bug#31796: 27.1; dired-do-find-regexp-and-replace fails to find multiline regexps

[Juri Linkov]

> Now that I've dug in a little, the situation seems difficult.
>
> -Pz does work, but it forces Grep to consider the file as one long
>  string. As a consequence, if we ask it to output the line number, the
>  number will always be 1. That's not a helpful mode of operation.
>
> Even if it worked differently, -P imposes a significant performance penalty
> from what I see, even when the extra syntax is not actually used. So we
> couldn't enable it by default.

When a grep input pattern contains a newline, then xref could use
the same algorithm as is used for 'M-.', i.e. run 'grep -Pzl'
to get the file names that contain the pattern, then return
these file names without line numbers.  This works exactly
like a new feature of extending xref-show-xrefs-function
with a new completion function was proposed recently on emacs-devel
(BTW, why it's not installed yet?)

So like this feature presenting such completions without line numbers:

  lisp/progmodes/project.el:(cl-defgeneric project-root)
  lisp/progmodes/project.el:(cl-defmethod project-root ((project (head 
transient))))
  lisp/progmodes/project.el:(cl-defmethod project-root ((project (head vc))))

xref for grep could work the same way without line numbers:

  lisp/progmodes/project.el:names"^Jproject--read-file-cpd-relative)
  lisp/progmodes/project.el:names"^Jproject--read-file-absolute)

Then visiting such grep hit should use Emacs search functions
to find the grep hit in the visited file.