Store flatten flag in container attributes

serde_derive/src/ser.rs

@@ -286,7 +286,7 @@ fn serialize_struct_as_struct(params: &Parameters, fields: &[Field], cattrs: &at
     }
 }
 
-fn serialize_struct_as_map(params: &Parameters, fields: &[Field], _cattrs: &attr::Container) -> Fragment {
+fn serialize_struct_as_map(params: &Parameters, fields: &[Field], cattrs: &attr::Container) -> Fragment {
     let serialize_fields = serialize_struct_visitor(
         fields,
         params,
@@ -301,11 +301,7 @@ fn serialize_struct_as_map(params: &Parameters, fields: &[Field], _cattrs: &attr
 
     let let_mut = mut_if(serialized_fields.peek().is_some());
 
-    let has_flatten = fields
+    let len = if cattrs.has_flatten() {
-        .iter()
-        .any(|field| field.attrs.flatten());
-
-    let len = if has_flatten {
         quote!(None)
     } else {
         let len = serialized_fields

serde_derive_internals/src/ast.rs

@@ -48,7 +48,7 @@ pub enum Style {
 
 impl<'a> Container<'a> {
     pub fn from_ast(cx: &Ctxt, item: &'a syn::DeriveInput) -> Container<'a> {
-        let attrs = attr::Container::from_ast(cx, item);
+        let mut attrs = attr::Container::from_ast(cx, item);
 
         let mut data = match item.data {
             syn::Data::Enum(ref data) => {
@@ -85,6 +85,10 @@ impl<'a> Container<'a> {
                               found #[serde(repr = \"{}\")]", attrs.repr()));
         }
 
+        if has_flatten {
+            attrs.mark_has_flatten();
+        }
+
         let item = Container {
             ident: item.ident,
             attrs: attrs,

serde_derive_internals/src/attr.rs

@@ -146,6 +146,7 @@ pub struct Container {
     remote: Option<syn::Path>,
     identifier: Identifier,
     repr: ContainerRepr,
+    has_flatten: bool,
 }
 
 /// Styles of representing an enum.
@@ -417,6 +418,7 @@ impl Container {
             remote: remote.get(),
             identifier: decide_identifier(cx, item, &field_identifier, &variant_identifier),
             repr: repr.get().unwrap_or(ContainerRepr::Auto),
+            has_flatten: false,
         }
     }
 
@@ -467,6 +469,14 @@ impl Container {
     pub fn repr(&self) -> ContainerRepr {
         self.repr
     }
+
+    pub fn has_flatten(&self) -> bool {
+        self.has_flatten
+    }
+
+    pub fn mark_has_flatten(&mut self) {
+        self.has_flatten = true;
+    }
 }
 
 fn decide_tag(