Avoid invalidating provenance of outstanding references in `alloc_ext…

…end`. Fixes #55

src/lib.rs

@@ -264,14 +264,16 @@ impl<T> Arena<T> {
                 i += 1;
             }
         }
-        let new_slice_ref = &mut chunks.current[next_item_index..];
 
         // Extend the lifetime from that of `chunks_borrow` to that of `self`.
         // This is OK because we’re careful to never move items
         // by never pushing to inner `Vec`s beyond their initial capacity.
         // The returned reference is unique (`&mut`):
         // the `Arena` never gives away references to existing items.
-        unsafe { mem::transmute::<&mut [T], &mut [T]>(new_slice_ref) }
+        unsafe {
+            let new_len = chunks.current.len() - next_item_index;
+            slice::from_raw_parts_mut(chunks.current.as_mut_ptr().add(next_item_index), new_len)
+        }
     }
 
     /// Allocates space for a given number of values, but doesn't initialize it.

src/test.rs

@@ -321,6 +321,17 @@ fn size_hint() {
     }
 }
 
+// Ensure that `alloc_extend` doesn't violate provenance of
+// existing references. (Note: This test is pointless except
+// under miri).
+#[test]
+fn check_extend_provenance() {
+    let arena = Arena::new();
+    let a = arena.alloc(0);
+    arena.alloc_extend([0]);
+    *a = 1;
+}
+
 #[test]
 fn size_hint_low_initial_capacities() {
     #[derive(Debug, PartialEq, Eq)]