Improve UI test coverage

With this commit I believe I've covered all `compile_error!`-based
errors.

test_suite/tests/ui/default-attribute/nameless_struct_fields_path.rs

@@ -0,0 +1,8 @@
+#[macro_use]
+extern crate serde_derive;
+
+#[derive(Deserialize)]
+#[serde(default = "default_t")]
+struct T(u8, u8);
+
+fn main() {}