master 8513ecc8a38 2/7: Restore predicate for detecting date stamps in ERC

[F. Jason Park]

branch: master
commit 8513ecc8a38f76b497348bbae61ae4c5ac5a9995
Author: F. Jason Park <jp@neverwas.me>
Commit: F. Jason Park <jp@neverwas.me>

    Restore predicate for detecting date stamps in ERC
    
    * etc/ERC-NEWS: Mention function `erc-stamp-inserting-date-stamp-p'.
    * lisp/erc/erc-stamp.el (erc-stamp-inserting-date-stamp-p): New
    function for third parties to detect whether the message being
    inserted is a date stamp.  Date stamps as independent messages were
    originally introduced as part of bug#60936.
---
 etc/ERC-NEWS          | 4 +++-
 lisp/erc/erc-stamp.el | 7 +++++++
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/etc/ERC-NEWS b/etc/ERC-NEWS
index 6dc8af3c514..f19255f8d75 100644
--- a/etc/ERC-NEWS
+++ b/etc/ERC-NEWS
@@ -474,7 +474,9 @@ also not appreciate that 'erc-timestamp-last-inserted-left' 
no longer
 records the final trailing newline in 'erc-timestamp-format-left'.  If
 these inconveniences prove too encumbering to deal with right away,
 see the escape hatch 'erc-stamp-prepend-date-stamps-p', which should
-help ease the transition.
+help ease the transition.  As for detecting these new stamp-only
+messages from members of 'erc-insert-modify-hook' and friends, see the
+function 'erc-stamp-inserting-date-stamp-p'.
 
 *** The role of a module's Custom group is now more clearly defined.
 Associating built-in modules with Custom groups and "provided" library
diff --git a/lisp/erc/erc-stamp.el b/lisp/erc/erc-stamp.el
index a65b564ba6c..cfd756ce625 100644
--- a/lisp/erc/erc-stamp.el
+++ b/lisp/erc/erc-stamp.el
@@ -678,6 +678,13 @@ value of t means the option's value doesn't require 
trimming.")
                                             0 erc-stamp--date-format-end)
                                erc-timestamp-format-left))))
 
+(defun erc-stamp-inserting-date-stamp-p ()
+  "Return non-nil if the narrowed buffer contains a date stamp.
+Expect to be called by members of `erc-insert-modify-hook' and
+`erc-insert-post-hook' to detect whether the message being
+inserted is a date stamp."
+  (erc--check-msg-prop 'erc--msg 'datestamp))
+
 ;; Calling `erc-display-message' from within a hook it's currently
 ;; running is roundabout, but it's a definite means of ensuring hooks
 ;; can act on the date stamp as a standalone message to do things like