(defun helm-completing-read-notmuch-tag (prompt collection test require init hist default inherit-input-method name buffer)
  "Completing read on notmuch tags, preferring those on this line"
  (let ((local-tag-list (notmuch-search-get-tags)))
    (or ; voodoo from helm-completing-read-symbols
     (helm
      :sources `(((name . "Local Tags")
                  (action . identity)
                  (candidates . ,local-tag-list))
                 ((name . "Global Tags")
                  (action . identity)
                  (candidates . ,collection)))
      :prompt prompt
      :buffer buffer
      :input init
      :history hist
      :resume 'noresume
      :default (or default ""))
     (helm-mode--keyboard-quit))))

(add-to-list 'helm-completing-read-handlers-alist '(notmuch-search-filter-by-tag . helm-completing-read-notmuch-tag))

(defun notmuch-search-filter-by-tag-at-point ()
  (interactive)
  (notmuch-search-filter-by-tag (thing-at-point 'symbol)))
(define-key notmuch-search-mode-map "T" 'notmuch-search-filter-by-tag-at-point)