Add regression tests for negative impls not showing their items

tests/rustdoc/negative-impl-no-items.rs

@@ -0,0 +1,26 @@
+// This test ensures that negative impls don't have items listed inside them.
+
+#![feature(negative_impls)]
+#![crate_name = "foo"]
+
+pub struct Thing;
+
+//@ has 'foo/struct.Thing.html'
+// We check the full path to ensure there is no `<details>` element.
+//@ has - '//div[@id="trait-implementations-list"]/section[@id="impl-Iterator-for-Thing"]/h3' \
+// 'impl !Iterator for Thing'
+impl !Iterator for Thing {}
+
+// This struct will allow us to compare both paths.
+pub struct Witness;
+
+//@ has 'foo/struct.Witness.html'
+//@ has - '//div[@id="trait-implementations-list"]/details//section[@id="impl-Iterator-for-Witness"]/h3' \
+// 'impl Iterator for Witness'
+impl Iterator for Witness {
+    type Item = u8;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        None
+    }
+}