Mark vars in codegen as generated to prevent warnings for __meta__

Closes #41

Author: michalmuskala

lib/encoder.ex

@@ -77,7 +77,7 @@ end
 defimpl Jason.Encoder, for: Any do
   defmacro __deriving__(module, struct, opts) do
     fields = fields_to_encode(struct, opts)
-    kv = Enum.map(fields, &{&1, Macro.var(&1, __MODULE__)})
+    kv = Enum.map(fields, &{&1, generated_var(&1, __MODULE__)})
     escape = quote(do: escape)
     encode_map = quote(do: encode_map)
     encode_args = [escape, encode_map]
@@ -94,6 +94,11 @@ defimpl Jason.Encoder, for: Any do
     end
   end
 
+  # The same as Macro.var/2 except it sets generated: true
+  defp generated_var(name, context) do
+    {name, [generated: true], context}
+  end
+
   def encode(%_{} = struct, _opts) do
     raise Protocol.UndefinedError,
       protocol: @protocol,

lib/helpers.ex

@@ -61,7 +61,7 @@ defmodule Jason.Helpers do
   """
   defmacro json_map_take(map, take) do
     take = Macro.expand(take, __CALLER__)
-    kv = Enum.map(take, &{&1, Macro.var(&1, Codegen)})
+    kv = Enum.map(take, &{&1, generated_var(&1, Codegen)})
     escape = quote(do: escape)
     encode_map = quote(do: encode_map)
     encode_args = [escape, encode_map]
@@ -82,4 +82,9 @@ defmodule Jason.Helpers do
       end
     end
   end
+
+  # The same as Macro.var/2 except it sets generated: true
+  defp generated_var(name, context) do
+    {name, [generated: true], context}
+  end
 end