Avoid compile warnings when implementing an empty protocol

lib/elixir/lib/protocol.ex

@@ -764,7 +764,7 @@ defmodule Protocol do
   end
 
   # TODO: Convert the following warnings into errors in future Elixir versions
-  def __before_compile__(env) do
+  defmacro __before_compile__(env) do
     # Callbacks
     callback_metas = callback_metas(env.module, :callback)
     callbacks = :maps.keys(callback_metas)
@@ -806,6 +806,16 @@ defmodule Protocol do
         nil
       )
     end
+
+    quote do
+      # Register the __impl__/1 callback to ensure the protocol is a behaviour.
+      # Without this, a protocol that defines no functions won't be detected
+      # as a behaviour by the Erlang compiler, and any implementations of that
+      # protocol will log a warning.
+      @callback __impl__(:for) :: module()
+      @callback __impl__(:target) :: module()
+      @callback __impl__(:protocol) :: module()
+    end
   end
 
   defp after_defprotocol do

lib/elixir/test/elixir/protocol_test.exs

@@ -150,6 +150,14 @@ defmodule ProtocolTest do
     assert args == [{:type, 23, :product, [{:user_type, 23, :t, []}]}, {:type, 23, :term, []}]
   end
 
+  test "protocol with no functions is a behaviour" do
+    defprotocol ProtoNoFunctions do
+    end
+
+    defimpl ProtoNoFunctions, for: Integer do
+    end
+  end
+
   test "protocol defines functions and attributes" do
     assert Sample.__protocol__(:module) == Sample
     assert Sample.__protocol__(:functions) == [ok: 1]