Require a trailing underscore to consider it a compiler variable

Author: josevalim

lib/elixir/src/elixir_env.erl

@@ -134,9 +134,9 @@ is_unused_var(Name) ->
     _ -> true
   end.
 
+is_compiler_var([$_]) -> true;
 is_compiler_var([Var | Rest]) when Var =:= $_; Var >= $A, Var =< $Z -> is_compiler_var(Rest);
-is_compiler_var([_ | _]) -> false;
-is_compiler_var([]) -> true.
+is_compiler_var(_) -> false.
 
 format_error({unused_var, Name}) ->
   case atom_to_list(Name) of

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

@@ -92,11 +92,13 @@ defmodule Kernel.WarningTest do
         Code.eval_string("""
         defmodule Sample do
           def hello(__MODULE___), do: :ok
+          def world(_R), do: :ok
         end
         """)
       end)
 
     assert output =~ "unknown compiler variable \"__MODULE___\""
+    refute output =~ "unknown compiler variable \"_R\""
   after
     purge(Sample)
   end