TO_NATIVE should always return self

* For simplicity, make Truffle::FFI::Struct a pointer for NFI interop,
  since it is only used internally.
* FFI::Struct does not need to define #to_native, as it can already
  be converted to a pointer with FFI's #to_ptr protocol and #get_pointer_value.

Author: eregon

lib/truffle/truffle/ffi/ffi_struct.rb

@@ -256,9 +256,19 @@ def to_ptr
       @pointer
     end
 
-    # For interop TO_NATIVE
+    # Every IS_POINTER object should also have TO_NATIVE
     def to_native
-      @pointer
+      self
+    end
+
+    # For NFI, to implement IS_POINTER
+    def pointer?
+      true
+    end
+
+    # For NFI, to implement AS_POINTER
+    def address
+      @pointer.address
     end
 
     def free

lib/truffle/truffle/ffi_backend/struct.rb

@@ -78,11 +78,6 @@ def initialize_copy(other)
       end
     end
 
-    # For interop TO_NATIVE
-    def to_native
-      @pointer
-    end
-
     private def pointer=(pointer)
       unless FFI::AbstractMemory === pointer
         raise TypeError, "wrong argument type #{pointer.class} (expected Pointer or Buffer)"