test Ref/RefMut protector interactions

tests/run-pass/stacked-borrows/refcell.rs

@@ -0,0 +1,68 @@
+use std::cell::{RefCell, Ref, RefMut};
+
+fn main() {
+    basic();
+    ref_protector();
+    ref_mut_protector();
+}
+
+fn basic() {
+    let c = RefCell::new(42);
+    {
+        let s1 = c.borrow();
+        let _x: i32 = *s1;
+        let s2 = c.borrow();
+        let _x: i32 = *s1;
+        let _y: i32 = *s2;
+        let _x: i32 = *s1;
+        let _y: i32 = *s2;
+    }
+    {
+        let mut m = c.borrow_mut();
+        let _z: i32 = *m;
+        {
+            let s: &i32 = &*m;
+            let _x = *s;
+        }
+        *m = 23;
+        let _z: i32 = *m;
+    }
+    {
+        let s1 = c.borrow();
+        let _x: i32 = *s1;
+        let s2 = c.borrow();
+        let _x: i32 = *s1;
+        let _y: i32 = *s2;
+        let _x: i32 = *s1;
+        let _y: i32 = *s2;
+    }
+}
+
+// Adding a Stacked Borrows protector for `Ref` would break this
+fn ref_protector() {
+    fn break_it(rc: &RefCell<i32>, r: Ref<'_, i32>) {
+        // `r` has a shared reference, it is passed in as argument and hence
+        // a protector is added that marks this memory as read-only for the entire
+        // duration of this function.
+        drop(r);
+        // *oops* here we can mutate that memory.
+        *rc.borrow_mut() = 2;
+    }
+
+    let rc = RefCell::new(0);
+    break_it(&rc, rc.borrow())
+}
+
+fn ref_mut_protector() {
+    fn break_it(rc: &RefCell<i32>, r: RefMut<'_, i32>) {
+        // `r` has a shared reference, it is passed in as argument and hence
+        // a protector is added that marks this memory as inaccessible for the entire
+        // duration of this function
+        drop(r);
+        // *oops* here we can mutate that memory.
+        *rc.borrow_mut() = 2;
+    }
+
+    let rc = RefCell::new(0);
+    break_it(&rc, rc.borrow_mut())
+}