[committed] Reject fix-it hints for various awkward boundary cases (PR c/82050)

[David Malcolm]

PR c/82050 reports a failed assertion deep within diagnostic_show_locus's
code for printing fix-it hints.

The root cause is a fix-it hint suggesting a textual replacement,
where the affected column numbers straddle the LINE_MAP_MAX_COLUMN_NUMBER
boundary, so that the start of the range has a column number, but the
end of the range doesn't.

The fix is to verify that the column numbers are sane when adding fix-it
hints to a rich_location, rejecting fix-it hints where they are not.

Successfully bootstrapped&regrtested on x86_64-pc-linux-gnu.
Committed to trunk as r255214.

libcpp/ChangeLog:
        PR c/82050
        * include/line-map.h (LINE_MAP_MAX_COLUMN_NUMBER): Move here.
        * line-map.c (LINE_MAP_MAX_COLUMN_NUMBER): ...from here.
        (rich_location::maybe_add_fixit): Reject fix-it hints in which
        the start column exceeds the next column.
---
 libcpp/include/line-map.h |  5 +++++
 libcpp/line-map.c         | 13 ++++++++-----
 2 files changed, 13 insertions(+), 5 deletions(-)

diff --git a/libcpp/include/line-map.h b/libcpp/include/line-map.h
index 8b7e5dc..1151484 100644
--- a/libcpp/include/line-map.h
+++ b/libcpp/include/line-map.h
@@ -280,6 +280,11 @@ enum lc_reason
    worked example in libcpp/location-example.txt.  */
 typedef unsigned int source_location;
 
+/* Do not track column numbers higher than this one.  As a result, the
+   range of column_bits is [12, 18] (or 0 if column numbers are
+   disabled).  */
+const unsigned int LINE_MAP_MAX_COLUMN_NUMBER = (1U << 12);
+
 /* Do not pack ranges if locations get higher than this.
    If you change this, update:
      gcc.dg/plugin/location-overflow-test-*.c.  */
diff --git a/libcpp/line-map.c b/libcpp/line-map.c
index 0e5804b..ac621e9 100644
--- a/libcpp/line-map.c
+++ b/libcpp/line-map.c
@@ -26,11 +26,6 @@ along with this program; see the file COPYING3.  If not see
 #include "internal.h"
 #include "hashtab.h"
 
-/* Do not track column numbers higher than this one.  As a result, the
-   range of column_bits is [12, 18] (or 0 if column numbers are
-   disabled).  */
-const unsigned int LINE_MAP_MAX_COLUMN_NUMBER = (1U << 12);
-
 /* Highest possible source location encoded within an ordinary or
    macro map.  */
 const source_location LINE_MAP_MAX_SOURCE_LOCATION = 0x70000000;
@@ -2352,6 +2347,14 @@ rich_location::maybe_add_fixit (source_location start,
       stop_supporting_fixits ();
       return;
     }
+  /* The columns must be in the correct order.  This can fail if the
+     endpoints straddle the boundary for which the linemap can represent
+     columns (PR c/82050).  */
+  if (exploc_start.column > exploc_next_loc.column)
+    {
+      stop_supporting_fixits ();
+      return;
+    }
 
   const char *newline = strchr (new_content, '\n');
   if (newline)
-- 
1.8.5.3