Deprecate opaque struct fields

* NEWS: Add entry.
* doc/ref/api-data.texi (Vtables, Structure Basics): Remove mention of
  opaque field protection.
* libguile/struct.c (scm_make_struct_layout, scm_make_struct_no_tail):
  Remove discussion of opaque fields.
  (set_vtable_layout_flags): Issue a deprecation warning when opaque
  fields are used.

1 files changed, 14 insertions, 0 deletions

diff --git a/NEWS b/NEWS
index 78e3b30c3..fec9af30f 100644
--- a/NEWS
+++ b/NEWS
@@ -67,6 +67,20 @@ structure.  However this was a little used complication without any use
 in Scheme code.  To replace it, just use "p" slots and initialize the
 slot values manually on initialization.
 
+** Struct fields with opaque ("o") protection deprecated
+
+Struct fields are declared with a "protection", meaning read-only ('r'),
+read-write ('w'), or opaque ('o').  There is also "hidden" ('o') which
+is read-write but which isn't initialized by arguments passed to
+`make-struct/no-tail', but that's a detail.  Opaque struct fields were
+used to allocate storage in a struct that could only be accessed by C.
+This facility was very rarely used (unused in Guile itself) but now that
+we are implementing more and more in Scheme, it is completely useless.
+
+To enforce permissions on struct fields, instead layer on an abstraction
+at a higher level, in the same way that immutable record fields are
+simply those which don't have an accessor.
+
 * Bug fixes
 
 ** Enable GNU Readline 7.0's support for "bracketed paste".