Raise when defining a bitstring field without a size with mysql

lib/ecto/adapters/myxql/connection.ex

@@ -1462,6 +1462,7 @@ if Code.ensure_loaded?(MyXQL) do
     defp ecto_cast_to_db(type, query), do: ecto_to_db(type, query)
 
     defp ecto_size_to_db(:binary), do: "varbinary"
+    defp ecto_size_to_db(:bitstring), do: "bit"
     defp ecto_size_to_db(type), do: ecto_to_db(type)
 
     defp ecto_to_db(type, query \\ nil)
@@ -1471,7 +1472,6 @@ if Code.ensure_loaded?(MyXQL) do
     defp ecto_to_db(:bigserial, _query), do: "bigint unsigned not null auto_increment"
     defp ecto_to_db(:binary_id, _query), do: "binary(16)"
     defp ecto_to_db(:string, _query), do: "varchar"
-    defp ecto_to_db(:bitstring, _query), do: "bit"
     defp ecto_to_db(:float, _query), do: "double"
     defp ecto_to_db(:binary, _query), do: "blob"
     # MySQL does not support uuid
@@ -1485,6 +1485,12 @@ if Code.ensure_loaded?(MyXQL) do
     defp ecto_to_db(:naive_datetime_usec, _query), do: "datetime"
     defp ecto_to_db(atom, _query) when is_atom(atom), do: Atom.to_string(atom)
 
+    defp ecto_to_db(:bitstring, _query) do
+      raise ArgumentError,
+            "type `bitstring` without the `:size` option is unsupported." <>
+              "The type can be used with an explicit size attribute (eg. `add my_field, :bitstring, size: 42`)"
+    end
+
     defp ecto_to_db(type, _query) do
       raise ArgumentError,
             "unsupported type `#{inspect(type)}`. The type can either be an atom, a string " <>