Fix ElaborateBoxDerefs on debug varinfo

[compiler-errors]

Slightly simplifies the ElaborateBoxDerefs pass to fix cases where it was applying the wrong projections to debug var infos containing places that deref boxes.

From what I can tell¹, we don't actually have any tests (or code anywhere, really) that exercise debug x => *(...: Box<T>), and it's very difficult to trigger this in surface Rust, so I wrote a custom MIR test.

What happens is that the pass was turning *(SOME_PLACE: Box<T>) into *(*((((SOME_PLACE).0: Unique<T>).0: NonNull<T>).0: *const T)) in debug var infos. In particular, notice the double deref, which was wrong.

This is the root cause of #128554, so this PR fixes #128554 as well. The reason that async closures was affected is because of the way that we compute the ByMove body, which resulted in *(...: Box<T>) in debug var info. But this really has nothing to do with async closures.

Footnotes

1. Validated by literally replacing the if elem == PlaceElem::Deref && base_ty.is_box() { ... } innards with a panic!(), which compiled all of stage2 without panicking. ↩

[rustbot]

r? @jieyouxu

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[rustbot]

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

[compiler-errors]

I think the problem here has to do with the indexing of this slice (and the other [last_deref..] below), but I think it was pretty difficult to follow so I simplified the logic to just build up the projections more simply, one at a time.

[jieyouxu]

Rerolling since I'm not familiar with this. r? mir

[saethlin]

we don't actually have any tests (or code anywhere, really) that exercise debug x => *(...: Box<T>)

Originally I was a bit surprised by this then I started looking at our debuginfo test suite more and now I'm not surprised at all. It's a Christmas Miracle that debuggers work as well with Rust as they do.

@bors r+ rollup=iffy

[bors]

📌 Commit 2e52d61 has been approved by saethlin

It is now in the queue for this repository.

[bors]

⌛ Testing commit 2e52d61 with merge 730d5d4...

[bors]

☀️ Test successful - checks-actions
Approved by: saethlin
Pushing 730d5d4 to master...

[rust-log-analyzer]

A job failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

  PR_MESSAGE: Automation to keep dependencies in `Cargo.lock` current.
following is the output from `cargo update`:
  COMMIT_MESSAGE: cargo update 
##[endgroup]
info: syncing channel updates for 'beta-2024-07-26-x86_64-unknown-linux-gnu'
info: latest update on 2024-07-26, rust version 1.81.0-beta.2 (08328a323 2024-07-25)
info: downloading component 'rust-std'
info: downloading component 'rustc'
info: installing component 'cargo'
info: installing component 'rust-std'
info: installing component 'rust-std'
info: installing component 'rustc'

  beta-2024-07-26-x86_64-unknown-linux-gnu installed - rustc 1.81.0-beta.2 (08328a323 2024-07-25)

info: using existing install for 'beta-2024-07-26-x86_64-unknown-linux-gnu'
info: default toolchain set to 'beta-2024-07-26-x86_64-unknown-linux-gnu'

  beta-2024-07-26-x86_64-unknown-linux-gnu unchanged - rustc 1.81.0-beta.2 (08328a323 2024-07-25)

##[group]Run cargo update 2>&1 | sed '/crates.io index/d' | tee -a cargo_update.log
cargo update 2>&1 | sed '/crates.io index/d' | tee -a cargo_update.log
---
    Updating zerocopy v0.7.34 -> v0.7.35
    Updating zerocopy-derive v0.7.34 -> v0.7.35
    Updating zerovec v0.10.2 -> v0.10.4
    Updating zerovec-derive v0.10.2 -> v0.10.3
note: pass `--verbose` to see 89 unchanged dependencies behind latest
##[group]Run echo -e "\nrustbook dependencies:" >> cargo_update.log
echo -e "\nrustbook dependencies:" >> cargo_update.log
cargo update --manifest-path src/tools/rustbook 2>&1 | sed '/crates.io index/d' | tee -a cargo_update.log

[compiler-errors]

wtf

[saethlin]

That's https://github.com/rust-lang/rust/blob/730d5d4095a264ef5f7c0a0781eea68c15431d45/.github/workflows/dependencies.yml it must be broken again, or still.

[saethlin]

Ah: https://github.com/rust-lang/rust/actions/runs/10335477626/job/28610080113#step:5:16

[rust-timer]

Finished benchmarking commit (730d5d4): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

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

Max RSS (memory usage)

Results (secondary 1.5%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	5.5%	[5.5%, 5.5%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.5%	[-2.5%, -2.5%]	1
All ❌✅ (primary)	-	-	0

Cycles

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

Binary size

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

Bootstrap: 756.781s -> 757.993s (0.16%)
Artifact size: 339.30 MiB -> 339.27 MiB (-0.01%)