TEST: Add test that ensures the MaybeUninit impl is used on nightly

Author: bluss

.travis.yml

@@ -19,13 +19,16 @@ matrix:
     - rust: nightly
       env:
         - NODEFAULT=1
+        - ARRAYVECTEST_ENSURE_UNION=1
     - rust: nightly
       env:
       - NODROP_FEATURES='use_needs_drop'
+      - ARRAYVECTEST_ENSURE_UNION=1
     - rust: nightly
       env:
       - FEATURES='serde use_union'
       - NODROP_FEATURES='use_union'
+      - ARRAYVECTEST_ENSURE_UNION=1
 branches:
   only:
     - master

tests/tests.rs

@@ -508,3 +508,12 @@ fn test_sizes_129_255() {
     ArrayVec::from([0u8; 255]);
 }
 
+
+#[test]
+fn test_nightly_uses_maybe_uninit() {
+    if option_env!("ARRAYVECTEST_ENSURE_UNION").map(|s| !s.is_empty()).unwrap_or(false) {
+        assert!(cfg!(has_manually_drop_in_union));
+        type ByteArray = ArrayVec<[u8; 4]>;
+        assert!(mem::size_of::<ByteArray>() == 5);
+    }
+}