OClosure: New function `function-documentation`

As mentioned in the original OClosure commit, OClosures (ab)use the
bytecode's docstring slot to hold the OClosure's type.  This currently
prevents OClosures from having their own docstring.

Introduce a new generic function `function-documentation` to fetch the
docstring of a function, which can then be implemented in various
different ways depending on the OClosure's type.

* lisp/simple.el (function-documentation): New generic function.
(bad-package-check): Strength-reduce `eval` to `symbol-value`.
* src/doc.c (Fdocumentation): Use it.

* lisp/emacs-lisp/oclosure.el (oclosure--accessor-docstring): New function.
* test/lisp/emacs-lisp/oclosure-tests.el (oclosure-test):
Add test for accessor's docstrings.

1 files changed, 6 insertions, 0 deletions

diff --git a/lisp/emacs-lisp/oclosure.el b/lisp/emacs-lisp/oclosure.el
index 3df64ad2806..90811199f25 100644
--- a/lisp/emacs-lisp/oclosure.el
+++ b/lisp/emacs-lisp/oclosure.el
@@ -505,6 +505,12 @@ This has 2 uses:
   "OClosure function to access a specific slot of an object."
   type slot)
 
+(defun oclosure--accessor-docstring (f)
+  ;; This would like to be a (cl-defmethod function-documentation ...)
+  ;; but for circularity reason the defmethod is in `simple.el'.
+  (format "Access slot \"%S\" of OBJ of type `%S'.\n\n(fn OBJ)"
+          (accessor--slot f) (accessor--type f)))
+
 (oclosure-define (oclosure-accessor
                   (:parent accessor)
                   (:copier oclosure--accessor-copy (type slot index)))