From 7bee7795147cd4144bc723fc71582a257954c377 Mon Sep 17 00:00:00 2001
From: Thomas de Zeeuw <thomasdezeeuw@gmail.com>
Date: Tue, 21 Feb 2017 00:15:00 +0100
Subject: [PATCH] Only allow #[serde(default)]` on structs

---
 serde_codegen_internals/src/attr.rs                      | 9 ++++++++-
 test_suite/tests/compile-fail/default-attribute/enum.rs  | 8 ++++++++
 .../default-attribute/nameless_struct_fields.rs          | 8 ++++++++
 3 files changed, 24 insertions(+), 1 deletion(-)
 create mode 100644 test_suite/tests/compile-fail/default-attribute/enum.rs
 create mode 100644 test_suite/tests/compile-fail/default-attribute/nameless_struct_fields.rs

diff --git a/serde_codegen_internals/src/attr.rs b/serde_codegen_internals/src/attr.rs
index 8abf469b..ae3af449 100644
--- a/serde_codegen_internals/src/attr.rs
+++ b/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")]`
diff --git a/test_suite/tests/compile-fail/default-attribute/enum.rs b/test_suite/tests/compile-fail/default-attribute/enum.rs
new file mode 100644
index 00000000..74c5d7ca
--- /dev/null
+++ b/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 },
+}
diff --git a/test_suite/tests/compile-fail/default-attribute/nameless_struct_fields.rs b/test_suite/tests/compile-fail/default-attribute/nameless_struct_fields.rs
new file mode 100644
index 00000000..2e8bccc6
--- /dev/null
+++ b/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() { }