start collecting some things ALLOWED by stacked borrows in a run-pass test

tests/run-pass/deref_partially_dangling_raw.rs

@@ -1,9 +0,0 @@
-// Deref a raw ptr to access a field of a large struct, where the field
-// is allocated but not the entire struct is.
-// For now, we want to allow this.
-
-fn main() {
-    let x = (1, 1);
-    let xptr = &x as *const _ as *const (i32, i32, i32);
-    let _val = unsafe { (*xptr).1 };
-}

tests/run-pass/stacked-borrows.rs

@@ -0,0 +1,27 @@
+// Test various stacked-borrows-related things.
+fn main() {
+    deref_partially_dangling_raw();
+    read_does_not_invalidate();
+}
+
+// Deref a raw ptr to access a field of a large struct, where the field
+// is allocated but not the entire struct is.
+// For now, we want to allow this.
+fn deref_partially_dangling_raw() {
+    let x = (1, 1);
+    let xptr = &x as *const _ as *const (i32, i32, i32);
+    let _val = unsafe { (*xptr).1 };
+}
+
+// Make sure that reading from an `&mut` does, like reborrowing to `&`,
+// NOT invalidate other reborrows.
+fn read_does_not_invalidate() {
+    fn foo(x: &mut (i32, i32)) -> &i32 {
+        let xraw = x as *mut (i32, i32);
+        let ret = unsafe { &(*xraw).1 };
+        let _val = x.1; // we just read, this does NOT invalidate the reborrows.
+        ret
+    }
+
+    foo(&mut (1, 2));
+}