Auto merge of #1789 - RalfJung:array-to-elem, r=RalfJung

stacked borrows: ensure array-to-elem casts behave correctly

When "as"-casting an entire array, that should create a raw ptr usable for all elements in the array, even if we immediately cast to the element type.

rust-version

@@ -1 +1 @@
-0309953232d9957aef4c7c5a24fcb30735b2066b
+1773f14a24c49356b384e45ebb45643bc9bef2c4

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

@@ -15,6 +15,7 @@ fn main() {
     shr_and_raw();
     disjoint_mutable_subborrows();
     raw_ref_to_part();
+    array_casts();
 }
 
 // Make sure that reading from an `&mut` does, like reborrowing to `&`,
@@ -174,3 +175,14 @@ fn raw_ref_to_part() {
     assert!(typed.extra == 42);
     drop(unsafe { Box::from_raw(whole) });
 }
+
+/// When casting an array reference to a raw element ptr, that should cover the whole array.
+fn array_casts() {
+    let mut x: [usize; 2] = [0, 0];
+    let p = &mut x as *mut usize;
+    unsafe { *p.add(1) = 1; }
+
+    let x: [usize; 2] = [0, 1];
+    let p = &x as *const usize;
+    assert_eq!(unsafe { *p.add(1) }, 1);
+}