From 77376f39eaca96eb12a6fe2c5bae3403ff478478 Mon Sep 17 00:00:00 2001
From: Armin Ronacher <armin.ronacher@active-4.com>
Date: Sun, 1 Apr 2018 22:11:21 +0200
Subject: [PATCH] Produce error message for use of flatten in tuple structs

---
 serde_derive_internals/src/check.rs              | 11 ++++++++++-
 .../conflict/flatten-newtype-struct.rs           | 16 ++++++++++++++++
 .../conflict/flatten-tuple-struct.rs             | 16 ++++++++++++++++
 3 files changed, 42 insertions(+), 1 deletion(-)
 create mode 100644 test_suite/tests/compile-fail/conflict/flatten-newtype-struct.rs
 create mode 100644 test_suite/tests/compile-fail/conflict/flatten-tuple-struct.rs

diff --git a/serde_derive_internals/src/check.rs b/serde_derive_internals/src/check.rs
index 31980a398..6db99a173 100644
--- a/serde_derive_internals/src/check.rs
+++ b/serde_derive_internals/src/check.rs
@@ -47,11 +47,20 @@ fn check_flatten(cx: &Ctxt, cont: &Container) {
         Data::Enum(_) => {
             assert!(!cont.attrs.has_flatten());
         }
-        Data::Struct(_, _) => {
+        Data::Struct(style, _) => {
             for field in cont.data.all_fields() {
                 if !field.attrs.flatten() {
                     continue;
                 }
+                match style {
+                    Style::Tuple => {
+                        cx.error("#[serde(flatten)] cannot be used on tuple structs");
+                    }
+                    Style::Newtype => {
+                        cx.error("#[serde(flatten)] cannot be used on newtype structs");
+                    }
+                    _ => {}
+                }
                 if field.attrs.skip_serializing() {
                     cx.error(
                         "#[serde(flatten] can not be combined with \
diff --git a/test_suite/tests/compile-fail/conflict/flatten-newtype-struct.rs b/test_suite/tests/compile-fail/conflict/flatten-newtype-struct.rs
new file mode 100644
index 000000000..07566f8c7
--- /dev/null
+++ b/test_suite/tests/compile-fail/conflict/flatten-newtype-struct.rs
@@ -0,0 +1,16 @@
+// Copyright 2018 Serde Developers
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#[macro_use]
+extern crate serde_derive;
+
+#[derive(Serialize)] //~ ERROR: proc-macro derive panicked
+//~^ HELP: #[serde(flatten)] cannot be used on newtype structs
+struct Foo(#[serde(flatten)] HashMap<String, String>);
+
+fn main() {}
diff --git a/test_suite/tests/compile-fail/conflict/flatten-tuple-struct.rs b/test_suite/tests/compile-fail/conflict/flatten-tuple-struct.rs
new file mode 100644
index 000000000..167bdbdac
--- /dev/null
+++ b/test_suite/tests/compile-fail/conflict/flatten-tuple-struct.rs
@@ -0,0 +1,16 @@
+// Copyright 2018 Serde Developers
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#[macro_use]
+extern crate serde_derive;
+
+#[derive(Serialize)] //~ ERROR: proc-macro derive panicked
+//~^ HELP: #[serde(flatten)] cannot be used on tuple structs
+struct Foo(u32, #[serde(flatten)] HashMap<String, String>);
+
+fn main() {}