mu4e: revamp emacs bookmark support

The support for emacs in bookmarks in mu4e-contrib has been broken for a while;
we removed it an now re-implement it as part of mu4e-proper.

Fixes: issue #2255.

mu4e/mu4e-headers.el

@@ -1190,6 +1190,9 @@ The following specs are supported:
         (add-function :before-until (local 'eldoc-documentation-function)
                       #'mu4e-headers-eldoc-function))))
 
+  ;; support bookmarks.
+  (set (make-local-variable 'bookmark-make-record-function)
+       'mu4e--make-bookmark-record)
   ;; maybe update the current headers upon indexing changes
   (add-hook 'mu4e-index-updated-hook #'mu4e~headers-maybe-auto-update)
   (setq

mu4e/mu4e-helpers.el

@@ -1,6 +1,6 @@
 ;;; mu4e-helpers.el -- part of mu4e -*- lexical-binding: t -*-
 
-;; Copyright (C) 2021 Dirk-Jan C. Binnema
+;; Copyright (C) 2022 Dirk-Jan C. Binnema
 
 ;; Author: Dirk-Jan C. Binnema <djcb@djcbsoftware.nl>
 ;; Maintainer: Dirk-Jan C. Binnema <djcb@djcbsoftware.nl>
@@ -30,6 +30,7 @@
 (require 'seq)
 (require 'ido)
 (require 'cl-lib)
+(require 'bookmark)
 
 (require 'mu4e-config)
 
@@ -521,6 +522,30 @@ Or go to the top level if there is none."
           ('mu4e-view-mode    "(mu4e)Message view")
           (_                  "mu4e"))))
 
+
+;;; bookmarks
+(defun mu4e--make-bookmark-record ()
+  "Create a bookmark for the message at point."
+  (let* ((msg     (mu4e-message-at-point))
+	 (subject (or (plist-get msg :subject) "No subject"))
+	 (date	  (plist-get msg :date))
+	 (date	  (if date (format-time-string "%F: " date) ""))
+	 (title	  (format "%s%s" date subject))
+	 (msgid	  (or (plist-get msg :message-id)
+		      (mu4e-error "Cannot bookmark message without message-id"))))
+    `(,title
+      ,@(bookmark-make-record-default 'no-file 'no-context)
+      (message-id . ,msgid)
+      (handler    . mu4e--jump-to-bookmark))))
+
+(declare-function mu4e-view-message-with-message-id "mu4e-view")
+(declare-function mu4e-message-at-point             "mu4e-message")
+
+(defun mu4e--jump-to-bookmark (bookmark)
+  "View the message referred to by BOOKMARK."
+  (when-let ((msgid (bookmark-prop-get bookmark 'message-id)))
+    (mu4e-view-message-with-message-id msgid)))
+
 ;;; Macros
 
 (defmacro mu4e-setq-if-nil (var val)

mu4e/mu4e-view.el

@@ -1052,6 +1052,11 @@ Based on Gnus' article-mode."
   (mu4e-context-minor-mode)
   (mu4e-search-minor-mode)
   (setq buffer-undo-list t);; don't record undo info
+
+  ;; support bookmarks.
+  (set (make-local-variable 'bookmark-make-record-function)
+       'mu4e--make-bookmark-record)
+
   ;; autopair mode gives error when pressing RET
   ;; turn it off
   (when (boundp 'autopair-dont-activate)

mu4e/mu4e.texi

@@ -2996,6 +2996,7 @@ with other tools.
 
 @menu
 * Emacs default::Making mu4e the default emacs e-mail program
+* Emacs bookmarks::Using Emacs' bookmark system
 * Org-mode links::Adding mu4e to your organized life
 * Org-contacts::Hooking up with org-contacts
 * BBDB::Hooking up with the Insidious Big Brother Database
@@ -3028,6 +3029,16 @@ mail, customize the variable @code{read-mail-command}.
 
 (@pxref{Top,,Emacs,Sending Mail, Mail Methods})
 
+
+@node Emacs bookmarks
+@section Emacs bookmarks
+
+@t{mu4e} supports linking to the message-at-point through the normal
+Emacs built-in bookmark system. The links are based on the message's
+message-id, and thus the bookmarks stay valid even if you move the
+message around.
+
+
 @node Org-mode links
 @section Org-mode links