bug#46859: 28.0.50; [PATCH]: Add option to truncate long lines in xref.el

[Dmitry Gutov <dgutov@yandex.ru>]

[-- Attachment #1: Type: text/plain, Size: 1376 bytes --]

On 07.03.2021 00:26, Dmitry Gutov wrote:
> - What do you think about making an effort to actually retain all the 
> matches in the output? That would mean interpreting the 
> xref-truncate-line-to value (or however the var could be renamed) as the 
> maximum number of chars to render on the line *per match*. And if there 
> is too much text between them, those parts can become "(truncated...)". 
> Your current implementation can cut off valid matches, and we probably 
> want to preserve them if feasible. OTOH, the default value could go down 
> to 200 with this approach.

Please try out the attached preparation patch.

It improves the performance of the "very long line" case drastically 
over here, while not doing any truncation yet. Looks like we regressed 
that case when we added rendering of multiple matches on the same line.

We can add the truncation feature on top of it.

Probably also in xref--collect-matches-1 (truncating the value of 
SUMMARY just before the xref-make-match call).

Alternatively, we could experiment with hiding parts of the long line 
using some display/visibility features (except the truncate-lines 
variable, that one keeps things slow). That could be done in 
xref--insert-xrefs or somewhere nearby. That is trickier, though, given 
that we'll probably want to unhide it (wholly or partially) when 
iterating over matches inside.

[-- Attachment #2: xref-insert-xrefs-sparingly.diff --]
[-- Type: text/x-patch, Size: 4839 bytes --]

diff --git a/lisp/progmodes/xref.el b/lisp/progmodes/xref.el
index 18fdd963fb..5c5a0508de 100644
--- a/lisp/progmodes/xref.el
+++ b/lisp/progmodes/xref.el
@@ -886,30 +886,24 @@ xref--insert-xrefs
                                (length (and line (format "%d" line)))))
            for line-format = (and max-line-width
                                   (format "%%%dd: " max-line-width))
-           with prev-line-key = nil
+           with prev-group = nil
+           with prev-line = nil
            do
            (xref--insert-propertized '(face xref-file-header xref-group t)
                                      group "\n")
            (cl-loop for (xref . more2) on xrefs do
                     (with-slots (summary location) xref
                       (let* ((line (xref-location-line location))
-                             (new-summary summary)
-                             (line-key (list (xref-location-group location) line))
                              (prefix
-                              (if line
-                                  (propertize (format line-format line)
-                                              'face 'xref-line-number)
-                                "  ")))
+                              (cond
+                               ((not line) "  ")
+                               ((equal line prev-line) "")
+                              (t (propertize (format line-format line)
+                                             'face 'xref-line-number)))))
                         ;; Render multiple matches on the same line, together.
-                        (when (and line (equal prev-line-key line-key))
-                          (when-let ((column (xref-location-column location)))
-                            (delete-region
-                             (save-excursion
-                               (forward-line -1)
-                               (move-to-column (+ (length prefix) column))
-                               (point))
-                             (point))
-                            (setq new-summary (substring summary column) prefix "")))
+                        (when (and (equal prev-group group)
+                                   (not (equal prev-line line)))
+                          (insert "\n"))
                         (xref--insert-propertized
                          (list 'xref-item xref
                                'mouse-face 'highlight
@@ -917,9 +911,10 @@ xref--insert-xrefs
                                'help-echo
                                (concat "mouse-2: display in another window, "
                                        "RET or mouse-1: follow reference"))
-                         prefix new-summary)
-                        (setq prev-line-key line-key)))
-                    (insert "\n"))))
+                         prefix summary)
+                        (setq prev-line line
+                              prev-group group))))
+           (insert "\n")))
 
 (defun xref--analyze (xrefs)
   "Find common filenames in XREFS.
@@ -1678,20 +1673,30 @@ xref--collect-matches
                                  syntax-needed)))))
 
 (defun xref--collect-matches-1 (regexp file line line-beg line-end syntax-needed)
-  (let (matches)
+  (let (match-pairs matches)
     (when syntax-needed
       (syntax-propertize line-end))
-    ;; FIXME: This results in several lines with the same
-    ;; summary. Solve with composite pattern?
     (while (and
             ;; REGEXP might match an empty string.  Or line.
-            (or (null matches)
+            (or (null match-pairs)
                 (> (point) line-beg))
             (re-search-forward regexp line-end t))
-      (let* ((beg-column (- (match-beginning 0) line-beg))
-             (end-column (- (match-end 0) line-beg))
+      (push (cons (match-beginning 0)
+                  (match-end 0))
+            match-pairs))
+    (setq match-pairs (nreverse match-pairs))
+    (while match-pairs
+      (let* ((beg-end (pop match-pairs))
+             (beg-column (- (car beg-end) line-beg))
+             (end-column (- (cdr beg-end) line-beg))
              (loc (xref-make-file-location file line beg-column))
-             (summary (buffer-substring line-beg line-end)))
+             (summary (buffer-substring (if matches (car beg-end) line-beg)
+                                        (if match-pairs
+                                            (caar match-pairs)
+                                          line-end))))
+        (when matches
+          (cl-decf beg-column (- (car beg-end) line-beg))
+          (cl-decf end-column (- (car beg-end) line-beg)))
         (add-face-text-property beg-column end-column 'xref-match
                                 t summary)
         (push (xref-make-match summary loc (- end-column beg-column))