Add GenServer.format_status/2 and use GenServer over :gen_server as the behaviour

lib/elixir/lib/gen_server.ex

@@ -382,6 +382,28 @@ defmodule GenServer do
     {:ok, new_state :: term} |
     {:error, reason :: term} when old_vsn: term | {:down, term}
 
+  @doc """
+  Invoked in some cases to retrieve a formatted version of the `GenServer` status.
+
+  This callback can be useful to control the *appearance* of the status of the
+  `GenServer`. For example, it can be used to return a compact representation of
+  the `GenServers`'s state to avoid having large state terms printed.
+
+    * one of `:sys.get_status/1` or `:sys.get_status/2` is invoked to get the
+      status of the `GenServer`; in such cases, `reason` is `:normal`
+
+    * the `GenServer` terminates abnormally and logs an error; in such cases,
+      `reason` is `:terminate`
+
+  `pdict_and_state` is a two-elements list `[pdict, state]` where `pdict` is a
+  list of `{key, value}` tuples representing the current process dictionary of
+  the `GenServer` and `state` is the current state of the `GenServer`.
+  """
+  @callback format_status(reason, pdict_and_state :: list) ::
+    term when reason: :normal | :terminate
+
+  @optional_callbacks format_status: 2
+
   @typedoc "Return values of `start*` functions"
   @type on_start :: {:ok, pid} | :ignore | {:error, {:already_started, pid} | term}
 
@@ -414,7 +436,7 @@ defmodule GenServer do
   @doc false
   defmacro __using__(_) do
     quote location: :keep do
-      @behaviour :gen_server
+      @behaviour GenServer
 
       @doc false
       def init(args) do

lib/elixir/test/elixir/module_test.exs

@@ -122,7 +122,7 @@ defmodule ModuleTest do
   ## Attributes
 
   test "reserved attributes" do
-    assert List.keyfind(ExUnit.Server.__info__(:attributes), :behaviour, 0) == {:behaviour, [:gen_server]}
+    assert List.keyfind(ExUnit.Server.__info__(:attributes), :behaviour, 0) == {:behaviour, [GenServer]}
   end
 
   test "registered attributes" do