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An assignment through a reference (`*x = y`, `*slot = None`) is rewritten into a reborrow `m1 = &mut *x` followed by `^m1 = y`, and the old value `*m1` was discarded without being dropped. When it carries a `&mut`, that borrow's prophecy was never resolved, leaving the lender unconstrained after the borrow ends. A value behind a reference cannot have been moved out, so drop it at the assignment. Local-rooted places are left alone since their old value may already have been moved. Fixes #310 Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01KXQQHZrWQHhtpyTtWNgBP3
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Fixes #310.
An assignment through a reference (
*x = y,*slot = None) is rewritten byReborrowVisitorintom1 = &mut *x; *m1 = y, andassign_to_localonly assumes^m1 = y. The old value*m1was thrown away without being dropped. When it contains a&mut, that borrow's prophecy was never resolved, so the lender was havoc'd after the borrow ended.Change
ReborrowVisitor::visit_assign: if the assigned place goes through aDerefof a reference, drop*m1(the old value) once the rvalue's reborrows have been processed. A value behind a reference can't have been moved out. Local-rooted places (s.f = ..,*boxed = ..) are left alone because their old value may already have been moved, and resolving it again would be unsound.Env::drop_place/Analyzer::drop_place: these are the same dropping assumption asdrop_local, applied to a place.tests/ui/{pass,fail}/mut_ref_overwrite.rs: a callee writes through*xand then retargets it.passwasUnsatbefore this change.Testing
cargo test: 386 passed (Z3 5.0.0 + the pinned PCSat wrapper)cargo fmt --check,cargo clippy -- -D warnings: clean&mut-carrying value through a reference (*x = y,*slot = None) never resolves the overwritten borrow's prophecy, so the lender is havoc'd — even writes made before the overwrite are lost #310 now verify, and their flipped assertions are stillUnsat.🤖 Generated with Claude Code
https://claude.ai/code/session_01KXQQHZrWQHhtpyTtWNgBP3
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