See https://lists.gnu.org/archive/html/emacs-devel/2017-10/msg00151.html
and its resulting thread.
* lisp/display-line-numbers.el (display-line-numbers): Add to the
convenience group.
* lisp/faces.el (line-number):
(line-number-current-line): Add to the display-line-numbers group.
Most of this change is to boilerplate commentary such as license URLs.
This change was prompted by ftp://ftp.gnu.org's going-away party,
planned for November. Change these FTP URLs to https://ftp.gnu.org
instead. Make similar changes for URLs to other organizations moving
away from FTP. Also, change HTTP to HTTPS for URLs to gnu.org and
fsf.org when this works, as this will further help defend against
man-in-the-middle attacks (for this part I omitted the MS-DOS and
MS-Windows sources and the test tarballs to keep the workload down).
HTTPS is not fully working to lists.gnu.org so I left those URLs alone
for now.
Functions prefixed with `turn-on-foo' are quite old, and in general
subsumed by the corresponding `foo-mode` command.
* lisp/display-line-numbers.el (turn-on-display-line-numbers-mode):
Rename with library prefix, mark as internal. Nix autoload.
* lisp/cus-start.el: Use the new display-line-numbers group.
* lisp/display-line-numbers.el: New file.
* doc/emacs/custom.texi (Init Rebinding): Re-add entry that used to
belong to linum-mode.
* doc/emacs/modes.texi (Minor Modes): Summarize the mode.
* etc/NEWS: Document display-line-numbers-mode and its customization
variables, and mention that display-line-numbers-width is
buffer-local.
* src/xdisp.c (syms_of_xdisp) <display-line-numbers-width>: Fix a
typo.
