Allow comment-indent-functions to specify exact indentation (Bug#385)

* lisp/newcomment.el (comment-choose-indent): Interpret a cons of two
integers as indicating a range of acceptable indentation.
(comment-indent): Don't apply `comment-inline-offset',
`comment-choose-indent' already does that.
(comment-indent-function):
* doc/emacs/programs.texi (Options for Comments): Document new
acceptable return values.
* etc/NEWS: Announce it.

1 files changed, 18 insertions, 17 deletions

diff --git a/lisp/newcomment.el b/lisp/newcomment.el
index 118549f421c..8772b52376d 100644
--- a/lisp/newcomment.el
+++ b/lisp/newcomment.el
@@ -142,9 +142,10 @@ Should be an empty string if comments are terminated by end-of-line.")
 ;;;###autoload
 (defvar comment-indent-function 'comment-indent-default
   "Function to compute desired indentation for a comment.
-This function is called with no args with point at the beginning of
-the comment's starting delimiter and should return either the desired
-column indentation or nil.
+This function is called with no args with point at the beginning
+of the comment's starting delimiter and should return either the
+desired column indentation, a range of acceptable
+indentation (MIN . MAX), or nil.
 If nil is returned, indentation is delegated to `indent-according-to-mode'.")
 
 ;;;###autoload
@@ -649,13 +650,20 @@ The criteria are (in this order):
 - prefer INDENT (or `comment-column' if nil).
 Point is expected to be at the start of the comment."
   (unless indent (setq indent comment-column))
-  ;; Avoid moving comments past the fill-column.
-  (let ((max (+ (current-column)
-                (- (or comment-fill-column fill-column)
-                   (save-excursion (end-of-line) (current-column)))))
-        (other nil)
-        (min (save-excursion (skip-chars-backward " \t")
-                             (if (bolp) 0 (+ comment-inline-offset (current-column))))))
+  (let ((other nil)
+        min max)
+    (pcase indent
+      (`(,lo . ,hi) (setq min lo) (setq max hi)
+       (setq indent comment-column))
+      (_ ;; Avoid moving comments past the fill-column.
+       (setq max (+ (current-column)
+                    (- (or comment-fill-column fill-column)
+                       (save-excursion (end-of-line) (current-column)))))
+       (setq min (save-excursion
+                   (skip-chars-backward " \t")
+                   ;; Leave at least `comment-inline-offset' space after
+                   ;; other nonwhite text on the line.
+                   (if (bolp) 0 (+ comment-inline-offset (current-column)))))))
     ;; Fix up the range.
     (if (< max min) (setq max min))
     ;; Don't move past the fill column.
@@ -750,13 +758,6 @@ If CONTINUE is non-nil, use the `comment-continue' markers if any."
 	  ;; If the comment is at the right of code, adjust the indentation.
 	  (unless (save-excursion (skip-chars-backward " \t") (bolp))
 	    (setq indent (comment-choose-indent indent)))
-	  ;; Update INDENT to leave at least one space
-	  ;; after other nonwhite text on the line.
-	  (save-excursion
-	    (skip-chars-backward " \t")
-	    (unless (bolp)
-	      (setq indent (max indent
-                                (+ (current-column) comment-inline-offset)))))
 	  ;; If that's different from comment's current position, change it.
 	  (unless (= (current-column) indent)
 	    (delete-region (point) (progn (skip-chars-backward " \t") (point)))