iface: require builder asset tags to be specified upfront the state data

src/interface/builder.rs

@@ -64,9 +64,9 @@ pub enum BuilderError {
     /// state `{0}` provided to the builder has invalid name.
     InvalidState(AssignmentType),
 
-    /// can't add asset of type `{0}`: you need to register the type with asset
-    /// type firtst using `add_asset_tag` method.
-    AssetTagUnknown(AssignmentType),
+    /// asset tag for the state `{0}` must be added before any fungible state of
+    /// the same type.
+    AssetTagSet(AssignmentType),
 
     /// interface doesn't specifies default operation name, thus an explicit
     /// operation type must be provided with `set_operation_type` method.
@@ -431,6 +431,10 @@ impl<Seal: ExposedSeal> OperationBuilder<Seal> {
             .assignments_type(&name, ty)
             .ok_or(BuilderError::AssignmentNotFound(name))?;
 
+        if self.fungible.contains_key(&type_id) {
+            return Err(BuilderError::AssetTagSet(type_id));
+        }
+
         self.asset_tags.insert(type_id, asset_tag)?;
         Ok(self)
     }