Load config/config.exs into escript

Closes #2643

Author: José Valim

lib/mix/lib/mix/tasks/escript.build.ex

@@ -191,29 +191,47 @@ defmodule Mix.Tasks.Escript.Build do
   end
 
   defp gen_main(name, module, app) do
+    config =
+      if File.regular?("config/config.exs") do
+        Mix.Config.read!("config/config.exs")
+      else
+        []
+      end
+
     {:module, ^name, binary, _} =
       defmodule name do
         @module module
+        @config config
         @app app
 
+        # We need to use Erlang modules at this point
+        # because we are not sure Elixir is available.
         def main(args) do
-          case Application.ensure_all_started(:elixir) do
+          case :application.ensure_all_started(:elixir) do
             {:ok, _} ->
+              load_config(@config)
               start_app(@app)
               args = Enum.map(args, &List.to_string(&1))
               Kernel.CLI.run fn _ -> @module.main(args) end, true
             _ ->
-              io_error "Elixir is not available, aborting."
-              System.halt(1)
+              :io.put_chars :standard_error, "Elixir is not available, aborting.\n"
+              :erlang.halt(1)
           end
         end
 
+        defp load_config(config) do
+          for {app, kw} <- config, {k, v} <- kw do
+            :application.set_env(app, k, v, persistent: true)
+          end
+          :ok
+        end
+
         defp start_app(nil) do
           :ok
         end
 
         defp start_app(app) do
-          case Application.ensure_all_started(app) do
+          case :application.ensure_all_started(app) do
             {:ok, _} -> :ok
             {:error, {app, reason}} ->
               io_error "Could not start application #{app}: " <>

lib/mix/test/fixtures/escripttest/lib/escripttest.ex

@@ -4,6 +4,6 @@ defmodule Escripttest do
   end
 
   def main(_args) do
-    IO.puts "TEST"
+    IO.puts Application.get_env(:foobar, :value, "TEST")
   end
 end

lib/mix/test/mix/tasks/escript_test.exs

@@ -42,7 +42,7 @@ defmodule Mix.Tasks.EscriptTest do
     end
   end
 
-  test "generate simple escript" do
+  test "generate escript" do
     Mix.Project.push Escript
 
     in_fixture "escripttest", fn ->
@@ -55,7 +55,20 @@ defmodule Mix.Tasks.EscriptTest do
     end
   end
 
-  test "generate simple escript with path" do
+  test "generate escript with config" do
+    Mix.Project.push Escript
+
+    in_fixture "escripttest", fn ->
+      File.write! "config/config.exs", """
+      [foobar: [value: "FROM CONFIG"]]
+      """
+      Mix.Tasks.Escript.Build.run []
+      assert_received {:mix_shell, :info, ["Generated escript escriptest with MIX_ENV=dev"]}
+      assert System.cmd("escript", ["escriptest"]) == {"FROM CONFIG\n", 0}
+    end
+  end
+
+  test "generate escript with path" do
     Mix.Project.push EscriptWithPath
 
     in_fixture "escripttest", fn ->