bug#15874: 24.3.50; exit! not properly font-locked in ruby-mode

[Dmitry Gutov <dgutov@yandex.ru>]

On 05.12.2013 04:50, Stefan Monnier wrote:
> Maybe a good way to handle them is via syntax-propertize,

Huh yes, that's an approach I haven't considered. But it seems more 
performance-intensive than the other options, at least as long as Emacs 
regexps don't support lookbehind.

If we could match "non-symbols", then the font-lock part of the problem 
would be solved, and appending "?" or "!" in the forward-token and 
backward-token functions would take negligible amount of time.

Say, why don't we have a "non-symbol" matcher? I would think that in 
many cases the regexp engine performs unnecessary work when it sees 
"\\_<" or "\\_>" because we often know in advance that the character 
we've already matched, or will have to match next, is a symbol 
constituent. But the engine has to check.

Most uses of `(regexp-opt '("boats" "boats" "boats") 'symbols)' would 
fall under this scenario.

 > then, especially
> if we can come up with a good regexp that distinguishes "method name"
> from "local variable name"?

Not really. References to local variables, unlike instance and global 
ones, are indistinguishable from method calls if we judge only by name.

We could backtrack until the beginning of block or method, look for 
assignments, etc, but that's a lot of work for marginal gain: we can 
just assume that each such reference is a method calls, because 
otherwise it would be syntax error.