1 files changed, 67 insertions, 0 deletions
diff --git a/src/test/ui/nll/issue-45696-scribble-on-boxed-borrow.rs b/src/test/ui/nll/issue-45696-scribble-on-boxed-borrow.rs
new file mode 100644
index 000000000..637cf278f
--- /dev/null
+++ b/src/test/ui/nll/issue-45696-scribble-on-boxed-borrow.rs
@@ -0,0 +1,67 @@
+// rust-lang/rust#45696: This test is checking that we *cannot* return
+// mutable borrows that would be scribbled over by destructors before
+// the return occurs.
+
+// ignore-compare-mode-polonius
+
+struct Scribble<'a>(&'a mut u32);
+
+impl<'a> Drop for Scribble<'a> { fn drop(&mut self) { *self.0 = 42; } }
+
+// this is okay: The `Scribble` here *has* to strictly outlive `'a`
+fn borrowed_scribble<'a>(s: &'a mut Scribble) -> &'a mut u32 {
+    &mut *s.0
+}
+
+// this, by analogy to previous case, is also okay.
+fn boxed_borrowed_scribble<'a>(s: Box<&'a mut Scribble>) -> &'a mut u32 {
+    &mut *(*s).0
+}
+
+// this, by analogy to previous case, is also okay.
+fn boxed_boxed_borrowed_scribble<'a>(s: Box<Box<&'a mut Scribble>>) -> &'a mut u32 {
+    &mut *(**s).0
+}
+
+// this is not okay: in between the time that we take the mutable
+// borrow and the caller receives it as a return value, the drop of
+// `s` will scribble on it, violating our aliasing guarantees.
+//
+// * (Maybe in the future the two-phase borrows system will be
+//   extended to support this case. But for now, it is an error in
+//   NLL, even with two-phase borrows.)
+fn scribbled<'a>(s: Scribble<'a>) -> &'a mut u32 {
+    &mut *s.0 //~ ERROR borrow may still be in use when destructor runs [E0713]
+}
+
+// This, by analogy to previous case, is *also* not okay.
+fn boxed_scribbled<'a>(s: Box<Scribble<'a>>) -> &'a mut u32 {
+    &mut *(*s).0 //~ ERROR borrow may still be in use when destructor runs [E0713]
+}
+
+// This, by analogy to previous case, is *also* not okay.
+fn boxed_boxed_scribbled<'a>(s: Box<Box<Scribble<'a>>>) -> &'a mut u32 {
+    &mut *(**s).0 //~ ERROR borrow may still be in use when destructor runs [E0713]
+}
+
+fn main() {
+    let mut x = 1;
+    {
+        let mut long_lived = Scribble(&mut x);
+        *borrowed_scribble(&mut long_lived) += 10;
+        // (Scribble dtor runs here, after `&mut`-borrow above ends)
+    }
+    {
+        let mut long_lived = Scribble(&mut x);
+        *boxed_borrowed_scribble(Box::new(&mut long_lived)) += 10;
+        // (Scribble dtor runs here, after `&mut`-borrow above ends)
+    }
+    {
+        let mut long_lived = Scribble(&mut x);
+        *boxed_boxed_borrowed_scribble(Box::new(Box::new(&mut long_lived))) += 10;
+        // (Scribble dtor runs here, after `&mut`-borrow above ends)
+    }
+    *scribbled(Scribble(&mut x)) += 10;
+    *boxed_scribbled(Box::new(Scribble(&mut x))) += 10;
+    *boxed_boxed_scribbled(Box::new(Box::new(Scribble(&mut x)))) += 10;
+}