generator layout: ignore fake borrows

[lcnr]

fixes #117059

We emit fake shallow borrows in case the scrutinee place uses a Deref and there is a match guard. This is necessary to prevent the match guard from mutating the scrutinee:

rust/compiler/rustc_mir_build/src/build/matches/mod.rs

Lines 1250 to 1265 in fab1054

	// Insert a borrows of prefixes of places that are bound and are
	// behind a dereference projection.
	//
	// These borrows are taken to avoid situations like the following:
	//
	// match x[10] {
	// _ if { x = &[0]; false } => (),
	// y => (), // Out of bounds array access!
	// }
	//
	// match *x {
	// // y is bound by reference in the guard and then by copy in the
	// // arm, so y is 2 in the arm!
	// y if { y == 1 && (x = &2) == () } => y,
	// _ => 3,
	// }

These fake borrows end up impacting the generator witness computation in mir_generator_witnesses, which causes the issue in #117059. This PR now completely ignores fake borrows during this computation. This is sound as thse are always removed after analysis and the actual computation of the generator layout happens afterwards.

Only the second commit impacts behavior, and could be backported by itself.

r? types

[rustbot]

This PR changes MIR

cc @oli-obk, @RalfJung, @JakobDegen, @davidtwco, @celinval, @vakaras

Some changes occurred to the core trait solver

cc @rust-lang/initiative-trait-system-refactor

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

This PR changes Stable MIR

cc @oli-obk, @celinval, @spastorino, @ouz-a

[BoxyUwU]

r? types

[lcnr]

Going to use a t-types fcp again, merging once the FCP period starts without waiting for 10 days.

@rfcbot fcp merge

[rfcbot]

Team member @lcnr has proposed to merge this. The next step is review by the rest of the tagged team members:

• @BoxyUwU
• @aliemjay
• @compiler-errors
• @jackh726
• @lcnr
• @nikomatsakis
• @oli-obk
• @spastorino

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

See this document for info about what commands tagged team members can give me.

[jackh726]

r=me when FCP completes

[lcnr]

(checked the box for @oli-obk as they're currently not available)

[aliemjay]

@rfcbot reviewed

[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔

[lcnr]

@bors r=jackh726 p=1

[bors]

⌛ Testing commit 92267c9 with merge 14575e5...

[apiraino]

Beta backport accepted as per compiler team on Zulip

@rustbot label +beta-accepted

[wesleywiser]

It is possible to only backport the second commit (after fixing the comment here which belongs in the next commit). However, T-compiler is fine with taking the whole PR as the rest of the rest of the changes are mechanical in nature.

[rust-log-analyzer]

The job x86_64-apple-1 failed! Check out the build log: (web) (plain)

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

SCCACHE_BUCKET=rust-lang-ci-sccache2
SCRIPT=./x.py --stage 2 test --skip tests/ui --skip tests/rustdoc --skip tests/run-make-fulldeps
SHELL=/bin/bash
SHLVL=3
SSH_AUTH_SOCK=/private/tmp/com.apple.launchd.A9bQDgyjTu/Listeners
STATS_EXT=true
STATS_EXTP=https://provjobdsettingscdn.blob.core.windows.net/settings/provjobdsettings-0.5.154/provjobd.data
STATS_RDCL=true
STATS_TIS=mining

[bors]

💔 Test failed - checks-actions

[lcnr]

@bors retry

seems spurious

[bors]

⌛ Testing commit 92267c9 with merge b7583d3...

[bors]

☀️ Test successful - checks-actions
Approved by: jackh726
Pushing b7583d3 to master...

[rust-timer]

Finished benchmarking commit (b7583d3): comparison URL.

Overall result: ✅ improvements - no action needed

@rustbot label: -perf-regression

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.4%	[-0.4%, -0.4%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.4%	[-0.4%, -0.4%]	1

Max RSS (memory usage)

Results

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)	0.6%	[0.4%, 0.9%]	7
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.6%	[-0.7%, -0.5%]	4
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.2%	[-0.7%, 0.9%]	11

Cycles

Results

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)	-	-	0
Improvements ✅ (primary)	-0.4%	[-0.5%, -0.4%]	2
Improvements ✅ (secondary)	-13.6%	[-13.8%, -13.4%]	2
All ❌✅ (primary)	-0.4%	[-0.5%, -0.4%]	2

Binary size

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

Bootstrap: 663.537s -> 662.476s (-0.16%)
Artifact size: 308.81 MiB -> 308.77 MiB (-0.01%)