feat: Print formatted file names in mix format --verbose

lib/mix/lib/mix/tasks/format.ex

@@ -77,6 +77,8 @@ defmodule Mix.Tasks.Format do
 
     * `--dry-run` - does not save files after formatting.
 
+    * `--verbose` - prints the names of files that were formatted.
+
     * `--dot-formatter` - path to the file with formatter configuration.
       Defaults to `.formatter.exs` if one is available. See the
       "Formatting options" section above for more information.
@@ -199,6 +201,7 @@ defmodule Mix.Tasks.Format do
     no_exit: :boolean,
     dot_formatter: :string,
     dry_run: :boolean,
+    verbose: :boolean,
     stdin_filename: :string,
     force: :boolean,
     migrate: :boolean
@@ -764,18 +767,27 @@ defmodule Mix.Tasks.Format do
         :ok
 
       true ->
-        write_or_print(file, input, output)
+        write_or_print(file, input, output, task_opts)
     end
   rescue
     exception ->
       {:exit, file, exception, __STACKTRACE__}
   end
 
-  defp write_or_print(file, input, output) do
+  defp write_or_print(file, input, output, task_opts) do
     cond do
-      file == :stdin -> IO.write(output)
-      input == output -> :ok
-      true -> File.write!(file, output)
+      file == :stdin ->
+        IO.write(output)
+
+      input == output ->
+        :ok
+
+      true ->
+        File.write!(file, output)
+
+        if task_opts[:verbose] do
+          Mix.shell().info([:green, "* formatting ", :reset, Path.relative_to_cwd(file)])
+        end
     end
 
     :ok

lib/mix/test/mix/tasks/format_test.exs

@@ -85,6 +85,35 @@ defmodule Mix.Tasks.FormatTest do
     end)
   end
 
+  test "prints formatted file names with --verbose", context do
+    in_tmp(context.test, fn ->
+      File.write!("a.ex", """
+      foo bar
+      """)
+
+      File.write!("b.ex", """
+      foo(bar)
+      """)
+
+      Mix.Tasks.Format.run(["a.ex", "b.ex", "--verbose"])
+
+      assert_received {:mix_shell, :info, ["* formatting a.ex"]}
+      refute_received {:mix_shell, :info, ["* formatting b.ex"]}
+    end)
+  end
+
+  test "does not print formatted file names by default", context do
+    in_tmp(context.test, fn ->
+      File.write!("a.ex", """
+      foo bar
+      """)
+
+      Mix.Tasks.Format.run(["a.ex"])
+
+      refute_received {:mix_shell, :info, ["* formatting a.ex"]}
+    end)
+  end
+
   test "does not try to format a directory that matches a given pattern", context do
     in_tmp(context.test, fn ->
       File.mkdir_p!("a.ex")