Re: Automatic (e)tags generation and incremental updates

[Dmitry Gutov]

Hi Eli,

On 07.01.2021 16:15, Eli Zaretskii wrote:
> Thanks for working on this branch.
>
> However, it is hard to check it out without investing an inordinate
> amount of time, without having some guidance: what features are added,
> how they work, and what are the relevant commands to invoke them and
> variables to control them.  Would it be possible to post a short
> summary of that?  The changes are large, and when I looked at them, it
> was hard for me to figure out what exactly are we gaining and how to
> test that.

The changes are now fairly small and non-invasive, you can review the 
diff like this: git diff master...scratch/etags-regen

They include:

- New file and new minor mode etags-regen-mode with 3 defcustom's (see 
the docstrings).

- A change to .dir-locals.el which sets the value of one of those 
options, so that the resulting tags table for the Emacs project itself 
looks close enough to what 'make tags' generates.

- A change to lib-src/etags.c which implements handing of '-L' flag, for 
compatibility with ctags.

To try it out:

- Start Emacs built from this branch and open, say, src/xdisp.c.

- Call 'M-x etags-regen-mode' to enable it.

- DO NOT call 'M-x visit-tags-table'.

- Call some xref command like 'M-.'. See message "Generating new tags 
table..." (it's saved in /tmp/...), then see navigation happen.

- Pressing C-M-i instead should also trigger tags table generation.

- Rename some function, try navigating to the new name. That should 
work. Navigation to the previous name should fail (unless there are 
other definitions for that name).

What doesn't work:

- When you switch between projects, the previously generated tags tables 
are discarded. It's not too hard to improve, but that would involve some 
choices/tradeoffs.

- When files are deleted, or otherwise changed outside of Emacs (perhaps 
with 'git checkout'), nothing is updated. I have a few new ideas, 
haven't started on them yet. Workaround: toggle etags-regen-mode off and 
on, which will result in full rescan when you use 'M-.'.