issue-46589 passes in Polonius and fails in NLL, duplicate it and manually check each outcome

src/test/ui/nll/issue-46589.rs

@@ -1,3 +1,9 @@
+// This tests passes in Polonius mode, so is skipped in the automated compare-mode.
+// We will manually check it passes in Polonius tests, as we can't have a test here
+// which conditionally passes depending on a test revision/compile-flags.
+
+// ignore-compare-mode-polonius
+
 struct Foo;
 
 impl Foo {

src/test/ui/nll/issue-46589.stderr

@@ -1,5 +1,5 @@
 error[E0499]: cannot borrow `**other` as mutable more than once at a time
-  --> $DIR/issue-46589.rs:17:21
+  --> $DIR/issue-46589.rs:23:21
    |
 LL |         *other = match (*other).get_self() {
    |                        -------- first mutable borrow occurs here

src/test/ui/nll/polonius/issue-46589.rs

@@ -0,0 +1,32 @@
+// This test is a copy of `ui/nll/issue-46589.rs` which fails in NLL but succeeds in Polonius.
+// As we can't have a test here which conditionally passes depending on a test
+// revision/compile-flags. We ensure here that it passes in Polonius mode.
+
+// check-pass
+// compile-flags: -Z borrowck=mir -Z polonius
+// ignore-compare-mode-nll
+
+struct Foo;
+
+impl Foo {
+    fn get_self(&mut self) -> Option<&mut Self> {
+        Some(self)
+    }
+
+    fn new_self(&mut self) -> &mut Self {
+        self
+    }
+
+    fn trigger_bug(&mut self) {
+        let other = &mut (&mut *self);
+
+        *other = match (*other).get_self() {
+            Some(s) => s,
+            None => (*other).new_self()
+        };
+
+        let c = other;
+    }
+}
+
+fn main() {}