Status of font-lock-fontify-region and ways to fontify a string

[Clément Pit--Claudel]

Hi all,

Emacs 25 complains when I use font-lock-fontify-buffer, suggesting I replace it 
by font-lock-flush or font-lock-ensure. I've been running into problems with 
both of these (I don't have good repros yet, but I've had cases where even 
calling both in succession didn't result in the buffer being fontified).

This causes me to wonder: what is the recommended way to fontify a string 
according to a given mode? font-lock-fontify region was not marked 
interactive-only; is it still a good idea to call it from lisp code? 
Alternatively, is the following snippet expected to always returned a fontified 
string?

(with-temp-buffer
  (my-fancy-mode)
  (insert some-text)
  (font-lock-ensure)
  (buffer-string))

Similarly, since this is a common operation in the mode that I'm currently 
working on, does font-lock provide a way to fontify a string as if it was the 
only text in the current buffer? At the moment I use

(with-temp-buffer
    (erase-buffer)
    (fundamental-mode)
    (when ref-buffer
      (cl-loop for var in '(font-lock-keywords
                            font-lock-keywords-only
                            font-lock-keywords-case-fold-search
                            font-lock-extra-managed-props
                            font-lock-mode
                            font-lock-set-defaults
                            font-lock-syntax-table
                            font-lock-syntactic-face-function)
               do (set (make-local-variable var)
                       (buffer-local-value var ref-buffer))))
    (setq-local font-lock-support-mode nil)
    (setq-local font-lock-fontified nil)
    (insert str)
    (let ((font-lock-major-mode major-mode))
      (font-lock-fontify-region (point-min) (point-max)))
    (buffer-string))

which seems very brittle.

Thanks,
Clément.

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