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

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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.

rustbot · 2025-10-18T18:47:43Z

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.

WaffleLapkin · 2025-10-18T20:04:11Z

compiler/rustc_mir_transform/src/elaborate_box_derefs.rs

@@ -1,26 +1,26 @@
 //! This pass transforms derefs of Box into a deref of the pointer inside Box.
 //!
 //! Box is not actually a pointer so it is incorrect to dereference it directly.


nit: Can you update docs here to also mention removal of ShallowBoxInit?

WaffleLapkin · 2025-10-18T20:07:50Z

compiler/rustc_mir_transform/src/validate.rs

+                if self.body.phase >= MirPhase::Runtime(RuntimePhase::Initial) {
+                    self.fail(location, format!("ShallowInitBox after ElaborateBoxDerefs"))
+                }


q: Why isn't this at the start of the block?

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

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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.

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

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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

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cjgillot added 2 commits October 18, 2025 16:30

Elaborate ShallowInitBox too.

fdd13b5

Introduce indexvec macro.

23999f0

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

WaffleLapkin reviewed Oct 18, 2025

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Forbid ShallowInitBox after box deref elaboration. #147687

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Forbid ShallowInitBox after box deref elaboration. #147687

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Overall result: no relevant changes - no action needed

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cjgillot commented Oct 14, 2025 •

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