Allowed Enum variants to be individually marked as untagged (#2403)

serde_derive/src/internals/ast.rs

@@ -140,6 +140,7 @@ fn enum_from_ast<'a>(
     variants: &'a Punctuated<syn::Variant, Token![,]>,
     container_default: &attr::Default,
 ) -> Vec<Variant<'a>> {
+    let mut seen_untagged = false;
     variants
         .iter()
         .map(|variant| {
@@ -153,8 +154,12 @@ fn enum_from_ast<'a>(
                 fields,
                 original: variant,
             }
-        })
-        .collect()
+        }).inspect(|variant| {
+            if !variant.attrs.untagged() && seen_untagged {
+                cx.error_spanned_by(&variant.ident, "all variants with the #[serde(untagged)] attribute must be placed at the end of the enum")
+            }
+            seen_untagged = variant.attrs.untagged()
+        }).collect()
 }
 
 fn struct_from_ast<'a>(

serde_derive/src/internals/attr.rs

@@ -740,6 +740,7 @@ pub struct Variant {
     serialize_with: Option<syn::ExprPath>,
     deserialize_with: Option<syn::ExprPath>,
     borrow: Option<BorrowAttribute>,
+    untagged: bool,
 }
 
 struct BorrowAttribute {
@@ -762,6 +763,7 @@ impl Variant {
         let mut serialize_with = Attr::none(cx, SERIALIZE_WITH);
         let mut deserialize_with = Attr::none(cx, DESERIALIZE_WITH);
         let mut borrow = Attr::none(cx, BORROW);
+        let mut untagged = BoolAttr::none(cx, UNTAGGED);
 
         for attr in &variant.attrs {
             if attr.path() != SERDE {
@@ -879,6 +881,8 @@ impl Variant {
                             cx.error_spanned_by(variant, msg);
                         }
                     }
+                } else if meta.path == UNTAGGED {
+                    untagged.set_true(&meta.path);
                 } else {
                     let path = meta.path.to_token_stream().to_string().replace(' ', "");
                     return Err(
@@ -905,6 +909,7 @@ impl Variant {
             serialize_with: serialize_with.get(),
             deserialize_with: deserialize_with.get(),
             borrow: borrow.get(),
+            untagged: untagged.get(),
         }
     }
 
@@ -956,6 +961,10 @@ impl Variant {
     pub fn deserialize_with(&self) -> Option<&syn::ExprPath> {
         self.deserialize_with.as_ref()
     }
+
+    pub fn untagged(&self) -> bool {
+        self.untagged
+    }
 }
 
 /// Represents field attribute information