test_helper files still need to be required upfront

Since they have to be required upfront, it makes no sense to have
a Code.require_file("test_helper.exs", __DIR__) in Mix projects
since the files can't run in isolation anyway.

Author: José Valim

CHANGELOG.md

@@ -10,7 +10,7 @@
   * [Kernel] Functions now points to the module and function they were defined when inspected
   * [Kernel] A documentation attached to a function that is never defined now prints warnings
   * [List] Add `List.keysort/2`
-  * [Mix] `:test_helper` project configuration did not affect `mix test` and was therefore removed
+  * [Mix] `:test_helper` project configuration did not affect `mix test` and was therefore removed. A `test/test_helper.exs` file is still necessary albeit it doesn't need to be automatically required in each test file
   * [Mix] Add manifests for yecc, leex and Erlang compilers, making it easier to detect dependencies in between compilers and providing a more useful clean behaviour
   * [Mix] `mix help` now outputs information about the default mix task
   * [Mix] Add `--no-deps-check` option to `mix run`, `mix compile` and friends to not check dependency status

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

@@ -218,8 +218,6 @@ defmodule Mix.Tasks.New do
   """
 
   embed_template :test_lib, """
-  Code.require_file "test_helper.exs", __DIR__
-
   defmodule <%= @mod %>Test do
     use ExUnit.Case
 

lib/mix/lib/mix/tasks/test.ex

@@ -29,15 +29,14 @@ defmodule Mix.Tasks.Test do
   @moduledoc """
   Run the tests for a project.
 
-  This task starts the current application and then requires
-  all files that match the given `test_pattern` in parallel.
+  This task starts the current application, loads up
+  `test/test_helper.exs` and then requires all files matching the
+  `test/**/_test.exs` pattern in parallel.
 
-  It is expected that each test file will properly setup the
-  test framework, usually by providing a `test/test_helper.exs`
-  file that is required at the top of the file.
+  A list of files can be given after the task name in order to select
+  the files to compile:
 
-  A list of files can be given after the task name in
-  order to select the files to compile.
+      mix test test/some/particular/file_test.exs
 
   ## Command line options
 
@@ -51,7 +50,8 @@ defmodule Mix.Tasks.Test do
 
   ## Configuration
 
-  * `:test_paths` - list of paths containing test files, defaults to `["test"]`
+  * `:test_paths` - list of paths containing test files, defaults to `["test"]`.
+                    it is expected all test paths to contain a `test_helper.exs` file
 
   * `:test_pattern` - a pattern to load test files, defaults to `*_test.exs`
 
@@ -109,13 +109,26 @@ defmodule Mix.Tasks.Test do
                 "please set the output directory as test_coverage: [output: \"PATH\"] instead"
     end
 
-    test_paths   = if files == [], do: project[:test_paths] || ["test"], else: files
-    test_pattern = project[:test_pattern] || "*_test.exs"
-
     :application.load(:ex_unit)
     ExUnit.configure(Dict.take(opts, [:trace, :max_cases, :color]))
 
+    test_paths = project[:test_paths] || ["test"]
+    Enum.each(test_paths, require_test_helper(&1))
+
+    test_paths   = if files == [], do: test_paths, else: files
+    test_pattern = project[:test_pattern] || "*_test.exs"
+
     files = Mix.Utils.extract_files(test_paths, test_pattern)
     Kernel.ParallelRequire.files files
   end
+
+  defp require_test_helper(dir) do
+    file = Path.join(dir, "test_helper.exs")
+
+    if File.exists?(file) do
+      Code.require_file file
+    else
+      raise Mix.Error, message: "Cannot run tests because test helper file #{inspect file} does not exist"
+    end
+  end
 end