Ensure full directory match on mix format, closes #12969

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

@@ -584,8 +584,15 @@ defmodule Mix.Tasks.Format do
 
   defp recur_formatter_opts_for_file(file, {formatter_opts, subs}) do
     Enum.find_value(subs, formatter_opts, fn {sub, formatter_opts_and_subs} ->
-      if String.starts_with?(file, sub) do
-        recur_formatter_opts_for_file(file, formatter_opts_and_subs)
+      size = byte_size(sub)
+
+      case file do
+        <<prefix::binary-size(size), dir_separator, _::binary>>
+        when prefix == sub and dir_separator in [?\\, ?/] ->
+          recur_formatter_opts_for_file(file, formatter_opts_and_subs)
+
+        _ ->
+          nil
       end
     end)
   end

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

@@ -573,11 +573,18 @@ defmodule Mix.Tasks.FormatTest do
   test "reads exported configuration from subdirectories", context do
     in_tmp(context.test, fn ->
       File.write!(".formatter.exs", """
-      [subdirectories: ["lib"]]
+      [subdirectories: ["li", "lib"]]
       """)
 
+      # We also create a directory called li to ensure files
+      # from lib won't accidentally match on li.
+      File.mkdir_p!("li")
       File.mkdir_p!("lib")
 
+      File.write!("li/.formatter.exs", """
+      [inputs: "**/*", locals_without_parens: [other_fun: 2]]
+      """)
+
       File.write!("lib/.formatter.exs", """
       [inputs: "a.ex", locals_without_parens: [my_fun: 2]]
       """)