bug#38104: 27.0.50; elixir-mode fontification is very slow

[Mattias Engdegård]

26 nov. 2019 kl. 17.26 skrev Dmitry Gutov <dgutov@yandex.ru>:

> elixir-mode does use rx, heavily. Albeit with a thin wrapper.

As it turned out, rx is fine (now); elixir-mode, not quite. In elixir-mode.el, 
we have

      (identifiers . ,(rx (one-or-more (any "A-Z" "a-z" "_"))
                          (zero-or-more (any "A-Z" "a-z" "0-9" "_"))
                          (optional (or "?" "!"))))

First, this regex is suboptimal: the first character of an identifier should 
occur exactly once, or you get bad backtracking behaviour. Just remove the 
one-or-more construct:

      (identifiers . ,(rx (any "A-Z" "a-z" "_")
                          (zero-or-more (any "A-Z" "a-z" "0-9" "_"))
                          (optional (or "?" "!"))))

This definition is then used in several places, but two in particular are of 
interest to us:

    ;; Module attributes
    (,(elixir-rx (and "@" (1+ identifiers)))

The construct (1+ identifiers) was perhaps meant to match multiple identifiers, 
but it doesn't (no separator); it just matches an identifier in several ways, 
which again leads to bad backtracking behaviour.
The same problem here:

    ;; Map keys
    (,(elixir-rx (group (and (one-or-more identifiers) ":")) space)

Remove the 1+ and one-or-more and it's fast again.

Why did this "work" with the old rx implementation? Because that code had a 
nasty bug: it does not bracket definitions in rx-constituents properly. Example:

(let ((rx-constituents (cons '(hello . "HELLO") rx-constituents)))
  (rx-to-string '(1+ hello) t))
=> "HELLO+"

The new rx implementation does not suffer from this bug.

The result in your case is that the old rx, when translating (1+ identifiers), 
only tacked the "+" onto whatever regexp 'identifiers' produced, resulting in

"[A-Z_a-z]+[0-9A-Z_a-z]*[!?]?+"

which is a lot faster, since only the final [!?] is repeated twice (and it 
probably doesn't match very often).