Support retagging of wildcard references in tree borrows #4707

royAmmerschuber · 2025-11-17T16:42:55Z

Adds proper suport for reborrowing wildcard pointers to tree borrows.

rustbot · 2025-11-17T16:42:58Z

Thank you for contributing to Miri! A reviewer will take a look at your PR, typically within a week or two.
Please remember to not force-push to the PR branch except when you need to rebase due to a conflict or when the reviewer asks you for it.

src/borrow_tracker/tree_borrows/tree.rs

RalfJung

Overall approach looks good. Here's a bunch of comments. :)

Please also add the 3 SB tests in this area: illegal_read/write_despite_exposed*.

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RalfJung · 2025-11-24T09:06:21Z

src/borrow_tracker/tree_borrows/tree.rs

-
-            let state = perm.or_insert(node.default_location_state());
+        #[cfg(feature = "expensive-consistency-checks")]
+        self.verify_wildcard_consistency(global);


We should only need this if a wildcard is involved in the access, right? I.e. if the tag is wildcard, or if there are multiple roots.

src/borrow_tracker/tree_borrows/tree.rs

RalfJung · 2025-11-24T10:02:48Z

src/borrow_tracker/tree_borrows/diagnostics.rs

+
+    /// Print extra text if the tag is exposed.
+    fn print_exposed(&self, exposed: bool) -> S {
+        if exposed { " Exposed" } else { "" }


Suggested change

if exposed { " Exposed" } else { "" }

if exposed { " (exposed)" } else { "" }

RalfJung · 2025-11-24T10:06:29Z

tests/pass/tree_borrows/wildcard/reborrow.rs

+    // We can still access both ref1, ref2.
+    assert_eq!(*ref2, 13);


Comment and code don't quite match... we can access both, after all.
You can just explain that this is technically UB we don't catch.

RalfJung · 2025-11-24T10:07:16Z

tests/pass/tree_borrows/wildcard/reborrow.rs

+    // We should be able to access any of the references.
+    assert_eq!(*ref2, 13);


Again this doesn't quite test what the comment says.

tests/pass/tree_borrows/wildcard/reborrow.rs

RalfJung · 2025-11-24T10:15:46Z

tests/fail/tree_borrows/wildcard/cross_tree_update_main_invalid_exposed.rs

Please explain in the comment that this tests the max_local_tag logic.

rustbot · 2025-11-24T10:24:23Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

RalfJung · 2025-11-25T10:05:59Z

src/borrow_tracker/tree_borrows/tree.rs

+            };
+            // Afterwards we check all tags in arbitrary order, so that we also catch
+            // protectors on different subtrees.
+            // (This unnecessarily checks the tags of `start_idx`s subtree again)


This comment seems outdated now.

RalfJung · 2025-11-25T10:08:14Z

src/borrow_tracker/tree_borrows/tree.rs

+                TreeVisitor { nodes: &mut self.nodes, loc }.traverse_this_parents_children_other(
+                    *root,
+                    // Visit all children, skipping none.
+                    |_| ContinueTraversal::Recurse,
+                    check_strong_protector,
+                )?;


This part is still duplicated between the two branches... maybe it makes more sense to share the entire visitor invocation, not just the check_strong_protector closure?

rustbot · 2025-11-25T14:10:12Z

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.

royAmmerschuber · 2025-11-27T15:37:37Z

@rustbot ready

RalfJung

Thanks! Looks good overall but I have plenty of minor comments.

View changes since this review

src/borrow_tracker/tree_borrows/diagnostics.rs

src/borrow_tracker/tree_borrows/tree.rs

RalfJung · 2025-11-28T15:21:56Z

src/borrow_tracker/tree_borrows/tree.rs

    ) -> InterpResult<'tcx> {
        let idx = self.tag_mapping.insert(new_tag);
-        let parent_idx = self.tag_mapping.get(&parent_tag).unwrap();
+


Suggested change

src/borrow_tracker/tree_borrows/tree.rs

tests/pass/tree_borrows/wildcard/reborrow.rs

tests/pass/tree_borrows/wildcard/undetected_ub.rs

royAmmerschuber · 2025-11-29T18:36:31Z

@rustbot ready

royAmmerschuber · 2025-11-29T18:41:30Z

src/borrow_tracker/tree_borrows/tree.rs

        access_kind: AccessKind,
-        access_cause: AccessCause,
-        access_range: Option<AllocRange>,
+        access_cause: AccessCause,        //diagnostics
+        access_range: Option<AllocRange>, //diagnostics
        relatedness: AccessRelatedness,
-        span: Span,
-        location_range: Range<u64>,
+        span: Span,                 //diagnostics
+        location_range: Range<u64>, //diagnostics
        protected: bool,


It might make sense to move all the diagnostics arguments into a single struct. Since these just get passed through unchanged all the way from Tree::perform_access.

That seems like a good idea for a future PR. :)

RalfJung

Thanks! However, there are still some comments that need further clarification.

View changes since this review

src/borrow_tracker/tree_borrows/tree.rs

RalfJung

So maybe something like this?

View changes since this review

src/borrow_tracker/tree_borrows/tree.rs

tests/pass/tree_borrows/wildcard/reborrow.rs

RalfJung · 2025-12-03T15:00:00Z

This looks great, thanks! Please squash the commits. You can squash manually if there are multiple independent commits you want to preserve, or use ./miri squash (make sure to pick a suitable commit message). Then write @rustbot ready after you force-pushed the squashed PR.

@rustbot author

royAmmerschuber · 2025-12-03T15:16:49Z

@rustbot ready

resolved

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royAmmerschuber force-pushed the feature/wildcard-root branch from 99dacc6 to 52f2087 Compare November 21, 2025 10:23

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royAmmerschuber mentioned this pull request Nov 25, 2025

preliminary refactor for wildcard reborrows #4723

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royAmmerschuber force-pushed the feature/wildcard-root branch from 8c7ac81 to 4097b0a Compare November 25, 2025 14:10

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Support retagging of wildcard references in tree borrows

e1c480e

royAmmerschuber force-pushed the feature/wildcard-root branch from 97353a8 to e1c480e Compare December 3, 2025 15:13

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RalfJung enabled auto-merge December 3, 2025 15:31

RalfJung added this pull request to the merge queue Dec 3, 2025

Merged via the queue into rust-lang:master with commit e2c7e74 Dec 3, 2025
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	if exposed { " Exposed" } else { "" }
	if exposed { " (exposed)" } else { "" }

		// We can still access both ref1, ref2.
		assert_eq!(*ref2, 13);

		// We should be able to access any of the references.
		assert_eq!(*ref2, 13);

Support retagging of wildcard references in tree borrows #4707

Support retagging of wildcard references in tree borrows #4707

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royAmmerschuber commented Nov 17, 2025

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royAmmerschuber commented Nov 29, 2025

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