Improve ERC's internal invisibility API

* etc/ERC-NEWS: Mention that line endings have moved from the end to
the beginning of hidden messages.
* lisp/erc/erc-fill.el (erc-fill--wrap-ensure-dependencies): Warn when
users have `erc-legacy-invisible-bounds-p' enabled, and force it to
its default value of nil in the current buffer.
(erc-fill-wrap-mode, erc-fill-wrap-enable): Move business involving
compat variable for enabling legacy hidden-message behavior to helper.
* lisp/erc/erc-match.el (erc-match--hide-fools-offset-bounds): Move
internal variable from to main library file and rename to
`erc-legacy-invisible-bounds-p'.  Also make obsolete and flip
semantics so a non-nil value enables the traditional behavior.
(erc-match--hide-message): Move to main library file and rename to
`erc--hide-message'.  Add a property-value parameter instead of
hard-coding to `erc-match'.  Also, condition behavior on renamed
compatibility flag `erc-legacy-invisible-bounds-p'.
(erc-hide-fools): Call `erc--hide-message' with own value for
`invisible' property specifically for fools.  That is, use
`match-fools' rather than `erc-match' or `erc-match-fools' to save
room when visually inspecting.  This retains the module name as a
prefix to hopefully minimize collisions with invisibility spec members
owned by non-ERC minor modes.  The `timestamp' spec member owned by
erc-stamp likewise lacks a namespace prefix, but its feature/group
affiliation is self-evident.
(erc-match--modify-invisibility-spec): Use toggle command
non-interactively for adding and removing invisibility spec member.
(erc-match-toggle-hidden-fools): Add explicit override argument and
defer to general helper for actually modifying spec.
(erc-match--toggle-hidden): New helper for toggling invisibility
spec.
* lisp/erc/erc.el (erc--merge-prop): If new value is a list, prepend
onto existing.  Add note about possible space optimization.
(erc-legacy-invisible-bounds-p): New obsolete compat variable to
enable traditional pre-5.6 invisibility interval on hidden messages.
Replaces `erc-match--hide-fools-offset-bounds-p' but has an inverted
meaning.  The new default value of nil means invisibility covers a
shifted interval consisting of the message body plus the line ending
immediately preceding it.
(erc--hide-message): New function, formerly `erc-match--hide-message'
from erc-match.el introduced in ERC 5.6.
* test/lisp/erc/erc-scenarios-match.el:
(erc-scenarios-match--invisible-stamp): Fix comment and use API
function in interactive convenience setup.
(erc-scenarios-match--find-bol): New test helper.
(erc-scenarios-match--find-eol): Fix bug affecting interactive use.
(erc-scenarios-match--stamp-left-fools-invisible,
erc-scenarios-match--stamp-right-fools-invisible,
erc-scenarios-match--stamp-right-invisible-fill-wrap,
erc-scenarios-match--stamp-both-invisible-fill-static): Update
`invisible' property from `erc-match' to `match-fools'.
(erc-scenarios-match--stamp-right-fools-invisible,
erc-scenarios-match--stamp-both-invisible-fill-static): Move test
body to function of same name for use in multiple cases.
(erc-scenarios-match--stamp-right-fools-invisible--nooffset,
erc-scenarios-match--stamp-both-invisible-fill-static--nooffset): New
test variants asserting proper hiding with old pre-5.6 invisibility
interval.
* test/lisp/erc/erc-tests.el (erc-tests--equal-including-properties):
Relocate macro higher in same file.
(erc--merge-prop): New test.  (Bug#64301)

lisp/erc/erc-fill.el

@@ -324,11 +324,17 @@ is 0, reset to value of `erc-fill-wrap-visual-keys'."
   ;; Not sure if this is problematic because `erc-bol' takes no args.
   "<remap> <erc-bol>" #'erc-fill--wrap-beginning-of-line)
 
-(defvar erc-match-mode)
 (defvar erc-button-mode)
-(defvar erc-match--hide-fools-offset-bounds)
+(defvar erc-legacy-invisible-bounds-p)
 
 (defun erc-fill--wrap-ensure-dependencies ()
+  (with-suppressed-warnings ((obsolete erc-legacy-invisible-bounds-p))
+    (when erc-legacy-invisible-bounds-p
+      (erc--warn-once-before-connect  'erc-fill-wrap-mode
+        "Module `fill-wrap' is incompatible with the obsolete compatibility"
+        " flag `erc-legacy-invisible-bounds-p'.  Disabling locally in %s."
+        (current-buffer))
+      (setq-local erc-legacy-invisible-bounds-p nil)))
   (let (missing-deps)
     (unless erc-fill-mode
       (push 'fill missing-deps)
@@ -389,9 +395,6 @@ them to every line."
    (setq erc-fill--function #'erc-fill-wrap)
    (add-function :after (local 'erc-stamp--insert-date-function)
                  #'erc-fill--wrap-stamp-insert-prefixed-date)
-   (when (or (bound-and-true-p erc-match-mode) (memq 'match erc-modules))
-     (require 'erc-match)
-     (setq erc-match--hide-fools-offset-bounds t))
    (when erc-fill-wrap-merge
      (add-hook 'erc-button--prev-next-predicate-functions
                #'erc-fill--wrap-merged-button-p nil t))

lisp/erc/erc-match.el

@@ -655,24 +655,10 @@ See `erc-log-match-format'."
 					(get-buffer (car buffer-cons))))))
     (switch-to-buffer buffer-name)))
 
-(defvar-local erc-match--hide-fools-offset-bounds nil)
-
 (defun erc-hide-fools (match-type _nickuserhost _message)
   "Hide comments from designated fools."
   (when (eq match-type 'fool)
-    (erc-match--hide-message)))
-
-(defun erc-match--hide-message ()
-  (progn ; FIXME raise sexp
-    (if erc-match--hide-fools-offset-bounds
-        (let ((beg (point-min))
-              (end (point-max)))
-          (save-restriction
-            (widen)
-            (erc--merge-prop (1- beg) (1- end) 'invisible 'erc-match)))
-      ;; Before ERC 5.6, this also used to add an `intangible'
-      ;; property, but the docs say it's now obsolete.
-      (erc--merge-prop (point-min) (point-max) 'invisible 'erc-match))))
+    (erc--hide-message 'match-fools)))
 
 (defun erc-beep-on-match (match-type _nickuserhost _message)
   "Beep when text matches.
@@ -682,19 +668,31 @@ This function is meant to be called from `erc-text-matched-hook'."
 
 (defun erc-match--modify-invisibility-spec ()
   "Add an `erc-match' property to the local spec."
+  ;; Hopefully, this will be extended to do the same for other
+  ;; invisible properties managed by this module.
   (if erc-match-mode
-      (add-to-invisibility-spec 'erc-match)
+      (erc-match-toggle-hidden-fools +1)
     (erc-with-all-buffers-of-server nil nil
-      (remove-from-invisibility-spec 'erc-match))))
+      (erc-match-toggle-hidden-fools -1))))
 
-(defun erc-match-toggle-hidden-fools ()
+(defun erc-match-toggle-hidden-fools (arg)
   "Toggle fool visibility.
-Expect `erc-hide-fools' or a function that does something similar
-to be in `erc-text-matched-hook'."
-  (interactive)
-  (if (memq 'erc-match (ensure-list buffer-invisibility-spec))
-      (remove-from-invisibility-spec 'erc-match)
-    (add-to-invisibility-spec 'erc-match)))
+Expect the function `erc-hide-fools' or similar to be present in
+`erc-text-matched-hook'."
+  (interactive "P")
+  (erc-match--toggle-hidden 'match-fools arg))
+
+(defun erc-match--toggle-hidden (prop arg)
+  "Toggle invisibility for spec member PROP.
+Treat ARG in a manner similar to mode toggles defined by
+`define-minor-mode'."
+  (when arg
+    (setq arg (prefix-numeric-value arg)))
+  (if (memq prop (ensure-list buffer-invisibility-spec))
+      (unless (natnump arg)
+        (remove-from-invisibility-spec prop))
+    (when (or (not arg) (natnump arg))
+      (add-to-invisibility-spec prop))))
 
 (provide 'erc-match)
 

lisp/erc/erc.el

@@ -3011,22 +3011,51 @@ If STRING is nil, the function does nothing."
 (defvar erc--compose-text-properties nil
   "Non-nil when `erc-put-text-property' defers to `erc--merge-prop'.")
 
+;; To save space, we could maintain a map of all readable property
+;; values and optionally dispense archetypal constants in their place
+;; in order to ensure all occurrences of some list (a b) across all
+;; text-properties in all ERC buffers are actually the same object.
 (defun erc--merge-prop (from to prop val &optional object)
-  "Compose existing PROP values with VAL between FROM and TO in OBJECT.
+  "Combine existing PROP values with VAL between FROM and TO in OBJECT.
 For spans where PROP is non-nil, cons VAL onto the existing
 value, ensuring a proper list.  Otherwise, just set PROP to VAL.
-See also `erc-button-add-face'."
+When VAL is itself a list, prepend its members onto an existing
+value.  See also `erc-button-add-face'."
   (let ((old (get-text-property from prop object))
         (pos from)
         (end (next-single-property-change from prop object to))
         new)
     (while (< pos to)
-      (setq new (if old (cons val (ensure-list old)) val))
+      (setq new (if old
+                    (if (listp val)
+                        (append val (ensure-list old))
+                      (cons val (ensure-list old)))
+                  val))
       (put-text-property pos end prop new object)
       (setq pos end
             old (get-text-property pos prop object)
             end (next-single-property-change pos prop object to)))))
 
+(defvar erc-legacy-invisible-bounds-p nil
+  "Whether to hide trailing rather than preceding newlines.
+Beginning in ERC 5.6, invisibility extends from a message's
+preceding newline to its last non-newline character.")
+(make-obsolete-variable 'erc-legacy-invisible-bounds-p
+                        "decremented interval now permanent" "30.1")
+
+(defun erc--hide-message (value)
+  "Apply `invisible' text-property with VALUE to current message.
+Expect to run in a narrowed buffer during message insertion."
+  (if erc-legacy-invisible-bounds-p
+      ;; Before ERC 5.6, this also used to add an `intangible'
+      ;; property, but the docs say it's now obsolete.
+      (erc--merge-prop (point-min) (point-max) 'invisible value)
+    (let ((beg (point-min))
+          (end (point-max)))
+      (save-restriction
+        (widen)
+        (erc--merge-prop (1- beg) (1- end) 'invisible value)))))
+
 (defun erc-display-message-highlight (type string)
   "Highlight STRING according to TYPE, where erc-TYPE-face is an ERC face.