Use text properties for color escape highlighting in Shell mode.

* ansi-color.el: Don't set comint-output-filter-functions; it is
now in the initial value defined in comint.el.
(ansi-color-apply-face-function): New variable.
(ansi-color-apply-on-region): Use it.
(ansi-color-apply-overlay-face): New function.

* comint.el: Require ansi-color.
(comint-output-filter-functions): Add ansi-color-process-output.

* shell.el (shell): No need to require ansi-color.
(shell-mode): Use ansi-color-apply-face-function to highlight
color escapes using font-lock-face property.

Fixes: debbugs:10835

1 files changed, 10 insertions, 1 deletions

diff --git a/lisp/shell.el b/lisp/shell.el
index b4b388655c8..1ed43863452 100644
--- a/lisp/shell.el
+++ b/lisp/shell.el
@@ -510,6 +510,16 @@ buffer."
   (set (make-local-variable 'shell-dirstack) nil)
   (set (make-local-variable 'shell-last-dir) nil)
   (shell-dirtrack-mode 1)
+
+  ;; By default, ansi-color applies faces using overlays.  This is
+  ;; very inefficient in Shell buffers (e.g. Bug#10835).  We use a
+  ;; custom `ansi-color-apply-face-function' to convert color escape
+  ;; sequences into `font-lock-face' properties.
+  (set (make-local-variable 'ansi-color-apply-face-function)
+       (lambda (beg end face)
+         (when face
+           (put-text-property beg end 'font-lock-face face))))
+
   ;; This is not really correct, since the shell buffer does not really
   ;; edit this directory.  But it is useful in the buffer list and menus.
   (setq list-buffers-directory (expand-file-name default-directory))
@@ -625,7 +635,6 @@ Otherwise, one argument `-i' is passed to the shell.
                       (read-directory-name
                        "Default directory: " default-directory default-directory
                        t nil))))))))
-  (require 'ansi-color)
   (setq buffer (if (or buffer (not (derived-mode-p 'shell-mode))
                        (comint-check-proc (current-buffer)))
                    (get-buffer-create (or buffer "*shell*"))