Ensure we do not treat all unions as not having any drop glue.

src/test/ui/union/union-custom-drop.rs

@@ -0,0 +1,19 @@
+// test for a union with a field that's a union with a manual impl Drop
+// Ensures we do not treat all unions as not having any drop glue.
+
+#![feature(untagged_unions)]
+
+union Foo {
+    bar: Bar, //~ ERROR unions may not contain fields that need dropping
+}
+
+union Bar {
+    a: i32,
+    b: u32,
+}
+
+impl Drop for Bar {
+    fn drop(&mut self) {}
+}
+
+fn main() {}

src/test/ui/union/union-custom-drop.stderr

@@ -0,0 +1,28 @@
+error[E0601]: `main` function not found in crate `union_custom_drop`
+  --> $DIR/union-custom-drop.rs:4:1
+   |
+LL | / #![feature(untagged_unions)]
+LL | |
+LL | | union Foo {
+LL | |     bar: Bar,
+...  |
+LL | |     }
+LL | | }
+   | |_^ consider adding a `main` function to `$DIR/union-custom-drop.rs`
+
+error[E0740]: unions may not contain fields that need dropping
+  --> $DIR/union-custom-drop.rs:7:5
+   |
+LL |     bar: Bar,
+   |     ^^^^^^^^
+   |
+note: `std::mem::ManuallyDrop` can be used to wrap the type
+  --> $DIR/union-custom-drop.rs:7:5
+   |
+LL |     bar: Bar,
+   |     ^^^^^^^^
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0601, E0740.
+For more information about an error, try `rustc --explain E0601`.