Autoinsert: Allow condition to be a function

Currently a condition can only be a regexp or a major-mode symbol but
there's no real reason to not allow any predicate as a condition.
* doc/misc/autotype.texi: Document the new condition type in
'auto-insert-alist'.
* etc/NEWS: Announce the new condition type in 'auto-insert-alist'.
* lisp/autoinsert.el (auto-insert-alist): Add the new condition type to
the variable documentation.
(auto-insert): Change the way the condition is matched to allow for
custom predicates.
* test/lisp/autoinsert-tests.el
(autoinsert-tests-auto-insert-lambda/-nil): Add two tests for
lambdas (nil and t cases)
(autoinsert-tests-auto-insert-predicate/-nil): Add two tests for named
predicates (nil and t cases) (Bug#79178)

doc/misc/autotype.texi

@@ -273,13 +273,15 @@ empty file is visited.  This is accomplished by putting
 
 @vindex auto-insert-alist
   What gets inserted, if anything, is determined by the variable
-@code{auto-insert-alist}.  The @sc{car} of each element of this list
-is either a mode name, making the element applicable when a buffer is
-in that mode, or a string, which is a regexp matched against a
-buffer's file name (the latter enables you to distinguish between
-different kinds of files that have the same mode in Emacs).  The
-@sc{car} of an element may also be a cons cell, consisting of mode
-name or regexp, as above, and an additional descriptive string.
+@code{auto-insert-alist}.  The @sc{car} of each element of this list is
+either a mode name, making the element applicable when a buffer is in
+that mode, or a string, which is a regexp matched against a buffer's
+file name (the latter enables you to distinguish between different kinds
+of files that have the same mode in Emacs).  It can also be a predicate
+declared through a plist of the form @code{(predicate @var{function})}.
+@var{function} should be a predicate function of no arguments.  The
+@sc{car} of an element may also be a cons cell, consisting of mode name
+or regexp, as above, and an additional descriptive string.
 
   When a matching element is found, the @sc{cdr} says what to do.  It may
 be a string, which is a file name, whose contents are to be inserted, if

etc/NEWS

@@ -700,6 +700,15 @@ you could already use 'C-u C-x C-n' to clear the goal column.
 
 * Changes in Specialized Modes and Packages in Emacs 31.1
 
+** Autoinsert
+
++++
+*** New condition for 'auto-insert-alist'.
+'auto-insert-alist' now also allows to have a predicate taking no
+argument as conditions.  These types of conditions should be declared
+with '(predicate FUNCTION)'.  This allows to trigger 'auto-insert'
+with finer grained control.
+
 ** Register
 
 *** The "*Register Preview*" buffer shows only suitable registers.

lisp/autoinsert.el

@@ -325,6 +325,9 @@ The document was typeset with
 Elements look like (CONDITION . ACTION) or ((CONDITION . DESCRIPTION) . ACTION).
 CONDITION may be a regexp that must match the new file's name, or it may be
 a symbol that must match the major mode for this element to apply.
+CONDITION can also be a custom predicate of no arguments declared with
+'(predicate FUNCTION)'.  Emacs will insert the text if the predicate
+function returns non-nil.
 Only the first matching element is effective.
 Optional DESCRIPTION is a string for filling `auto-insert-prompt'.
 ACTION may be a skeleton to insert (see `skeleton-insert'), an absolute
@@ -368,12 +371,24 @@ Matches the visited file name against the elements of `auto-insert-alist'."
                 (pcase-lambda (`(,cond . ,action))
                   (if (atom cond)
                       (setq desc cond)
-                    (setq desc (cdr cond)
-                          cond (car cond)))
-                  (when (if (symbolp cond)
-                            (derived-mode-p cond)
+                    ;; if `cond' is a predicate, don't split it but set `desc' to a custom string
+                    (if (and (consp cond) (equal (car cond) 'predicate))
+                        (setq desc "predicate")
+                      (setq desc (cdr cond)
+                            cond (car cond))))
+                  (when (cond
+                         ;; `cond' should be a major-mode variable
+                         ((symbolp cond)
+                          (derived-mode-p cond))
+
+                         ;; `cond' should be a predicate that takes no argument
+                         ((and (consp cond) (equal (car cond) 'predicate))
+                          (funcall (cadr cond)))
+
+                         ;; cond should be a regexp
+                         (t
                           (and buffer-file-name
-                               (string-match cond buffer-file-name)))
+                               (string-match cond buffer-file-name))))
                     action))
                 auto-insert-alist)))
          (goto-char 1)

test/lisp/autoinsert-tests.el

@@ -76,6 +76,40 @@
       (auto-insert)
       (should (equal (buffer-string) "2nd")))))
 
+(ert-deftest autoinsert-tests-auto-insert-lambda ()
+  (let ((auto-insert-alist
+         '(((predicate (lambda () t)) . (lambda () (insert "foo")))))
+        (auto-insert-query nil))
+    (with-temp-buffer
+      (auto-insert)
+      (should (equal (buffer-string) "foo")))))
+
+(ert-deftest autoinsert-tests-auto-insert-predicate ()
+  (defun predicate () t)
+  (let ((auto-insert-alist
+         '(((predicate predicate) . (lambda () (insert "foo")))))
+        (auto-insert-query nil))
+    (with-temp-buffer
+      (auto-insert)
+      (should (equal (buffer-string) "foo")))))
+
+(ert-deftest autoinsert-tests-auto-insert-lambda-nil ()
+  (let ((auto-insert-alist
+         '(((predicate (lambda () nil)) . (lambda () (insert "foo")))))
+        (auto-insert-query nil))
+    (with-temp-buffer
+      (auto-insert)
+      (should (equal (buffer-string) "")))))
+
+(ert-deftest autoinsert-tests-auto-insert-predicate-nil ()
+  (defun predicate () nil)
+  (let ((auto-insert-alist
+         '(((predicate predicate) . (lambda () (insert "foo")))))
+        (auto-insert-query nil))
+    (with-temp-buffer
+      (auto-insert)
+      (should (equal (buffer-string) "")))))
+
 (ert-deftest autoinsert-tests-define-auto-insert-before ()
   (let ((auto-insert-alist
          (list (cons 'text-mode (lambda () (insert "foo")))))