diagnostics: fix ICE on fix-it hints on very long lines [PR99323]

PR c/99323 describes an ICE due to a failed assertion deep inside the
fix-it printing machinery, where the fix-it hints on one line have not
been properly sorted in layout's constructor.

The underlying issue occurs when multiple fix-it hints affect a line
wider that LINE_MAP_MAX_COLUMN_NUMBER, where the location_t values for
characters after that threshold fall back to having column zero.

It's not meaningful to try to handle fix-it hints without column
information, so this patch rejects them as they are added to the
rich_location, falling back to the "no fix-it hints on this diagnostic"
case, fixing the crash.

gcc/ChangeLog:
	PR c/99323
	* diagnostic-show-locus.c
	(selftest::test_one_liner_many_fixits_2): Fix accidental usage of
	column 0.

gcc/testsuite/ChangeLog:
	PR c/99323
	* gcc.dg/pr99323-1.c: New test.
	* gcc.dg/pr99323-2.c: New test.

libcpp/ChangeLog:
	PR c/99323
	* line-map.c (rich_location::maybe_add_fixit): Reject fix-it hints
	at column 0.

1 files changed, 8 insertions, 0 deletions

diff --git a/libcpp/line-map.c b/libcpp/line-map.c
index ccabd51..a003af8 100644
--- a/libcpp/line-map.c
+++ b/libcpp/line-map.c
@@ -2431,6 +2431,14 @@ rich_location::maybe_add_fixit (location_t start,
       stop_supporting_fixits ();
       return;
     }
+  /* If we have very long lines, tokens will eventually fall back to
+     having column == 0.
+     We can't handle fix-it hints that use such locations.  */
+  if (exploc_start.column == 0 || exploc_next_loc.column == 0)
+    {
+      stop_supporting_fixits ();
+      return;
+    }
 
   const char *newline = strchr (new_content, '\n');
   if (newline)