Re: highlight failed part of isearch input

[Richard Stallman]

    > We could simply try to find the longest prefix which does have a match.
    > Can't be that hard.

    The rest of my message said that I do not think such a fix is even a good
    idea. I probably should not even have said that I don't know how to fix
    that, since I don't think it should be "fixed".

    Starting again with `C-s C-s' is, IMO, asking Emacs to find the whole search
    string; it is not asking it to find the part that can be found. At least,
    that's how I interpret such a user intention.

I think that is correct behavior.

The fact that C-M-w can be used to delete chars from the string does
not invalidate it.  You can also use M-e to edit the search string in
any which way, but I don't think that matters here, and neither does
C-M-w.

Putting this together with the fact that it is hard to determine the
right "failing part" in this case, we get a thoroughly convincing
reason not to try.