Add a __struct__ field on defstruct and ensure proper message on struct body access

Author: José Valim

lib/elixir/lib/inspect.ex

@@ -555,10 +555,10 @@ defimpl Inspect, for: Any do
     rescue
       _ -> Inspect.Map.inspect(map, opts)
     else
-      inner ->
-        outer = :maps.remove(:__struct__, map)
-        if :maps.keys(inner) == :maps.keys(outer) do
-          Inspect.Map.inspect(outer, Inspect.Atom.inspect(struct, opts), opts)
+      dunder ->
+        if :maps.keys(dunder) == :maps.keys(map) do
+          Inspect.Map.inspect(:maps.remove(:__struct__, map),
+            Inspect.Atom.inspect(struct, opts), opts)
         else
           Inspect.Map.inspect(map, opts)
         end

lib/elixir/lib/kernel.ex

@@ -3201,7 +3201,7 @@ defmodule Kernel do
       end)
 
       def __struct__() do
-        %{ unquote_splicing(opts) }
+        %{ unquote_splicing(opts), __struct__: __MODULE__ }
       end
     end
   end

lib/elixir/src/elixir_map.erl

@@ -23,7 +23,17 @@ expand_struct(Meta, Left, Right, E) ->
   EMeta =
     case lists:member(ELeft, E#elixir_env.context_modules) andalso
          elixir_module:is_open(ELeft) of
-      true  -> [{ struct, context }|Meta];
+      true  ->
+        case (ELeft == E#elixir_env.module) and
+             (E#elixir_env.function == nil) of
+          true ->
+            compile_error(Meta, E#elixir_env.file,
+              "cannot access struct ~ts in body of the module that defines it as "
+              "the struct fields are not yet accessible",
+              [elixir_aliases:inspect(ELeft)]);
+          false ->
+            [{ struct, context }|Meta]
+        end;
       false -> elixir_aliases:ensure_loaded(Meta, ELeft, E), Meta
     end,
 

lib/elixir/test/elixir/inspect_test.exs

@@ -266,7 +266,7 @@ defmodule Inspect.MapTest do
 
   defmodule Public do
     def __struct__ do
-      %{key: 0}
+      %{key: 0, __struct__: Public}
     end
   end
 

lib/elixir/test/elixir/kernel/errors_test.exs

@@ -195,6 +195,18 @@ defmodule Kernel.ErrorsTest do
       '%{ :a, :b }{ a: :b }'
   end
 
+  test :struct_access_on_body do
+    assert_compile_fail CompileError,
+      "nofile:3: cannot access struct TZ in body of the module that defines it " <>
+      "as the struct fields are not yet accessible",
+      '''
+      defmodule TZ do
+        defstruct %{name: "Brasilia"}
+        %TZ{}
+      end
+      '''
+  end
+
   test :unbound_map_key_var do
     assert_compile_fail CompileError,
       "nofile:1: cannot bind variable x in map key",