From 96393bfcc7324eca877002157ca6f44d066013b6 Mon Sep 17 00:00:00 2001
From: Armin Ronacher <armin.ronacher@active-4.com>
Date: Tue, 20 Mar 2018 21:48:25 +0100
Subject: [PATCH] Added checks for flatten attribute

---
 serde_derive_internals/src/check.rs | 35 +++++++++++++++++++++++++++++
 1 file changed, 35 insertions(+)

diff --git a/serde_derive_internals/src/check.rs b/serde_derive_internals/src/check.rs
index c3df861b..7afe42ca 100644
--- a/serde_derive_internals/src/check.rs
+++ b/serde_derive_internals/src/check.rs
@@ -14,6 +14,7 @@ use Ctxt;
 /// object. Simpler checks should happen when parsing and building the attrs.
 pub fn check(cx: &Ctxt, cont: &Container) {
     check_getter(cx, cont);
+    check_flatten(cx, cont);
     check_identifier(cx, cont);
     check_variant_skip_attrs(cx, cont);
     check_internal_tag_field_name_conflict(cx, cont);
@@ -40,6 +41,40 @@ fn check_getter(cx: &Ctxt, cont: &Container) {
     }
 }
 
+/// Flattening has some restrictions we can test.
+fn check_flatten(cx: &Ctxt, cont: &Container) {
+    match cont.data {
+        Data::Enum(_) => {
+            if cont.attrs.has_flatten() {
+                cx.error("#[serde(flatten)] is not allowed in an enum");
+            }
+        }
+        Data::Struct(_, _) => {
+            for field in cont.data.all_fields() {
+                if !field.attrs.flatten() {
+                    continue;
+                }
+                if field.attrs.skip_serializing() {
+                    cx.error(
+                        "#[serde(flatten] can not be combined with \
+                         #[serde(skip_serializing)]"
+                    );
+                } else if field.attrs.skip_serializing_if().is_some() {
+                    cx.error(
+                        "#[serde(flatten] can not be combined with \
+                         #[serde(skip_serializing_if = \"...\")]"
+                    );
+                } else if field.attrs.skip_deserializing() {
+                    cx.error(
+                        "#[serde(flatten] can not be combined with \
+                         #[serde(skip_deserializing)]"
+                    );
+                }
+            }
+        }
+    }
+}
+
 /// The `other` attribute must be used at most once and it must be the last
 /// variant of an enum that has the `field_identifier` attribute.
 ///