Raise if app in mix new already exists, closes #11300

Author: josevalim

lib/mix/lib/mix/tasks/app.tree.ex

@@ -80,9 +80,8 @@ defmodule Mix.Tasks.App.Tree do
   end
 
   defp load(app, type) do
-    case Application.load(app) do
+    case Application.ensure_loaded(app) do
       :ok -> true
-      {:error, {:already_loaded, ^app}} -> true
       _ when type == :optional -> false
       _ -> Mix.raise("could not find application #{app}")
     end

lib/mix/lib/mix/tasks/new.ex

@@ -28,7 +28,6 @@ defmodule Mix.Tasks.New do
   An `--umbrella` option can be given to generate an
   umbrella project.
 
-
   ## Examples
 
       mix new hello_world
@@ -167,10 +166,9 @@ defmodule Mix.Tasks.New do
   end
 
   defp check_application_name!(name, inferred?) do
-    unless name =~ ~r/^[a-z][a-z0-9_]*$/ do
+    if message = invalid_app(name) || reserved_app(name) do
       Mix.raise(
-        "Application name must start with a lowercase ASCII letter, followed by " <>
-          "lowercase ASCII letters, numbers, or underscores, got: #{inspect(name)}" <>
+        message <>
           if inferred? do
             ". The application name is inferred from the path, if you'd like to " <>
               "explicitly name the application then use the \"--app APP\" option"
@@ -181,6 +179,19 @@ defmodule Mix.Tasks.New do
     end
   end
 
+  defp invalid_app(name) do
+    unless name =~ ~r/^[a-z][a-z0-9_]*$/ do
+      "Application name must start with a lowercase ASCII letter, followed by " <>
+        "lowercase ASCII letters, numbers, or underscores, got: #{inspect(name)}"
+    end
+  end
+
+  defp reserved_app(name) do
+    if name |> String.to_atom() |> Application.ensure_loaded() == :ok do
+      "Cannot use application name #{inspect(name)} because it is already used by Erlang/OTP or Elixir"
+    end
+  end
+
   defp check_mod_name_validity!(name) do
     unless name =~ ~r/^[A-Z]\w*(\.[A-Z]\w*)*$/ do
       Mix.raise(

lib/mix/test/mix/tasks/new_test.exs

@@ -16,7 +16,6 @@ defmodule Mix.Tasks.NewTest do
       assert_file("hello_world/.gitignore")
 
       assert_file("hello_world/lib/hello_world.ex", ~r/defmodule HelloWorld do/)
-
       assert_file("hello_world/test/test_helper.exs", ~r/ExUnit.start()/)
 
       assert_file("hello_world/test/hello_world_test.exs", fn file ->
@@ -160,6 +159,12 @@ defmodule Mix.Tasks.NewTest do
                      Mix.Tasks.New.run(["007invalid"])
                    end
 
+      assert_raise Mix.Error,
+                   ~r"Cannot use application name \"tools\" because it is already used by Erlang/OTP or Elixir",
+                   fn ->
+                     Mix.Tasks.New.run(["tools"])
+                   end
+
       assert_raise Mix.Error,
                    ~r"followed by lowercase ASCII letters, numbers, or underscores",
                    fn ->
@@ -185,18 +190,6 @@ defmodule Mix.Tasks.NewTest do
       end
     end)
 
-    in_tmp("new with an already taken application name", fn ->
-      assert_raise Mix.Error, ~r"Module name \w+ is already taken", fn ->
-        Mix.Tasks.New.run(["mix"])
-      end
-    end)
-
-    in_tmp("new with an already taken application name from the app option", fn ->
-      assert_raise Mix.Error, ~r"Module name \w+ is already taken", fn ->
-        Mix.Tasks.New.run(["valid", "--app", "mix"])
-      end
-    end)
-
     in_tmp("new with an already taken module name from the module options", fn ->
       assert_raise Mix.Error, ~r"Module name \w+ is already taken", fn ->
         Mix.Tasks.New.run(["valid", "--module", "Mix"])