(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.

3 files changed, 103 insertions, 0 deletions

diff --git a/test/lisp/loadhist-resources/loadhist--bar.el b/test/lisp/loadhist-resources/loadhist--bar.el
new file mode 100644
index 00000000000..5c8914ed573
--- /dev/null
+++ b/test/lisp/loadhist-resources/loadhist--bar.el
@@ -0,0 +1,27 @@
+;;; loadhist--bar.el --- Dummy package for loadhist-tests  -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2022  Free Software Foundation, Inc.
+
+;; Author: Stefan Monnier <monnier@iro.umontreal.ca>
+
+;; This program is free software; you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; This program is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Code:
+
+(autoload 'loadhist--foo-inc "loadhist--foo")
+
+(defun loadhist--bar-dec (x) (1- x))
+
+(provide 'loadhist--bar)
+;;; loadhist--bar.el ends here
diff --git a/test/lisp/loadhist-resources/loadhist--foo.el b/test/lisp/loadhist-resources/loadhist--foo.el
new file mode 100644
index 00000000000..3574c220135
--- /dev/null
+++ b/test/lisp/loadhist-resources/loadhist--foo.el
@@ -0,0 +1,29 @@
+;;; loadhist--foo.el --- Dummy package for loadhist-tests  -*- lexical-binding: t; -*-
+
+;; Copyright (C) 2022  Free Software Foundation, Inc.
+
+;; Author: Stefan Monnier <monnier@iro.umontreal.ca>
+
+;; This program is free software; you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation, either version 3 of the License, or
+;; (at your option) any later version.
+
+;; This program is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;;; Code:
+
+(autoload 'loadhist--bar-dec "loadhist--bar")
+
+(defun loadhist--foo-inc (x) (1+ x))
+
+(provide 'loadhist--foo)
+;;; loadhist--foo.el ends here
diff --git a/test/lisp/loadhist-tests.el b/test/lisp/loadhist-tests.el
index a941ac06320..ef5fc164d34 100644
--- a/test/lisp/loadhist-tests.el
+++ b/test/lisp/loadhist-tests.el
@@ -54,4 +54,51 @@
   (should-error (unload-feature 'dired))
   (unload-feature 'dired-x))
 
+(defvar loadhist--tests-dir (file-name-directory (macroexp-file-name)))
+
+(ert-deftest loadhist-tests-unload-feature-nested ()
+  (add-to-list 'load-path (expand-file-name
+                           "loadhist-resources/"
+                           loadhist--tests-dir))
+  (declare-function loadhist--foo-inc "loadhist--foo")
+  (declare-function loadhist--bar-dec "loadhist--dec")
+  (load "loadhist--foo" nil t)
+  (should (and (functionp 'loadhist--bar-dec) (functionp 'loadhist--foo-inc)))
+  (should (autoloadp (symbol-function 'loadhist--bar-dec)))
+  (load "loadhist--bar" nil t)
+  (should (and (functionp 'loadhist--bar-dec) (functionp 'loadhist--foo-inc)))
+  (should (not (autoloadp (symbol-function 'loadhist--bar-dec))))
+  (should (not (autoloadp (symbol-function 'loadhist--foo-inc))))
+  (should (equal (list 40 42)
+                 (list (loadhist--bar-dec 41) (loadhist--foo-inc 41))))
+  (unload-feature 'loadhist--bar)
+  (should (and (functionp 'loadhist--bar-dec) (functionp 'loadhist--foo-inc)))
+  (should (autoloadp (symbol-function 'loadhist--bar-dec)))
+  (should (not (autoloadp (symbol-function 'loadhist--foo-inc))))
+  (unload-feature 'loadhist--foo)
+  (should (null (symbol-function 'loadhist--bar-dec)))
+  (should (null (symbol-function 'loadhist--foo-inc)))
+  (should (null (get 'loadhist--bar-dec 'function-history)))
+  (should (null (get 'loadhist--foo-inc 'function-history))))
+
+(ert-deftest loadhist-tests-unload-feature-notnested ()
+  (add-to-list 'load-path (expand-file-name
+                           "loadhist-resources/"
+                           loadhist--tests-dir))
+  (load "loadhist--foo" nil t)
+  (load "loadhist--bar" nil t)
+  (should (equal (list 40 42)
+                 (list (loadhist--bar-dec 41) (loadhist--foo-inc 41))))
+  (unload-feature 'loadhist--foo)
+  (should (functionp 'loadhist--bar-dec))
+  (should (not (autoloadp (symbol-function 'loadhist--bar-dec))))
+  (should  (let ((f (symbol-function 'loadhist--foo-inc)))
+             ;; Both choices seem acceptable.
+             (or (null f) (autoloadp f))))
+  (unload-feature 'loadhist--bar)
+  (should (null (symbol-function 'loadhist--bar-dec)))
+  (should (null (symbol-function 'loadhist--foo-inc)))
+  (should (null (get 'loadhist--bar-dec 'function-history)))
+  (should (null (get 'loadhist--foo-inc 'function-history))))
+
 ;;; loadhist-tests.el ends here