShallowInitBox rvalue. Its principal usage is to allow for in-place initialization of boxes. Having it is necessary for drop elaboration to be correct with that in-place initialization.

As part of analysis->runtime MIR lowering, we canonicalize deref of boxes to use the stored raw pointer. But we did not perform the same change to the construction of the box.

This PR replaces ShallowInitBox by the pointer manipulation it represents.

Alternatives:

fully remove ShallowInitBox and implement Box in-place initialization differently;
remove the ElaborateBoxDeref pass and keep dereferencing Box in runtime MIR.

cjgillot · 2025-10-15T02:09:17Z

@bors try @rust-timer queue

Forbid ShallowInitBox after box deref elaboration.

rust-bors · 2025-10-15T04:21:40Z

☀️ Try build successful (CI)
Build commit: 472fdbf (472fdbff5e8adcc0c5d6905b613add926cbfec40, parent: 235a4c083eb2a2bfe8779d211c3232f39396de00)

rust-timer · 2025-10-15T05:43:15Z

Finished benchmarking commit (472fdbf): comparison URL.

Overall result: no relevant changes - 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

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

Max RSS (memory usage)

Results (primary -0.3%)

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

	mean	range	count
Regressions ❌ (primary)	2.1%	[1.2%, 3.0%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-5.0%	[-5.0%, -5.0%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.3%	[-5.0%, 3.0%]	3

Cycles

Results (secondary -2.0%)

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)	3.9%	[3.9%, 3.9%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-5.0%	[-6.0%, -3.9%]	2
All ❌✅ (primary)	-	-	0

Binary size

Results (primary 0.0%, secondary 0.1%)

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

	mean	range	count
Regressions ❌ (primary)	0.1%	[0.0%, 0.4%]	14
Regressions ❌ (secondary)	0.1%	[0.0%, 0.1%]	2
Improvements ✅ (primary)	-0.2%	[-0.3%, -0.0%]	9
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.0%	[-0.3%, 0.4%]	23

Bootstrap: 474.707s -> 474.445s (-0.06%)
Artifact size: 388.17 MiB -> 388.15 MiB (-0.00%)

rustbot · 2025-10-16T19:05:36Z

Some changes occurred in compiler/rustc_codegen_cranelift

cc @bjorn3

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

Some changes occurred in compiler/rustc_codegen_ssa

cc @WaffleLapkin

rustbot · 2025-10-16T19:05:38Z

r? @nnethercote

rustbot has assigned @nnethercote.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

nnethercote · 2025-10-17T06:39:58Z

compiler/rustc_index/src/lib.rs

+
+#[macro_export]
+macro_rules! indexvec {
+    ($expr:expr; $n:expr) => {


We also have IndexVec::from_elem_n. Probably doesn't hurt to have this as well, just thought I'd mention it.

nnethercote · 2025-10-17T06:44:09Z

I don't see anything objectionable here but my knowledge of this part of the code is very thin, so it's a very shallow approval, and I have zero sense of whether this approach is better or worse than the alternatives mentioned in the PR description. So I would be happy if someone who knows more (@bjorn3? @WaffleLapkin?) wants to weigh in.

bors · 2025-10-18T14:01:35Z

☔ The latest upstream changes (presumably #147654) made this pull request unmergeable. Please resolve the merge conflicts.

compiler/rustc_mir_transform/src/elaborate_box_derefs.rs

compiler/rustc_mir_transform/src/validate.rs

nnethercote · 2025-10-21T10:46:33Z

Can you squash the 3rd commit into the 1st commit? r=me with that.

rustbot · 2025-10-22T00:53:03Z

This PR was rebased onto a different master 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.

cjgillot · 2025-10-22T00:53:26Z

@bors r=nnethercote

bors · 2025-10-22T00:53:29Z

📌 Commit 0156eaf has been approved by nnethercote

It is now in the queue for this repository.

bors · 2025-10-22T09:53:53Z

⌛ Testing commit 0156eaf with merge f5e2df7...

bors · 2025-10-22T13:10:38Z

☀️ Test successful - checks-actions
Approved by: nnethercote
Pushing f5e2df7 to master...

github-actions · 2025-10-22T13:13:17Z

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 b2ee1b3 (parent) -> f5e2df7 (this PR)

Test differences

Show 2 test diffs

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

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard f5e2df741b4a9820a7579f0c8eccc951706a8782 --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-android: 1202.1s -> 1642.5s (36.6%)
dist-apple-various: 3442.2s -> 4066.8s (18.1%)
dist-aarch64-apple: 7285.9s -> 6113.6s (-16.1%)
aarch64-gnu: 6289.0s -> 7268.6s (15.6%)
dist-armhf-linux: 5092.0s -> 5824.0s (14.4%)
x86_64-rust-for-linux: 2812.9s -> 3172.6s (12.8%)
dist-s390x-linux: 4957.5s -> 5524.0s (11.4%)
dist-x86_64-windows-gnullvm: 5344.9s -> 4850.1s (-9.3%)
x86_64-gnu-llvm-20-3: 6196.9s -> 6722.7s (8.5%)
dist-powerpc64le-linux-gnu: 5722.2s -> 5252.0s (-8.2%)

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 · 2025-10-22T14:21:10Z

Finished benchmarking commit (f5e2df7): comparison URL.

Overall result: ❌✅ regressions and 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)	0.4%	[0.4%, 0.4%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.2%	[-0.2%, -0.2%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.1%	[-0.2%, 0.4%]	2

Max RSS (memory usage)

Results (primary 1.4%)

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

	mean	range	count
Regressions ❌ (primary)	2.4%	[2.1%, 2.9%]	3
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-1.6%	[-1.6%, -1.6%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.4%	[-1.6%, 2.9%]	4

Cycles

Results (secondary -3.1%)

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)	-3.1%	[-3.8%, -2.4%]	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)	0.1%	[0.0%, 0.8%]	11
Regressions ❌ (secondary)	0.0%	[0.0%, 0.0%]	2
Improvements ✅ (primary)	-0.2%	[-0.3%, -0.0%]	11
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.1%]	1
All ❌✅ (primary)	-0.0%	[-0.3%, 0.8%]	22

Bootstrap: 472.274s -> 472.978s (0.15%)
Artifact size: 390.70 MiB -> 390.71 MiB (0.00%)

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Oct 14, 2025

This comment has been minimized.

Sign in to view

rust-bors bot added a commit that referenced this pull request Oct 15, 2025

Auto merge of #147687 - cjgillot:noshallow-init-box, r=<try>

472fdbf

Forbid ShallowInitBox after box deref elaboration.

This comment has been minimized.

Sign in to view

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Oct 15, 2025

This comment has been minimized.

Sign in to view

rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Oct 15, 2025

cjgillot marked this pull request as ready for review October 16, 2025 19:05

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Oct 16, 2025

rustbot assigned nnethercote Oct 16, 2025

nnethercote reviewed Oct 17, 2025

View reviewed changes

cjgillot force-pushed the noshallow-init-box branch from 8bfd97c to 23999f0 Compare October 18, 2025 18:47

This comment has been minimized.

Sign in to view

WaffleLapkin reviewed Oct 18, 2025

View reviewed changes

compiler/rustc_mir_transform/src/elaborate_box_derefs.rs Show resolved Hide resolved

compiler/rustc_mir_transform/src/validate.rs Show resolved Hide resolved

nnethercote added the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Oct 21, 2025

cjgillot added 2 commits October 22, 2025 00:52

Elaborate ShallowInitBox too.

51275e8

Introduce indexvec macro.

0156eaf

cjgillot force-pushed the noshallow-init-box branch from e4228be to 0156eaf Compare October 22, 2025 00:52

bors added the merged-by-bors This PR was explicitly merged by bors. label Oct 22, 2025

bors merged commit f5e2df7 into rust-lang:master Oct 22, 2025
12 checks passed

rustbot added this to the 1.92.0 milestone Oct 22, 2025

bors mentioned this pull request Oct 22, 2025

Extend GVN to perform local value numbering. #143333

Draft

cjgillot deleted the noshallow-init-box branch October 22, 2025 21:39

Forbid ShallowInitBox after box deref elaboration. #147687

Forbid ShallowInitBox after box deref elaboration. #147687

Uh oh!

Conversation

cjgillot commented Oct 14, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

cjgillot commented Oct 15, 2025

Uh oh!

This comment has been minimized.

This comment has been minimized.

rust-bors bot commented Oct 15, 2025

Uh oh!

This comment has been minimized.

rust-timer commented Oct 15, 2025

Overall result: no relevant changes - no action needed

Uh oh!

rustbot commented Oct 16, 2025

Uh oh!

rustbot commented Oct 16, 2025

Uh oh!

Choose a reason for hiding this comment

Uh oh!

nnethercote commented Oct 17, 2025

Uh oh!

bors commented Oct 18, 2025

Uh oh!

This comment has been minimized.

Uh oh!

Uh oh!

nnethercote commented Oct 21, 2025

Uh oh!

rustbot commented Oct 22, 2025

Uh oh!

cjgillot commented Oct 22, 2025

Uh oh!

bors commented Oct 22, 2025

Uh oh!

bors commented Oct 22, 2025

Uh oh!

bors commented Oct 22, 2025

Uh oh!

Uh oh!

github-actions bot commented Oct 22, 2025

Test differences

Job duration changes

Uh oh!

rust-timer commented Oct 22, 2025

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

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

6 participants

cjgillot commented Oct 14, 2025 •

edited

Loading