Raise on conflicting .formatter.exs files (elixir-lang#8323)

Warn on conflicting .formatter.exs files

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

@@ -325,7 +325,7 @@ defmodule Mix.Tasks.Format do
 
     dot_formatter
     |> expand_dot_inputs([], formatter_opts_and_subs, %{})
-    |> Enum.uniq()
+    |> Enum.map(fn {file, {_dot_formatter, formatter_opts}} -> {file, formatter_opts} end)
   end
 
   defp expand_args(files_and_patterns, _dot_formatter, {formatter_opts, subs}) do
@@ -360,15 +360,35 @@ defmodule Mix.Tasks.Format do
     map =
       for input <- List.wrap(formatter_opts[:inputs]),
           file <- Path.wildcard(Path.join(prefix ++ [input]), match_dot: true),
-          do: {file, formatter_opts},
+          do: {expand_relative_to_cwd(file), {dot_formatter, formatter_opts}},
           into: %{}
 
-    Enum.reduce(subs, Map.merge(acc, map), fn {sub, formatter_opts_and_subs}, acc ->
+    acc =
+      Map.merge(acc, map, fn file, {dot_formatter1, _}, {dot_formatter2, formatter_opts} ->
+        Mix.shell().error(
+          "Both #{dot_formatter1} and #{dot_formatter2} specify the file " <>
+            "#{Path.relative_to_cwd(file)} in their :inputs option. To resolve the " <>
+            "conflict, the configuration in #{dot_formatter1} will be ignored. " <>
+            "Please change the list of :inputs in one of the formatter files so only " <>
+            "one of them matches #{Path.relative_to_cwd(file)}"
+        )
+
+        {dot_formatter2, formatter_opts}
+      end)
+
+    Enum.reduce(subs, acc, fn {sub, formatter_opts_and_subs}, acc ->
       sub_formatter = Path.join(sub, ".formatter.exs")
       expand_dot_inputs(sub_formatter, [sub], formatter_opts_and_subs, acc)
     end)
   end
 
+  defp expand_relative_to_cwd(path) do
+    case File.cwd() do
+      {:ok, cwd} -> Path.expand(path, cwd)
+      _ -> path
+    end
+  end
+
   defp find_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

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

@@ -451,6 +451,48 @@ defmodule Mix.Tasks.FormatTest do
     end)
   end
 
+  test "prints an error on conflicting .formatter.exs files", context do
+    in_tmp(context.test, fn ->
+      File.write!(".formatter.exs", """
+      [inputs: "lib/**/*.{ex,exs}", subdirectories: ["lib", "foo"]]
+      """)
+
+      File.mkdir_p!("lib")
+
+      File.write!("lib/.formatter.exs", """
+      [inputs: "a.ex", locals_without_parens: [my_fun: 2]]
+      """)
+
+      File.mkdir_p!("foo")
+
+      File.write!("foo/.formatter.exs", """
+      [inputs: "../lib/a.ex", locals_without_parens: [my_fun: 2]]
+      """)
+
+      File.write!("lib/a.ex", """
+      my_fun :foo, :bar
+      other_fun :baz
+      """)
+
+      Mix.Tasks.Format.run([])
+
+      message1 =
+        "Both .formatter.exs and lib/.formatter.exs specify the file lib/a.ex in their " <>
+          ":inputs option. To resolve the conflict, the configuration in .formatter.exs " <>
+          "will be ignored. Please change the list of :inputs in one of the formatter files " <>
+          "so only one of them matches lib/a.ex"
+
+      message2 =
+        "Both lib/.formatter.exs and foo/.formatter.exs specify the file lib/a.ex in their " <>
+          ":inputs option. To resolve the conflict, the configuration in lib/.formatter.exs " <>
+          "will be ignored. Please change the list of :inputs in one of the formatter files " <>
+          "so only one of them matches lib/a.ex"
+
+      assert_received {:mix_shell, :error, [^message1]}
+      assert_received {:mix_shell, :error, [^message2]}
+    end)
+  end
+
   test "raises on invalid arguments", context do
     in_tmp(context.test, fn ->
       assert_raise Mix.Error, ~r"Expected one or more files\/patterns to be given", fn ->