new compile-fail test involving non-reborrowing-cast-to-raw

Author: RalfJung

tests/compile-fail/stacked_borrows/illegal_read8.rs

@@ -0,0 +1,13 @@
+// Make sure that creating a raw ptr next to a shared ref works
+// but the shared ref still gets invalidated when the raw ptr is used for writing.
+
+fn main() { unsafe {
+    use std::mem;
+    let x = &mut 0;
+    let y1: &i32 = mem::transmute(&*x); // launder lifetimes
+    let y2 = x as *mut _;
+    let _val = *y2;
+    let _val = *y1;
+    *y2 += 1;
+    let _fail = *y1; //~ ERROR borrow stack
+} }

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

@@ -136,7 +136,6 @@ fn shr_and_raw() { unsafe {
     let y2 = x as *mut _;
     let _val = *y1;
     *y2 += 1;
-    // TODO: Once this works, add compile-fail test that tries to read from y1 again.
 } }
 
 fn disjoint_mutable_subborrows() {