test another version of 'creating a shared ref must not leak the Unique'

tests/compile-fail/stacked_borrows/illegal_read7.rs

@@ -0,0 +1,20 @@
+// Creating a shared reference does not leak the data to raw pointers,
+// not even when interior mutability is involved.
+
+use std::cell::Cell;
+use std::ptr;
+
+fn main() { unsafe {
+    let x = &mut Cell::new(0);
+    let raw = x as *mut Cell<i32>;
+    let x = &mut *raw;
+    let _shr = &*x;
+    // The state here is interesting because the top of the stack is [Unique, SharedReadWrite],
+    // just like if we had done `x as *mut _`.
+    // If we said that reading from a lower item is fine if the top item is `SharedReadWrite`
+    // (one way to maybe preserve a stack discipline), then we could now read from `raw`
+    // without invalidating `x`.  That would be bad!  It would mean that creating `shr`
+    // leaked `x` to `raw`.
+    let _val = ptr::read(raw);
+    let _val = *x.get_mut(); //~ ERROR borrow stack
+} }