Add mix test --dry-run flag

lib/ex_unit/lib/ex_unit/runner.ex

@@ -44,12 +44,27 @@ defmodule ExUnit.Runner do
     config = configure(opts, manager, self(), stats_pid)
     :erlang.system_flag(:backtrace_depth, Keyword.fetch!(opts, :stacktrace_depth))
 
-    start_time = System.monotonic_time()
-    EM.suite_started(config.manager, opts)
-
     modules_to_restore =
       if Keyword.fetch!(opts, :repeat_until_failure) > 0, do: {[], []}, else: nil
 
+    modules_to_restore =
+      if not opts[:dry_run] do
+        do_run(config, modules_to_restore, opts, load_us)
+      else
+        nil
+      end
+
+    stats = ExUnit.RunnerStats.stats(stats_pid)
+    EM.stop(config.manager)
+    after_suite_callbacks = Application.fetch_env!(:ex_unit, :after_suite)
+    Enum.each(after_suite_callbacks, fn callback -> callback.(stats) end)
+    {stats, modules_to_restore}
+  end
+
+  defp do_run(config, modules_to_restore, opts, load_us) do
+    start_time = System.monotonic_time()
+    EM.suite_started(config.manager, opts)
+
     {async_stop_time, modules_to_restore} = async_loop(config, %{}, false, modules_to_restore)
     stop_time = System.monotonic_time()
 
@@ -65,11 +80,7 @@ defmodule ExUnit.Runner do
     times_us = %{async: async_us, load: load_us, run: run_us}
     EM.suite_finished(config.manager, times_us)
 
-    stats = ExUnit.RunnerStats.stats(stats_pid)
-    EM.stop(config.manager)
-    after_suite_callbacks = Application.fetch_env!(:ex_unit, :after_suite)
-    Enum.each(after_suite_callbacks, fn callback -> callback.(stats) end)
-    {stats, modules_to_restore}
+    modules_to_restore
   end
 
   defp configure(opts, manager, runner_pid, stats_pid) do
@@ -88,7 +99,8 @@ defmodule ExUnit.Runner do
       seed: opts[:seed],
       stats_pid: stats_pid,
       timeout: opts[:timeout],
-      trace: opts[:trace]
+      trace: opts[:trace],
+      dry_run: opts[:dry_run]
     }
   end
 

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

@@ -121,6 +121,10 @@ defmodule Mix.Tasks.Test do
 
     * `--cover` - runs coverage tool. See "Coverage" section below
 
+    * `--dry-run` *(since v1.19.0)* - prints which tests would be run based on current options,
+      but does not actually run any tests. This combines with all other options
+      like `--stale`, `--only`, `--exclude`, and so on.
+
     * `--exclude` - excludes tests that match the filter. This option may be given
       several times to apply different filters, such as `--exclude ci --exclude slow`
 
@@ -494,7 +498,8 @@ defmodule Mix.Tasks.Test do
     warnings_as_errors: :boolean,
     profile_require: :string,
     exit_status: :integer,
-    repeat_until_failure: :integer
+    repeat_until_failure: :integer,
+    dry_run: :boolean
   ]
 
   @cover [output: "cover", tool: Mix.Tasks.Test.Coverage]
@@ -847,7 +852,8 @@ defmodule Mix.Tasks.Test do
     :only_test_ids,
     :test_location_relative_path,
     :exit_status,
-    :repeat_until_failure
+    :repeat_until_failure,
+    :dry_run
   ]
 
   @doc false

lib/mix/test/mix/tasks/test_test.exs

@@ -770,6 +770,42 @@ defmodule Mix.Tasks.TestTest do
     end
   end
 
+  # TODO: Get to pass after deciding on implementation
+  @tag :skip
+  describe "--dry-run" do
+    test "prints which tests would run without executing them" do
+      in_fixture("test_stale", fn ->
+        File.write!("test/dry_run_test.exs", """
+        defmodule DryRunTest do
+          use ExUnit.Case
+
+          test "passing test" do
+            assert true
+          end
+
+          test "failing test" do
+            assert false
+          end
+        end
+        """)
+
+        assert {output, 0} = mix_code(["test", "--dry-run", "--stale"])
+        assert output =~ "DRY RUN"
+        assert output =~ "test/dry_run_test.exs"
+        refute output =~ "Finished in"
+      end)
+    end
+
+    test "prints message when no tests would run" do
+      in_fixture("test_stale", fn ->
+        assert {output, 0} = mix_code(["test", "--dry-run", "non_existent_test.exs"])
+        assert output =~ "DRY RUN"
+        assert output =~ "No tests would run"
+        refute output =~ "Finished in"
+      end)
+    end
+  end
+
   defp receive_until_match(port, expected, acc) do
     receive do
       {^port, {:data, output}} ->