(function-history): New symbol property (bug#53632)

Rework the code we have in Fdefalias that tries to keep track
of definitions so as to be able to undo them later.

We used to store in `load-history` when an autoload is redefined as
a non-autoload and in the `autoload` symbol property we used to store
the autoload data that used to be used before it got overriden.

Instead, store the history of the function definition of
a symbol in its `function-history` symbol property.
To make this list cheap in the default case, the latest value is not stored
in the list (since it's in the `symbol-function`) and neither is the first
file.  So if there's only been a single definition (the most common case),
the list is empty and the property is just not present at all.

The patch also gets rid of the `autoload` vs `defun` distinction in
`load-history` which seems unnecessary (a significant part of the
motivation for this patch was to get rid of the special handling of
autoloads in this part of the code).

* src/data.c (add_to_function_history): New function.
(defalias): Use it.  Don't add the `t` entries for autoloads and always
use `defun` regardless of the kind of definition.
Change `Vautoload_queue` to only hold the function
symbols since the rest is now available from `function-history`.
* src/eval.c (un_autoload): Adjust accordingly.

* src/lread.c (load-history): Udate docstring.

* lisp/loadhist.el (loadhist-unload-filename): New var.
(unload-feature): Bind it.
(loadhist-unload-element): Document its availability.
(loadhist--restore-autoload): Delete var.
(loadhist--unload-function): Delete function.
(loadhist-unload-element): Delete the `t` and `autoload` methods.
Rewrite the `defun` method using `function-history`.

* lisp/help-fns.el: Require `seq`.
(help-fns--autoloaded-p): Rewrite.
(help-fns-function-description-header): Adjust call accordingly.

* doc/lispref/loading.texi (Where Defined): Remove `autoload` and `t`
entries from `load-history` since we don't generate them any more.
Document the `function-history` which replaces the `autoload` property.
(Unloading): Adjust symbol property name accordingly.

* test/lisp/loadhist-resources/loadhist--bar.el:
* test/lisp/loadhist-resources/loadhist--foo.el: New files.
* test/lisp/loadhist-tests.el (loadhist-tests-unload-feature-nested)
(loadhist-tests-unload-feature-notnested): New tests.

1 files changed, 5 insertions, 9 deletions

diff --git a/src/eval.c b/src/eval.c
index b083a00a791..1076985d097 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -2250,21 +2250,17 @@ this does nothing and returns nil.  */)
 static void
 un_autoload (Lisp_Object oldqueue)
 {
-  Lisp_Object queue, first, second;
-
   /* Queue to unwind is current value of Vautoload_queue.
      oldqueue is the shadowed value to leave in Vautoload_queue.  */
-  queue = Vautoload_queue;
+  Lisp_Object queue = Vautoload_queue;
   Vautoload_queue = oldqueue;
   while (CONSP (queue))
     {
-      first = XCAR (queue);
-      second = Fcdr (first);
-      first = Fcar (first);
-      if (EQ (first, make_fixnum (0)))
-	Vfeatures = second;
+      Lisp_Object first = XCAR (queue);
+      if (CONSP (first) && EQ (XCAR (first), make_fixnum (0)))
+	Vfeatures = XCDR (first);
       else
-	Ffset (first, second);
+	Ffset (first, Fcar (Fcdr (Fget (first, Qfunction_history))));
       queue = XCDR (queue);
     }
 }