Only allow #[serde(default)]` on structs

test_suite/tests/compile-fail/default-attribute/enum.rs

@@ -0,0 +1,8 @@
+#[macro_use]
+extern crate serde_derive;
+
+#[derive(Deserialize)] //~ ERROR: proc-macro derive panicked
+#[serde(default)] //~^ HELP: #[serde(default)] can only be used on structs
+enum E {
+    S { f: u8 },
+}

test_suite/tests/compile-fail/default-attribute/nameless_struct_fields.rs

@@ -0,0 +1,8 @@
+#[macro_use]
+extern crate serde_derive;
+
+#[derive(Deserialize)] //~ ERROR: proc-macro derive panicked
+#[serde(default)] //~^ HELP: #[serde(default)] can only be used on structs
+struct T(u8, u8);
+
+fn main() { }