fix(struct): address code review feedback

- Formatting: add field/hydrate/discord_struct to locals_without_parens
  in .formatter.exs so the no-paren data-DSL style is the formatted form;
  mix format then wrapped the long codec encode_value/2 clause. Both
  previously-unformatted files now satisfy mix format --check-formatted.
- Empty-struct guard: __before_compile__ now raises ArgumentError when a
  module does 'use Dexcord.Struct' with no field/hydrate declarations,
  instead of silently generating a bare defstruct []. Added a compile-time
  test mirroring the existing Code.compile_string validation tests.

.formatter.exs

@@ -1,4 +1,5 @@
 # Used by "mix format"
 [
-  inputs: ["{mix,.formatter}.exs", "{config,lib,test}/**/*.{ex,exs}"]
+  inputs: ["{mix,.formatter}.exs", "{config,lib,test}/**/*.{ex,exs}"],
+  locals_without_parens: [field: 2, field: 3, hydrate: 2, discord_struct: 1]
 ]

lib/dexcord/struct.ex

@@ -66,13 +66,22 @@ defmodule Dexcord.Struct do
   defmacro __before_compile__(env) do
     module = env.module
 
+    raw_fields = Module.get_attribute(module, :dexcord_fields) || []
+    raw_hydrates = Module.get_attribute(module, :dexcord_hydrates) || []
+
+    if raw_fields == [] and raw_hydrates == [] do
+      raise ArgumentError,
+            "#{inspect(module)}: `use Dexcord.Struct` requires a `discord_struct do ... end` " <>
+              "block declaring at least one `field` or `hydrate`"
+    end
+
     fields =
-      (Module.get_attribute(module, :dexcord_fields) || [])
+      raw_fields
       |> Enum.reverse()
       |> Enum.map(fn {name, type_ast, opts} -> build_field(name, type_ast, opts, env) end)
 
     hydrates =
-      (Module.get_attribute(module, :dexcord_hydrates) || [])
+      raw_hydrates
       |> Enum.reverse()
       |> Enum.map(fn {name, opts} -> build_hydrate(name, opts, env) end)
 

lib/dexcord/struct/codec.ex

@@ -88,7 +88,10 @@ defmodule Dexcord.Struct.Codec do
   # structs delegate straight back here, while hand-written fallback structs
   # (UnknownChannel, Component.Unknown — no __fields__/0) provide their own.
   defp encode_value({:struct, _mod}, %_{} = v), do: v.__struct__.to_map(v)
-  defp encode_value({:list, inner}, v) when is_list(v), do: Enum.map(v, &encode_element(inner, &1))
+
+  defp encode_value({:list, inner}, v) when is_list(v),
+    do: Enum.map(v, &encode_element(inner, &1))
+
   defp encode_value({:enum, mod}, v) when is_atom(v) and not is_nil(v), do: mod.encode(v)
   defp encode_value({:enum, _mod}, {:unknown, n}) when is_integer(n), do: n
   # raw passthrough: junk scalars that decode kept raw re-encode raw; a raw

test/dexcord/struct_test.exs

@@ -126,6 +126,16 @@ defmodule Dexcord.StructTest do
         """)
       end
     end
+
+    test "use Dexcord.Struct without any field/hydrate raises at compile time" do
+      assert_raise ArgumentError, ~r/requires a `discord_struct`? do/, fn ->
+        Code.compile_string("""
+        defmodule Dexcord.Test.EmptyStruct do
+          use Dexcord.Struct
+        end
+        """)
+      end
+    end
   end
 
   describe "from_map decode (Task 5)" do