Also remove undefined warnings when loading projects outside the CLI

Author: José Valim

lib/mix/lib/mix/cli.ex

@@ -37,10 +37,10 @@ defmodule Mix.CLI do
     file = System.get_env("MIX_EXS") || "mix.exs"
 
     if File.regular?(file) do
-      old_value = Code.get_compiler_option(:no_warn_undefined)
+      old_undefined = Code.get_compiler_option(:no_warn_undefined)
       Code.put_compiler_option(:no_warn_undefined, :all)
       Code.compile_file(file)
-      Code.put_compiler_option(:no_warn_undefined, old_value)
+      Code.put_compiler_option(:no_warn_undefined, old_undefined)
     end
   end
 

lib/mix/lib/mix/project.ex

@@ -721,15 +721,17 @@ defmodule Mix.Project do
 
       {new_proj, file} =
         if File.regular?(file) do
+          old_undefined = Code.get_compiler_option(:no_warn_undefined)
+
           try do
-            Code.compiler_options(relative_paths: false)
+            Code.compiler_options(relative_paths: false, no_warn_undefined: :all)
             _ = Code.compile_file(file)
             get()
           else
             ^old_proj -> Mix.raise("Could not find a Mix project at #{file}")
             new_proj -> {new_proj, file}
           after
-            Code.compiler_options(relative_paths: true)
+            Code.compiler_options(relative_paths: true, no_warn_undefined: old_undefined)
           end
         else
           push(nil, file, app)