Allow characters and single-char strings in rx charsets

The `not' and `intersection' forms, and `or' inside these forms,
now accept characters and single-character strings as arguments.
Previously, they had to be wrapped in `any' forms.
This does not add expressive power but is a convenience and is easily
understood.

* doc/lispref/searching.texi (Rx Constructs): Amend the documentation.
* etc/NEWS: Announce the change.
* lisp/emacs-lisp/rx.el (rx--charset-p, rx--translate-not)
(rx--charset-intervals, rx): Accept characters and 1-char strings in
more places.
* test/lisp/emacs-lisp/rx-tests.el (rx-not, rx-charset-or)
(rx-def-in-charset-or, rx-intersection): Test the change.

1 files changed, 6 insertions, 5 deletions

diff --git a/doc/lispref/searching.texi b/doc/lispref/searching.texi
index 0c6c7cc68b5..700880c2289 100644
--- a/doc/lispref/searching.texi
+++ b/doc/lispref/searching.texi
@@ -1214,8 +1214,9 @@ Corresponding string regexp: @samp{[@dots{}]}
 @item @code{(not @var{charspec})}
 @cindex @code{not} in rx
 Match a character not included in @var{charspec}.  @var{charspec} can
-be an @code{any}, @code{not}, @code{or}, @code{intersection},
-@code{syntax} or @code{category} form, or a character class.
+be a character, a single-character string, an @code{any}, @code{not},
+@code{or}, @code{intersection}, @code{syntax} or @code{category} form,
+or a character class.
 If @var{charspec} is an @code{or} form, its arguments have the same
 restrictions as those of @code{intersection}; see below.@*
 Corresponding string regexp: @samp{[^@dots{}]}, @samp{\S@var{code}},
@@ -1224,9 +1225,9 @@ Corresponding string regexp: @samp{[^@dots{}]}, @samp{\S@var{code}},
 @item @code{(intersection @var{charset}@dots{})}
 @cindex @code{intersection} in rx
 Match a character included in all of the @var{charset}s.
-Each @var{charset} can be an @code{any} form without character
-classes, or an @code{intersection}, @code{or} or @code{not} form whose
-arguments are also @var{charset}s.
+Each @var{charset} can be a character, a single-character string, an
+@code{any} form without character classes, or an @code{intersection},
+@code{or} or @code{not} form whose arguments are also @var{charset}s.
 
 @item @code{not-newline}, @code{nonl}
 @cindex @code{not-newline} in rx