Do not reborrow the operand of PtrMetadata

src/analyze/basic_block/visitor/reborrow.rs

@@ -95,6 +95,16 @@ impl<'a, 'tcx, 'ctx> mir::visit::MutVisitor<'tcx> for ReborrowVisitor<'a, 'tcx,
         self.super_assign(place, rvalue, location);
     }
 
+    fn visit_rvalue(&mut self, rvalue: &mut mir::Rvalue<'tcx>, location: mir::Location) {
+        // `PtrMetadata` only reads the length out of the reference it is applied to, so its
+        // operand needs no reborrow. `analyze_assignment` takes the shared borrow it needs.
+        if let mir::Rvalue::UnaryOp(mir::UnOp::PtrMetadata, _) = rvalue {
+            return;
+        }
+
+        self.super_rvalue(rvalue, location);
+    }
+
     // TODO: is it always true that the operand is not referred again in rvalue
     fn visit_operand(&mut self, operand: &mut mir::Operand<'tcx>, location: mir::Location) {
         let Some(p) = operand.place() else {

tests/ui/fail/slice_len_guard_mut.rs

@@ -0,0 +1,12 @@
+//@error-in-other-file: Unsat
+//@compile-flags: -C debug-assertions=off -C opt-level=2
+
+#[thrust::callable]
+fn check(v: &mut [i32], i: usize) {
+    if i < v.len() {
+        v[i] = 7;
+        assert!(v[i] == 8);
+    }
+}
+
+fn main() {}

tests/ui/pass/slice_len_guard_mut.rs

@@ -0,0 +1,16 @@
+//@check-pass
+//@compile-flags: -C debug-assertions=off -C opt-level=2
+
+// At `-C opt-level=1` and above rustc reads slice metadata straight off the `&mut [i32]`
+// local (`_len = PtrMetadata(copy _v)`) instead of through a shared reborrow, so this pins
+// down that the length `len()` returns still describes the referent the guarded index
+// reads.
+#[thrust::callable]
+fn check(v: &mut [i32], i: usize) {
+    if i < v.len() {
+        v[i] = 7;
+        assert!(v[i] == 7);
+    }
+}
+
+fn main() {}