Add :root option for formatter_for_file (#12541)

2 files changed, 43 insertions, 2 deletions

diff --git a/lib/mix/lib/mix/tasks/format.ex b/lib/mix/lib/mix/tasks/format.ex
index 6d4afc874..a164f3aac 100644
--- a/lib/mix/lib/mix/tasks/format.ex
+++ b/lib/mix/lib/mix/tasks/format.ex
@@ -251,10 +251,12 @@ defmodule Mix.Tasks.Format do
   def formatter_for_file(file, opts \\ []) do
     {dot_formatter, formatter_opts} = eval_dot_formatter(opts)
 
+    prefix = Keyword.get(opts, :root, [])
+
     {formatter_opts_and_subs, _sources} =
-      eval_deps_and_subdirectories(dot_formatter, [], formatter_opts, [dot_formatter])
+      eval_deps_and_subdirectories(dot_formatter, prefix, formatter_opts, [dot_formatter])
 
-    find_formatter_and_opts_for_file(file, formatter_opts_and_subs)
+    find_formatter_and_opts_for_file(file, prefix, formatter_opts_and_subs)
   end
 
   @doc """
@@ -275,6 +277,10 @@ defmodule Mix.Tasks.Format do
       File.regular?(".formatter.exs") ->
         {".formatter.exs", eval_file_with_keyword_list(".formatter.exs")}
 
+      opts[:root] && File.regular?(Path.join(opts[:root], ".formatter.exs")) ->
+        dot_formatter = Path.join(opts[:root], ".formatter.exs")
+        {dot_formatter, eval_file_with_keyword_list(dot_formatter)}
+
       true ->
         {".formatter.exs", []}
     end
@@ -582,6 +588,12 @@ defmodule Mix.Tasks.Format do
     {find_formatter_for_file(file, formatter_opts), formatter_opts}
   end
 
+  defp find_formatter_and_opts_for_file(file, prefix, formatter_opts_and_subs) do
+    split = file |> Path.relative_to(prefix) |> Path.split()
+    formatter_opts = recur_formatter_opts_for_file(split, formatter_opts_and_subs)
+    {find_formatter_for_file(file, formatter_opts), formatter_opts}
+  end
+
   defp recur_formatter_opts_for_file(split, {formatter_opts, subs}) do
     Enum.find_value(subs, formatter_opts, fn {sub, formatter_opts_and_subs} ->
       if List.starts_with?(split, Path.split(sub)) do
diff --git a/lib/mix/test/mix/tasks/format_test.exs b/lib/mix/test/mix/tasks/format_test.exs
index cf6f6b544..05be1212e 100644
--- a/lib/mix/test/mix/tasks/format_test.exs
+++ b/lib/mix/test/mix/tasks/format_test.exs
@@ -523,6 +523,35 @@ defmodule Mix.Tasks.FormatTest do
     end)
   end
 
+  test "uses inputs and configuration from :root path", context do
+    in_tmp(context.test, fn ->
+      File.write!(".formatter.exs", """
+      [
+        locals_without_parens: [foo: 1]
+      ]
+      """)
+
+      File.mkdir_p!("lib")
+
+      File.write!("lib/a.ex", """
+      foo bar baz
+      """)
+
+      root = File.cwd!()
+
+      {formatter_function, _options} =
+        File.cd!("lib", fn ->
+          Mix.Tasks.Format.formatter_for_file("lib/a.ex", root: root)
+        end)
+
+      assert formatter_function.("""
+             foo bar baz
+             """) == """
+             foo bar(baz)
+             """
+    end)
+  end
+
   test "reads exported configuration from subdirectories", context do
     in_tmp(context.test, fn ->
       File.write!(".formatter.exs", """