vc-find-revision-no-save calls set-auto-mode, and then we repeat that call
in diff-syntax-fontify-props.
Which is a problem with CC Mode because it parses the buffer inside the
major mode function, and ends up doing that twice, at least the first time
the buffer is initialized before being saved to revision cache. Not exactly
double the work, but more work nevertheless.
This is a split off from https://debbugs.gnu.org/61667#587. I thought
I recalled an existing report of vc-diff being slow, but apparently not.
@@ -2883,9 +2883,6 @@ diff--get-revision-properties
(buffer (cdr entry)))
(if (buffer-live-p buffer)
(progn
- ;; Don't re-initialize the buffer (which would throw
- ;; away the previous fontification work).
- (setq file nil)
(setq diff--cached-revision-buffers
(cons entry
(delq entry diff--cached-revision-buffers))))
@@ -2905,7 +2902,8 @@ diff--get-revision-properties
(diff--cache-schedule-clean)
(and buffer
(with-current-buffer buffer
- (diff-syntax-fontify-props file text line-nb)))))
+ ;; major mode is set in vc-find-revision-no-save already.
+ (diff-syntax-fontify-props nil text line-nb)))))