Pass :sigils to mix format plugins invoked for a sigil

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

@@ -349,24 +349,17 @@ defmodule Mix.Tasks.Format do
       end
     end
 
-    sigils =
+    flatten_sigils =
       for plugin <- plugins,
           sigil <- find_sigils_from_plugins(plugin, formatter_opts),
           do: {sigil, plugin}
 
-    sigils =
-      sigils
+    formatter_opts =
+      flatten_sigils
       |> Enum.group_by(&elem(&1, 0), &elem(&1, 1))
-      |> Enum.map(fn {sigil, plugins} ->
-        {sigil,
-         fn input, opts ->
-           Enum.reduce(plugins, input, fn plugin, input ->
-             plugin.format(input, opts ++ formatter_opts)
-           end)
-         end}
-      end)
+      |> prepend_sigils(Keyword.delete(formatter_opts, :sigils))
 
-    {Keyword.put(formatter_opts, :sigils, sigils),
+    {formatter_opts,
      Enum.map(subs, fn {path, formatter_opts_and_subs} ->
        {path, load_plugins(formatter_opts_and_subs, opts)}
      end)}
@@ -384,6 +377,24 @@ defmodule Mix.Tasks.Format do
     plugins
   end
 
+  # A sigil formatter must receive :sigils itself, so a plugin can format sigils
+  # nested inside the sigil it was given. The list is rebuilt on each invocation.
+  defp prepend_sigils(grouped_sigils, formatter_opts_without_sigils) do
+    sigils =
+      Enum.map(grouped_sigils, fn {sigil, plugins} ->
+        {sigil,
+         fn input, opts ->
+           formatter_opts = prepend_sigils(grouped_sigils, formatter_opts_without_sigils)
+
+           Enum.reduce(plugins, input, fn plugin, input ->
+             plugin.format(input, opts ++ formatter_opts)
+           end)
+         end}
+      end)
+
+    [sigils: sigils] ++ formatter_opts_without_sigils
+  end
+
   @typedoc """
   Options for `formatter_for_file/2`.
   """

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

@@ -572,6 +572,66 @@ defmodule Mix.Tasks.FormatTest do
     end)
   end
 
+  defmodule Elixir.SigilOuterPlugin do
+    @behaviour Mix.Tasks.Format
+
+    def features(opts) do
+      assert opts[:from_formatter_exs] == :yes
+      [sigils: [:O]]
+    end
+
+    def format(contents, opts) do
+      assert opts[:from_formatter_exs] == :yes
+      assert opts[:sigil] == :O
+      [Code.format_string!(contents, opts), ?\n]
+    end
+  end
+
+  defmodule Elixir.SigilInnerPlugin do
+    @behaviour Mix.Tasks.Format
+
+    def features(opts) do
+      assert opts[:from_formatter_exs] == :yes
+      [sigils: [:I]]
+    end
+
+    def format(contents, opts) do
+      assert opts[:from_formatter_exs] == :yes
+      assert opts[:sigil] == :I
+      String.upcase(contents)
+    end
+  end
+
+  test "uses sigil plugins for sigils nested inside sigils", context do
+    in_tmp(context.test, fn ->
+      File.write!(".formatter.exs", """
+      [
+        inputs: ["a.ex"],
+        plugins: [SigilOuterPlugin, SigilInnerPlugin],
+        from_formatter_exs: :yes
+      ]
+      """)
+
+      File.write!("a.ex", """
+      def nested_sigil_test do
+        ~O'''
+        inner(~I"foo bar")
+        '''
+      end
+      """)
+
+      Mix.Tasks.Format.run([])
+
+      assert File.read!("a.ex") == """
+             def nested_sigil_test do
+               ~O'''
+               inner(~I"FOO BAR")
+               '''
+             end
+             """
+    end)
+  end
+
   test "customizes plugin loading", context do
     in_tmp(context.test, fn ->
       File.write!(".formatter.exs", """