Use comment-search-forward to see if a delimiter starts a comment.

(looking-at comment-start-skip) is not always enough, because some
major modes set `comment-start-skip' to not exactly what we expect.

Fixes #35.

rainbow-delimiters.el

@@ -96,6 +96,7 @@
 ;; - Intelligent support for other languages: Ruby, LaTeX tags, et al.
 
 ;;; Code:
+(require 'newcomment)
 
 ;;; Customize interface:
 
@@ -437,8 +438,19 @@ Returns t if char at loc meets one of the following conditions:
    (nth 4 ppss)                ; inside comment?
    (save-excursion             ; starting a comment?
      (goto-char loc)
-     (let ((inhibit-changing-match-data t))
-       (looking-at comment-start-skip)))
+     (and
+      ;; Fast path: if this test fails, it's not a comment, and we avoid a
+      ;; costly `comment-search-forward' call.
+      (let ((inhibit-changing-match-data t))
+        (looking-at comment-start-skip))
+      ;; Some major modes set `comment-start-skip' to not exactly what we
+      ;; expect, catching more than actual comments. Use
+      ;; `comment-search-forward' to see if it's really a comment.
+      ;; NOTE: is lookahead of five characters enough for all languages? I hope
+      ;; there's no language with 6-character comment delimiters.
+      (save-match-data
+        (let ((comment-start-pos (comment-search-forward (min (+ 5 loc) (point-max)) t)))
+          (and comment-start-pos (= loc comment-start-pos))))))
    (and rainbow-delimiters-escaped-char-predicate
         (funcall rainbow-delimiters-escaped-char-predicate loc))))
 
@@ -531,6 +543,8 @@ Used by font-lock for dynamic highlighting."
 
 (defun rainbow-delimiters-mode-turn-on ()
   "Set up `rainbow-delimiters-mode'."
+  ;; Necessary for our use of `comment-search-forward'.
+  (comment-normalize-vars t)
   ;; Flush the ppss cache now in case there's something left in there.
   (setq rainbow-delimiters-parse-partial-sexp-cache nil)
   (add-hook 'before-change-functions 'rainbow-delimiters-syntax-ppss-flush-cache t t)