Properly annotate key errors for structs, closes #7935

Signed-off-by: José Valim <jose.valim@plataformatec.com.br>

lib/elixir/lib/exception.ex

@@ -1161,7 +1161,7 @@ defmodule KeyError do
   end
 
   defp map_with_atom_keys_only?(term) do
-    is_map(term) and Enum.all?(term, fn {k, _} -> is_atom(k) end)
+    is_map(term) and Enum.all?(Map.to_list(term), fn {k, _} -> is_atom(k) end)
   end
 
   defp available_keys(term) when is_map(term), do: Map.keys(term)

lib/elixir/test/elixir/exception_test.exs

@@ -504,6 +504,13 @@ defmodule ExceptionTest do
              """
     end
 
+    test "annotates key error with suggestions for structs" do
+      message = blame_message(%URI{}, fn map -> map.schema end)
+      assert message =~ "key :schema not found in: %URI{"
+      assert message =~ "Did you mean one of:"
+      assert message =~ "* :scheme"
+    end
+
     if :erlang.system_info(:otp_release) >= '21' do
       test "annotates +/1 arithmetic errors" do
         assert blame_message(:foo, &(+&1)) == "bad argument in arithmetic expression: +(:foo)"