Fix derive with _ key

Summary:

Test Plan:

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, generated_var(&1, __MODULE__)})
+ kv = Enum.map(fields, &{&1, generated_var(&1)})
  escape = quote(do: escape)
  encode_map = quote(do: encode_map)
  encode_args = [escape, encode_map]
@@ -94,9 +94,12 @@ 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}
+ # The same as Macro.var/2 except it sets generated: true and handles _ key
+ defp generated_var(:_) do
+ {:__, [generated: true], __MODULE__.Underscore}
+ end
+ defp generated_var(name) do
+ {name, [generated: true], __MODULE__}
  end
 
  def encode(%_{} = struct, _opts) do

test/encode_test.exs

@@ -135,6 +135,11 @@ defmodule Jason.EncoderTest do
  defstruct name: "", size: 0
  end
 
+ defmodule DerivedWeirdKey do
+ @derive Encoder
+ defstruct [:_]
+ end
+
  defmodule NonDerived do
  defstruct name: ""
  end
@@ -154,6 +159,9 @@ defmodule Jason.EncoderTest do
 
  derived_using_except = %DerivedUsingExcept{name: "derived using :except", size: 10}
  assert to_json(derived_using_except) == ~s({"size":10})
+
+ derived_weird_key = %DerivedWeirdKey{}
+ assert to_json(derived_weird_key) == ~s({"_":null})
  end
 
  test "@derive validate `except:`" do