bug#23179: 25.0.92; Restore `M-,' to continue etags search

[Dmitry Gutov <dgutov@yandex.ru>]

On 04/02/2016 10:46 PM, Anders Lindgren wrote:

> I think that we would be making a big mistake if we would release Emacs
> 25 with an "xref" without searching and query-replace, but with key
> bindings that, for most tags users, break existing use patterns.

As already mentioned, we have multiple solutions for searching, and one 
unified command for query-replace, invoked from the xref buffer.

> Today, many people use "tags" as a simple project file. They don't want
> to redo this process with another tool ("project") and a dired approach
> might not match a project layout at all.

project-or-external-find-regexp will take the tags files into account, 
as long as the current major mode hasn't overridden 
project-vc-external-roots-function, or the current project 
implementation hasn't overridden project-vc-external-roots-function.

"redo this process"? Which one?

> Maybe I'm being naive, but a tags file (and presumably all other xref
> backends) must represent a list of files. A free text search and
> query-replace across those files would be very straight forward to
> specify, wouldn't it?

An xref backend aims to represent the current coding environment; it 
could combine the source files in the current project, the library files 
external to the project (which could be compiled, zipped, etc), the 
information available in the currently running REPL.

Yes, there probably is a list of files in there a backend could search, 
but it should be specified better than that. Search only inside source 
code, but not documentation, resources, etc? Including any external 
files that do not belong to this project (try imagining a different xref 
backend for C code; it would probably include the installed libraries)?

And again, what do you see as the main advantage of the new command over 
project-or-external-find-regexp?

> I have given this some thought -- if we decide to really do make a
> change, maybe we should try to make the xref search command more
> isearch-like, so that a user could be able to continue an xref search
> using `C-s' rather than `M-,'.

That doesn't sound clear to me at all. You mean pressing C-s in an xref 
buffer? The place where you can currently use `n', `p', or `,' and `.'?

If you mean in a source buffer, what about next-error and previous-error?