Warn if duplicate key is found in struct (#9452)

Warning is the preferable solution for this, if we raise instead then it
is likely that there are some existing libraries that use duplicate
struct keys that will break.

lib/elixir/lib/kernel/utils.ex

@@ -94,6 +94,9 @@ defmodule Kernel.Utils do
     fields = :lists.map(mapper, fields)
     enforce_keys = List.wrap(Module.get_attribute(module, :enforce_keys))
 
+    # TODO: Make it raise on v2.0
+    warn_on_duplicate_struct_key(:lists.keysort(1, fields))
+
     foreach = fn
       key when is_atom(key) ->
         :ok
@@ -108,6 +111,19 @@ defmodule Kernel.Utils do
     {struct, enforce_keys, Module.get_attribute(module, :derive)}
   end
 
+  defp warn_on_duplicate_struct_key([]) do
+    :ok
+  end
+
+  defp warn_on_duplicate_struct_key([{key, _} | [{key, _} | _] = rest]) do
+    IO.warn("duplicate key #{inspect(key)} found in struct")
+    warn_on_duplicate_struct_key(rest)
+  end
+
+  defp warn_on_duplicate_struct_key([_ | rest]) do
+    warn_on_duplicate_struct_key(rest)
+  end
+
   @doc """
   Announcing callback for defstruct.
   """

lib/elixir/test/elixir/kernel/warning_test.exs

@@ -1770,6 +1770,18 @@ defmodule Kernel.WarningTest do
     end
   end
 
+  test "defstruct warns with duplicate keys" do
+    assert capture_err(fn ->
+             Code.eval_string("""
+             defmodule TestMod do
+               defstruct [:foo, :foo, :bar]
+             end
+             """)
+           end) =~ "duplicate key :foo found in struct"
+  after
+    purge(TestMod)
+  end
+
   defp purge(list) when is_list(list) do
     Enum.each(list, &purge/1)
   end