Re: <filename> changed on disk; really edit the buffer?

[Richard Stallman]

    `make-auto-save-file-name' already tries to substitute the
    default-directory in

                        (expand-file-name
                         (format "#%s#" buffer-name)
                         ;; Try a few alternative directories, to get one we can
                         ;; write it.
                         (cond
                          ((file-writable-p default-directory) 
default-directory)

That is in the branch for non-file buffers.