(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, 3 insertions, 6 deletions

diff --git a/src/lread.c b/src/lread.c
index 9910db27de7..713c03243cb 100644
--- a/src/lread.c
+++ b/src/lread.c
@@ -5240,12 +5240,9 @@ for symbols and features not associated with any file.
 The remaining ENTRIES in the alist element describe the functions and
 variables defined in that file, the features provided, and the
 features required.  Each entry has the form `(provide . FEATURE)',
-`(require . FEATURE)', `(defun . FUNCTION)', `(autoload . SYMBOL)',
-`(defface . SYMBOL)', `(define-type . SYMBOL)',
-`(cl-defmethod METHOD SPECIALIZERS)', or `(t . SYMBOL)'.
-Entries like `(t . SYMBOL)' may precede a `(defun . FUNCTION)' entry,
-and mean that SYMBOL was an autoload before this file redefined it
-as a function.  In addition, entries may also be single symbols,
+`(require . FEATURE)', `(defun . FUNCTION)', `(defface . SYMBOL)',
+ `(define-type . SYMBOL)', or `(cl-defmethod METHOD SPECIALIZERS)'.
+In addition, entries may also be single symbols,
 which means that symbol was defined by `defvar' or `defconst'.
 
 During preloading, the file name recorded is relative to the main Lisp