Fix source map generation for the encoder.

src/utils/object_serializer.cr

@@ -37,7 +37,7 @@ module Mint
       Codegen.join ["((_)=>", js.class_of(node.name), ".encode(_))"]
     end
 
-    def encoder(node : TypeChecker::Variable)
+    def encoder(node : TypeChecker::Variable) : Codegen::Node
       # This should never happen because of the typechecker!
       raise "Cannot generate an encoder for a type variable!"
     end
@@ -47,11 +47,20 @@ module Mint
       when "Time"
         "Encoder.time"
       when "Array"
-        "Encoder.array(#{encoder(node.parameters.first)})"
+        item_encoder =
+          (encoder(node.parameters.first) || "").as(Codegen::Node)
+
+        Codegen.join ["Encoder.array(", item_encoder, ")"]
       when "Maybe"
-        "Encoder.maybe(#{encoder(node.parameters.first)})"
+        item_encoder =
+          (encoder(node.parameters.first) || "").as(Codegen::Node)
+
+        Codegen.join ["Encoder.maybe(", item_encoder, ")"]
       when "Map"
-        "Encoder.map(#{encoder(node.parameters.last)})"
+        item_encoder =
+          (encoder(node.parameters.last) || "").as(Codegen::Node)
+
+        Codegen.join ["Encoder.map(", item_encoder, ")"]
       end
     end