[RFC PATCH] Fix up cpp_check_xid_property (used by rust)

[Jakub Jelinek]

Hi!

I believe cpp_check_xid_property (function added to libcpp for the Rust
FE) is incorrect.
The bits are documented
  /* Valid in a C++23 identifier?  */
  CXX23 = 32,
  /* Valid in a C++23 identifier, but not as the first character?  */
  NXX23 = 64,
So, characters which have both the Unicode XID_Start and XID_Continue
derived properties have just CXX23 bit set, and characters which
have just XID_Continue derived property and not XID_Start have
CXX23|NXX23 bits set.
So, I believe the function (except for the ASCII fast path which is correct)
incorrectly returns CPP_XID_START | CPP_XID_CONTINUE for any characters
which have XID_Continue derived property, and only ever returns just
CPP_XID_CONTINUE in the ASCII [0-9_] cases.

The following untested patch should fix that, I guess it would be nice to
have a testcase, pick some character in XID_Continue and not in XID_Start,
random choice e.g. SUNDANESE CONSONANT SIGN PASANGAN WA, and check if
it can be used in the second+ character of identifier (probably it can and
should continue to do so) and as the first character of identifier
(likely incorrectly accepted right now).

2026-06-11  Jakub Jelinek  <jakub@redhat.com>

	* charset.cc (cpp_check_xid_property): Return
	CPP_XID_START | CPP_XID_CONTINUE only if CXX23 bit is set and NXX23
	is not.  If both are set, return CPP_XID_CONTINUE.

--- libcpp/charset.cc.jj	2026-04-21 18:24:44.122033404 +0200
+++ libcpp/charset.cc	2026-06-11 13:33:51.356096060 +0200
@@ -1366,9 +1366,9 @@ cpp_check_xid_property (cppchar_t c)
 
   unsigned short flags = ucnranges[mn].flags;
 
-  if (flags & CXX23)
+  if ((flags & (CXX23 | NXX23)) == CXX23)
     return CPP_XID_START | CPP_XID_CONTINUE;
-  if (flags & NXX23)
+  if ((flags & (CXX23 | NXX23)) == (CXX23 | NXX23))
     return CPP_XID_CONTINUE;
   return 0;
 }

	Jakub