Resolver: Introduce `CmRef` which has a speclative borrow variant for `CmRefCell` by LorrensP-2158466 · Pull Request #160271 · rust-lang/rust

LorrensP-2158466 · 2026-07-31T09:47:18Z

We now introduce CmRef that is returned by CmRefCell::borrow, which does a normal borrow of the underlying T if we are in speculative resolution. We do not alter state during speculative resolution and because it will be run in parallel in the future, updating the RefCell borrow counters is not possible. Thus we just return &T that is "untracked".

Also allows us to remove the CmRefCell wrapper around the external module resolution table.

r? @petrochenkov

… the underlying `RefCell`

LorrensP-2158466 · 2026-07-31T09:49:42Z

-        pub(crate) fn borrow(&self) -> Ref<'_, T> {
-            self.0.borrow()
+        pub(crate) fn borrow<'ra, 'tcx>(&self, r: &Resolver<'ra, 'tcx>) -> CmRef<'_, T> {
+            if r.assert_speculative {
+                // SAFETY: We are in `speculative_resolution`, so we cannot mutate the `CmRefCell`,
+                // not even the borrow counters. However, because of the way we use
+                // `assert_speculative`, we know no borrow to the underlying value is alive. Thus
+                // making this "unguarded_borrow" safe.
+                //
+                // Note however, that it is unsafe to borrow this `CmRefCell` and then immediately
+                // flip the `assert_speculative` flag, as that will cause aliased borrows. No
+                // idea how to mitigate this...
+                let unguarded_borrow = unsafe {
+                    self.0.try_borrow_unguarded().expect("`CmRefCell` already mutably borrowed.")
+                };
+                CmRef::Normal(unguarded_borrow)
+            } else {
+                CmRef::Ref(self.0.borrow())
+            }


Because of the way we do speculative resolution, this is "safe". However it is perfectly fine to do the following:

let mut resolver = Resolver { assert_speculative: true, data: CmRefCell::new(10), }; // we have a spec resolver and take a speculative borrow: let spec_bor = resolver.data.borrow(&resolver); // we then switch to non-speculative: resolver.assert_speculative = false; // and then we take a mutable borrow on `data`: let mut mut_bor = resolver.data.borrow_mut(&resolver); // Miri will report an error here: // Undefined Behavior: trying to retag from <tag> for SharedReadOnly permission at allocxxxxx[0x8], // but that tag does not exist in the borrow stack for this location // Basically telling us that `spec_bor` is still live *mut_bor = 20;

View changes since the review

petrochenkov · 2026-07-31T10:49:47Z

@bors try @rust-timer queue

Resolver: Introduce `CmRef` which has a speclative borrow variant for `CmRefCell`

petrochenkov · 2026-07-31T11:04:02Z

+        /// A tracked borrow of a [`CmRefCell`]
+        Ref(Ref<'b, T>),
+        /// An untracked or normal reference (not dynamically borrow-checked by `RefCell`)
+        Normal(&'b T),


Maybe rename the variants to Tracked and Untracked as the comments say?

View changes since the review

petrochenkov · 2026-07-31T11:04:18Z

+
+        fn deref(&self) -> &Self::Target {
+            match self {
+                CmRef::Ref(r) => r.deref(),


Suggested change

CmRef::Ref(r) => r.deref(),

CmRef::Ref(r) => r,

Nit: the coercion should work here.

View changes since the review

petrochenkov · 2026-07-31T11:07:44Z

+                // flip the `assert_speculative` flag, as that will cause aliased borrows. No
+                // idea how to mitigate this...
+                let unguarded_borrow = unsafe {
+                    self.0.try_borrow_unguarded().expect("`CmRefCell` already mutably borrowed.")


Suggested change

self.0.try_borrow_unguarded().expect("`CmRefCell` already mutably borrowed.")

self.0.try_borrow_unguarded().unwrap()

expect on Results usually duplicates the message that Result::Err already reports on unwrap.

View changes since the review

petrochenkov · 2026-07-31T11:11:07Z

        #[track_caller]
-        pub(crate) fn borrow(&self) -> Ref<'_, T> {
-            self.0.borrow()
+        pub(crate) fn borrow<'ra, 'tcx>(&self, r: &Resolver<'ra, 'tcx>) -> CmRef<'_, T> {


Technically this change is unnecessary for removing CmRefCell from Resolutions (at least until parallelism is enabled).
We could just always return CmRef::Ref for Resolutions::Local.

View changes since the review

Yes, that is true. Do you want to keep it here or in a later pr?

Let's wait for the perf results.

Let's revert it for now and benchmark again.

rust-bors · 2026-07-31T12:56:34Z

☀️ Try build successful (CI)
Build commit: caf2dd1 (caf2dd11317e37ec790b3746057fae8a36779c87)
Base parent: 922325b (922325bb13bfea5b41454318563f2a65e83c2336)

rust-timer · 2026-07-31T14:08:25Z

Finished benchmarking commit (caf2dd1): comparison URL.

Overall result: ❌ regressions - please read:

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

@bors rollup=never rustc-perf
@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)	0.2%	[0.2%, 0.3%]	34
Regressions ❌ (secondary)	0.4%	[0.1%, 0.8%]	22
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.2%	[-0.3%, -0.1%]	3
All ❌✅ (primary)	0.2%	[0.2%, 0.3%]	34

Max RSS (memory usage)

Results (primary 1.5%, secondary 1.7%)

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

	mean	range	count
Regressions ❌ (primary)	3.4%	[3.4%, 3.4%]	1
Regressions ❌ (secondary)	2.8%	[0.8%, 6.2%]	4
Improvements ✅ (primary)	-0.4%	[-0.4%, -0.4%]	1
Improvements ✅ (secondary)	-0.5%	[-0.6%, -0.4%]	2
All ❌✅ (primary)	1.5%	[-0.4%, 3.4%]	2

Cycles

Results (primary 0.3%, secondary -1.2%)

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

	mean	range	count
Regressions ❌ (primary)	1.2%	[0.4%, 2.7%]	7
Regressions ❌ (secondary)	1.7%	[0.5%, 3.5%]	15
Improvements ✅ (primary)	-1.0%	[-2.2%, -0.5%]	5
Improvements ✅ (secondary)	-5.1%	[-10.8%, -0.6%]	11
All ❌✅ (primary)	0.3%	[-2.2%, 2.7%]	12

Binary size

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

Bootstrap: 490.333s -> 490.941s (0.12%)
Artifact size: 392.58 MiB -> 390.55 MiB (-0.52%)

LorrensP-2158466 added 2 commits July 31, 2026 11:23

implement CmRef for CmRefCell, having 2 different borrow types of…

0759b71

… the underlying `RefCell`

remove CmRefCell wrapper around external resolution tables

88cdffa

rustbot assigned petrochenkov Jul 31, 2026

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Jul 31, 2026

LorrensP-2158466 commented Jul 31, 2026

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rust-bors Bot pushed a commit that referenced this pull request Jul 31, 2026

Auto merge of #160271 - LorrensP-2158466:res-cm-refcell, r=<try>

caf2dd1

Resolver: Introduce `CmRef` which has a speclative borrow variant for `CmRefCell`

petrochenkov reviewed Jul 31, 2026

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petrochenkov added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jul 31, 2026

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rustbot added perf-regression Performance regression. and removed S-waiting-on-perf Status: Waiting on a perf run to be completed. labels Jul 31, 2026

	self.0.try_borrow_unguarded().expect("`CmRefCell` already mutably borrowed.")
	self.0.try_borrow_unguarded().unwrap()

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rust-timer commented Jul 31, 2026

Overall result: ❌ regressions - please read:

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