Comment on the purpose(s) of NodeId in VariantData

src/librustc_front/hir.rs

@@ -1161,6 +1161,17 @@ impl StructFieldKind {
     }
 }
 
+/// Fields and Ids of enum variants and structs
+///
+/// For enum variants: `NodeId` represents both an Id of the variant itself (relevant for all
+/// variant kinds) and an Id of the variant's constructor (not relevant for `Struct`-variants).
+/// One shared Id can be successfully used for these two purposes.
+/// Id of the whole enum lives in `Item`.
+///
+/// For structs: `NodeId` represents an Id of the structure's constructor, so it is not actually
+/// used for `Struct`-structs (but still presents). Structures don't have an analogue of "Id of
+/// the variant itself" from enum variants.
+/// Id of the whole struct lives in `Item`.
 #[derive(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Debug)]
 pub enum VariantData {
     Struct(Vec<StructField>, NodeId),

src/libsyntax/ast.rs

@@ -1741,6 +1741,17 @@ impl StructFieldKind {
     }
 }
 
+/// Fields and Ids of enum variants and structs
+///
+/// For enum variants: `NodeId` represents both an Id of the variant itself (relevant for all
+/// variant kinds) and an Id of the variant's constructor (not relevant for `Struct`-variants).
+/// One shared Id can be successfully used for these two purposes.
+/// Id of the whole enum lives in `Item`.
+///
+/// For structs: `NodeId` represents an Id of the structure's constructor, so it is not actually
+/// used for `Struct`-structs (but still presents). Structures don't have an analogue of "Id of
+/// the variant itself" from enum variants.
+/// Id of the whole struct lives in `Item`.
 #[derive(Clone, PartialEq, Eq, RustcEncodable, RustcDecodable, Hash, Debug)]
 pub enum VariantData {
     Struct(Vec<StructField>, NodeId),