How to highlight “prettified” symbols?

Question

I would light to highlight where prettify-symbols-mode has prettified a symbol. For example, if Emacs displays lambda as λ it could be displayed with a gray background, but if a literal λ occurs in the text, the background would remain white.

How can I achieve this?

Answer 1

You could add another matcher using the same regexp as prettify-symbols-mode, i.e. something like this:

(font-lock-add-keywords
 nil
 `((,(regexp-opt (mapcar 'car prettify-symbols-alist) t)
    (0 (when (funcall prettify-symbols-compose-predicate
                      (match-beginning 0)
                      (match-end 0)
                      (match-string 0))
         '(:background "grey80"))
       append)))
 'append)