Native display of line numbers

[Eli Zaretskii]

I generally consider display of line numbers for each line in the
buffer un-Emacsy, something that came from the dark era when we needed
to count lines to be able to tell the then existing editors something
like "move down N lines, then DO SOMETHING".  Emacs pioneered today's
world where all this is unnecessary, and the rest is history.

However, many users want line numbers to be displayed in Emacs.  I
don't know why they want it, perhaps they were "spoiled" by other IDEs
or something.  In any case, given the popularity of the various add-on
modes which insist on displaying line numbers using all kinds of
tricks, I don't think we can ignore this tendency any longer, even if
we agree that this is a disease.  The "epidemic" has spread too far
and too wide for us to continue looking the other way.  And since
Emacs doesn't make it easy for a Lisp program to invade the holy realm
of redisplay, those modes use tricks that make Emacs slower and
frequently interfere with other packages.  They also make demands on
the display engine that make Emacs harder to develop and maintain.

So with that in mind, I came up with an implementation of a native
display of line numbers.  You can find it in the scratch/line-numbers
branch of the Emacs repository.  Compared with linum-mode,
nlinum-mode, and other similar modes, it has the following main
advantages:

  . it works significantly faster (almost twice as fast as linum-mode,
    50% faster that relative-line-numbers-mode)
  . it doesn't use the display margins
  . it works with R2L text, something the old modes never did

The main features of the line-numbering modes are all supported by
this new feature.

Please give it a try and report any bugs you find.  I'm sure there
will be quite a few bugs, given the sheer amount of display feature
which could be affected, and which I didn't have time to test.

Once the flood of serious bug reports dries out, I will look into
merging this to master.  If, miraculously, no such flood arrives, I
will merge in a couple of weeks.  When that happens, we will declare
all the other modes which do the same job deprecated.

TIA