mu4e-compose: allow including parent mime parts

mu4e-compose-reply-include-mime-types specifies the mime-types for
message attachments that should be included in replies, with the default
set to "text/x-patch" -- i.e., patches are included in replies, so you
can comment on them.

As mentioned in #2896.

mu4e/mu4e-compose.el

@@ -380,6 +380,8 @@ non-nil, make it a \"wide\" reply (a.k.a. \"reply-to-all\")."
        (with-current-buffer (mu4e--message-call #'message-reply to wide)
          (message-goto-body)
          (insert (mu4e--compose-cite parent))
+         ;; include matching MIME parts from the parent
+         (mu4e--reply-insert-mime-parts parent)
          (current-buffer))))))
 
 ;;;###autoload

mu4e/mu4e-draft.el

@@ -26,6 +26,7 @@
 ;; look a little convoluted since we need to subvert the gnus/message
 ;; functions a bit to work with mu4e.
 (require 'message)
+(require 'mm-decode)
 (require 'mu4e-config)
 (require 'mu4e-helpers)
 (require 'mu4e-contacts)
@@ -186,6 +187,17 @@ you don't consider that reasonable, set to nil."
   :safe 'booleanp
   :group 'mu4e-compose)
 
+(defcustom mu4e-compose-reply-include-mime-types '("text/x-patch")
+  "MIME types from the parent message to include when replying.
+
+When replying to a message, MIME parts with content-types in this
+list are automatically included in the the reply message. This is
+useful for carrying over patches or other structured text parts.
+
+Set to nil to disable."
+  :type '(repeat string)
+  :group 'mu4e-compose)
+
 ;;
 ;; display the ready-to-go display buffer in the desired way.
 ;;
@@ -774,6 +786,50 @@ Returns the new buffer."
       (mu4e--prepare-post oldframe oldwinconf)
       draft-buffer)))
 
+(defun mu4e--collect-mime-parts (handles types)
+  "Collect MIME parts from HANDLES with content-types in TYPES.
+
+HANDLES is as returned by `mm-dissect-buffer'. TYPES is a list of
+MIME type strings.
+
+Returns a list of matching leaf handles."
+  (cond
+   ;; Leaf handle (buffer in car).
+   ((bufferp (car handles))
+    (when (member (mm-handle-media-type handles) types)
+      (list handles)))
+   ;; Composite handle, e.g. multipart/* (type string in car).
+   ((stringp (car handles))
+    (mapcan (lambda (h) (mu4e--collect-mime-parts h types))
+            (cdr handles)))
+   ;; Plain list of handles.
+   (t
+    (mapcan (lambda (h) (mu4e--collect-mime-parts h types))
+            handles))))
+
+(defun mu4e--reply-insert-mime-parts (msg)
+  "Insert MIME parts from MSG matching `mu4e-compose-reply-include-mime-types'.
+Matching parts are inserted inline at point so that the user can
+comment on them directly."
+  (when mu4e-compose-reply-include-mime-types
+    (let ((handles
+           (with-temp-buffer
+             (insert-file-contents-literally
+              (mu4e-message-readable-path msg))
+             (mm-dissect-buffer t t))))
+      (when handles
+        (unwind-protect
+            (dolist (part (mu4e--collect-mime-parts
+                          handles mu4e-compose-reply-include-mime-types))
+              (let ((filename (mm-handle-filename part))
+                    (content (mm-get-part part)))
+                (insert "\n")
+                (when filename
+                  (insert filename ":\n"))
+                (insert content)
+                (unless (bolp) (insert "\n"))))
+          (mm-destroy-parts handles))))))
+
 (defun mu4e--draft-with-parent (compose-type parent compose-func)
   "Draft a message based on some parent message.
 COMPOSE-TYPE, COMPOSE-FUNC and PARENT are as in `mu4e--draft',

mu4e/mu4e.texi

@@ -1814,6 +1814,11 @@ that, you can set (or @code{let}-bind) @code{message-cite-function} to
 
 See @ref{(message) Reply} and @ref{(message) Wide Reply} for further details.
 
+@code{mu4e-compose-reply-include-mime-types} specifies the MIME-types for
+message attachments that should be included in replies, with the default set to
+@code{"text/x-patch"}-- i.e., patches are included in replies, so you can
+comment on them. Set to @code{nil} to disable.
+
 @subsubsection Interactively deciding whom to reply to
 
 In older @t{mu4e} versions, @code{mu4e-compose-reply} would @emph{ask} whether