Add test for new DOM for empty impls

src/test/rustdoc/empty-impls.rs

@@ -0,0 +1,19 @@
+#![crate_name = "foo"]
+
+// @has foo/struct.Foo.html
+// @has - '//div[@id="synthetic-implementations-list"]/h3[@id="impl-Send"]' 'impl Send for Foo'
+pub struct Foo;
+
+pub trait EmptyTrait {}
+
+// @has - '//div[@id="trait-implementations-list"]/h3[@id="impl-EmptyTrait"]' 'impl EmptyTrait for Foo'
+impl EmptyTrait for Foo {}
+
+pub trait NotEmpty {
+    fn foo(&self);
+}
+
+// @has - '//div[@id="trait-implementations-list"]/details/summary/h3[@id="impl-NotEmpty"]' 'impl NotEmpty for Foo'
+impl NotEmpty for Foo {
+    fn foo(&self) {}
+}