&Mod.fun/arity are valid app terms, closes #14891

Author: josevalim

lib/mix/lib/mix/tasks/compile.app.ex

@@ -239,7 +239,20 @@ defmodule Mix.Tasks.Compile.App do
     [?#, ?{, inner, ?}]
   end
 
-  defp to_erl_term(term) when is_function(term) or is_reference(term) or is_pid(term) do
+  defp to_erl_term(function) when is_function(function) do
+    fun_info = Function.info(function)
+
+    if fun_info[:type] == :external and fun_info[:env] == [] do
+      :io_lib.print(function)
+    else
+      Mix.raise(
+        "\"def application\" has a function which cannot be written to .app files: #{inspect(function)}" <>
+          " (only functions in the form &Mod.fun/arity can be part of the application environment)"
+      )
+    end
+  end
+
+  defp to_erl_term(term) when is_reference(term) or is_pid(term) do
     Mix.raise(
       "\"def application\" has a term which cannot be written to .app files: #{inspect(term)}"
     )

lib/mix/test/mix/tasks/compile.app_test.exs

@@ -110,7 +110,7 @@ defmodule Mix.Tasks.Compile.AppTest do
   test "uses custom application settings" do
     in_fixture("no_mixfile", fn ->
       Mix.Project.push(CustomProject)
-      env = [foo: [:one, "two", 3, 4, "฿"], bar: [{} | %{foo: :bar}]]
+      env = [foo: [:one, "two", 3, 4, "฿", &List.flatten/1], bar: [{} | %{foo: :bar}]]
 
       Process.put(:application,
         maxT: :infinity,
@@ -132,7 +132,10 @@ defmodule Mix.Tasks.Compile.AppTest do
       assert properties[:applications] ==
                [:kernel, :stdlib, :elixir, :logger, :ex_unit, :example_app, :mix]
 
-      assert properties[:env] == [foo: [:one, "two", 3, 4, "฿"], bar: [{} | %{foo: :bar}]]
+      assert properties[:env] == [
+               foo: [:one, "two", 3, 4, "฿", &List.flatten/1],
+               bar: [{} | %{foo: :bar}]
+             ]
 
       refute Keyword.has_key?(properties, :extra_applications)
     end)