scratch/bug-44858 4e7c5a6 2/6: Improve filling of generated docstring lines

[Stefan Kangas]

branch: scratch/bug-44858
commit 4e7c5a62d02fa0a170ad86c95a505584c150f377
Author: Stefan Kangas <stefan@marxist.se>
Commit: Stefan Kangas <stefan@marxist.se>

    Improve filling of generated docstring lines
    
    * lisp/subr.el (internal--fill-string-single-line): Improve filling to
    use full width.  Fix bug where line was not wrapped correctly.
---
 lisp/subr.el | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/lisp/subr.el b/lisp/subr.el
index 2d2794c..2a163b7 100644
--- a/lisp/subr.el
+++ b/lisp/subr.el
@@ -6418,12 +6418,19 @@ seconds."
 This is intended for very simple filling while bootstrapping
 Emacs itself, and does not support all the customization options
 of fill.el (for example `fill-region')."
-  (if (< (string-width str) fill-column)
+  (if (< (length str) fill-column)
       str
-    (let ((fst (substring str 0 fill-column))
-          (lst (substring str fill-column)))
-      (if (string-match ".*\\( \\(.+\\)\\)$" fst)
-          (setq fst (replace-match "\n\\2" nil nil fst 1)))
+    (let* ((limit (min fill-column (length str)))
+           (fst (substring str 0 limit))
+           (lst (substring str limit)))
+      (cond ((string-match "\\( \\)$" fst)
+             (setq fst (replace-match "\n" nil nil fst 1)))
+            ((string-match "^ \\(.*\\)" lst)
+             (setq fst (concat fst "\n"))
+             (setq lst (match-string 1 lst)))
+            ((string-match ".*\\( \\(.+\\)\\)$" fst)
+             (setq lst (concat (match-string 2 fst) lst))
+             (setq fst (replace-match "\n" nil nil fst 1))))
       (concat fst (internal--fill-string-single-line lst)))))
 
 (defun internal--format-docstring-line (string &rest objects)