"Raw" string literals for elisp

[Anna Glasgall]

[My previous message appears to have been eaten, or at least it's not
showing up in the archive; resending from a different From: address.
Apologies for any duplication]

Hello Emacs developers,

I've long been annoyed by the number of backslashes needed when using
string literals in elisp for certain things (regexes, UNC paths, etc),
so I started work on a patch (WIP attached) to implement support for
"raw" string literals, a la Python r-strings. These are string literals
that work exactly like normal string literals, with the exception that
backslash escapes (except for \") are not processed; \ may freely
appear in the string without need to escape. I've made good progress,
but unfortunately I've run into a roadblock and am not sure what to do
next.

I've successfully taught the elisp reader (read1 in lread.c) how to
read r-strings. I thought I had managed to make lisp-mode/elisp-mode
happy by allowing "r" to be a prefix character (C-x C-e and the
underlying forward-sexp/backward-sexp seemed to work fine at first),
but realized that I ran into trouble with strings containing the
sequence of characters '\\"'.

The reader correctly reads r"a\\"" as a string containing the sequence
of characters 'a', '\', '"', and M-: works. Unfortunately, if I try
sexp-based navigation or e.g. C-x C-e, it falls apart. The parser in
syntax.c, which afaict is what lisp-mode is using to try and find sexps
in buffer text, doesn't seem to know what to do with this expression.
I've spent some time staring at syntax.c, but I must confess that I'm
entirely defeated in terms of what changes need to be made here to
teach this other parser about prefixed strings in where the prefix has
meaning that affects the interpretation of the characters between
string fences.

I've attached a copy of my WIP patch; it's definitely not near final
code quality and doesn't have documentation yet, all of which I would
take care of before submitting for inclusion. I also haven't filled out
the copyright assignment paperwork yet, but should this work reach a
point where it was likely to be accepted, I'd be happy to do that.


I'd very much appreciate some pointers on what to try next here, or
some explanation of how syntax.c/syntax.el works beyond what's in the
reference manual. If this is a fool's errand I'm tilting at here, I'd
also appreciate being told that before I sink more time into it :)

thanks,

Anna Glasgall

rstrings.patch
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