describe-bindings hack added by sytse on Sat Dec 16 09:28:01 2017

Make a separate byte-compiled function for each file, using closure support in emacs with lexical bindings:

(defun sw/make-find-file-function (file)
  (lambda ()
    (find-file file)))

At emacs startup, make an uninterned symbol with the filename as its name, and fset its function definition to this closure:

(defmacro sw/define-file-key (keymap key file)
  (unless (stringp file)
    (signal 'wrong-type-argument (list 'stringp file)))
  (let ((fn (sw/make-find-file-function file)))
    `(let ((sym (make-symbol ,file)))
       (fset sym ',fn)
       (define-key ,keymap ,key sym))))

Byte code for (sw/define-file-key ctl-x-map [?t ?f] "~/git/foo/tests/foo.el"):

190     constant  make-symbol
193     constant  "~/git/foo/tests/foo.el"
196     call      1         ; Make an uninterned symbol
197     dup                 ; that gets displayed as the
                            ; keybinding
198     constant  <compiled-function>
      args: nil
     interactive: nil
    0       constant  find-file
    1       constant  "~/git/foo/tests/foo.el"
    2       call      1
    3       return    

201     fset                ; Set the function slot to a
                            ; function constant that calls
                            ; find-file on this file
202     discard   
203     constant  define-key
206     varref    ctl-x-map
207     constant  [116 102]
210     stack-ref 3
211     call      3         ; Call define-key to bind
                            ; C-x tf to this symbol
212     discardN  2

Now describe-bindings and helm-descbinds show the filename instead of "??".