Consider .formatter.exs when formatting stdin

Closes #7411

Signed-off-by: José Valim <jose.valim@plataformatec.com.br>

Author: José Valim

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

@@ -308,7 +308,7 @@ defmodule Mix.Tasks.Format do
     |> Enum.uniq()
   end
 
-  defp expand_args(files_and_patterns, _dot_formatter, formatter_opts_and_subs) do
+  defp expand_args(files_and_patterns, _dot_formatter, {formatter_opts, subs}) do
     files =
       for file_or_pattern <- files_and_patterns,
           file <- stdin_or_wildcard(file_or_pattern),
@@ -324,10 +324,10 @@ defmodule Mix.Tasks.Format do
 
     for file <- files do
       if file == :stdin do
-        {file, []}
+        {file, formatter_opts}
       else
         split = file |> Path.relative_to_cwd() |> Path.split()
-        {file, find_formatter_opts_for_file(split, formatter_opts_and_subs)}
+        {file, find_formatter_opts_for_file(split, {formatter_opts, subs})}
       end
     end
   end

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

@@ -90,6 +90,23 @@ defmodule Mix.Tasks.FormatTest do
     end)
   end
 
+  test "reads file from stdin and prints to stdout with formatter", context do
+    in_tmp(context.test, fn ->
+      File.write!(".formatter.exs", """
+      [locals_without_parens: [foo: 1]]
+      """)
+
+      output =
+        capture_io("foo :bar", fn ->
+          Mix.Tasks.Format.run(["-"])
+        end)
+
+      assert output == """
+             foo :bar
+             """
+    end)
+  end
+
   test "checks if file is formatted with --check-formatted", context do
     in_tmp(context.test, fn ->
       File.write!("a.ex", """