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

serde_codegen_internals/src/attr.rs

@@ -167,7 +167,14 @@ impl Item {
 
                     // Parse `#[serde(default)]`
                     MetaItem(Word(ref name)) if name == "default" => {
-                        default.set_true();
+                        match item.body {
+                            syn::Body::Struct(_) => {
+                                default.set_true();
+                            }
+                            _ => {
+                                cx.error("#[serde(default)] can only be used on structs")
+                            }
+                        }
                     }
 
                     // Parse `#[serde(bound="D: Serialize")]`

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() { }