errors: show carets

* src/locations.c (caret_info): New, persistant information useful
for...
(location_caret): New, print a caret.
(cleanup_caret): Release caret_info cleanly, call it...
* src/main.c (main): Here.
* src/complain.c (error_message): Call location_caret here.

src/complain.c

@@ -74,11 +74,15 @@ error_message (location *loc,
   vfprintf (stderr, message, args);
   {
     size_t l = strlen (message);
-    if (l < 2 || message[l-2] != ':' || message[l-1] != ' ') {
+    if (l < 2 || message[l - 2] != ':' || message[l - 1] != ' ')
+      {
         putc ('\n', stderr);
         fflush (stderr);
+        if (loc && feature_flag & feature_caret)
+          location_caret (stderr, *loc);
       }
   }
+  fflush (stderr);
 }
 
 /** Wrap error_message() with varargs handling. */

src/getargs.c

@@ -646,7 +646,7 @@ getargs (int argc, char *argv[])
 	exit (EXIT_SUCCESS);
 
       case 'f':
-        FLAGS_ARGMATCH (flag, optarg);
+        FLAGS_ARGMATCH (feature, optarg);
         break;
 
       case 'W':

src/gram.c

@@ -307,6 +307,10 @@ grammar_rules_useless_report (const char *message)
   rule_number r;
   for (r = 0; r < nrules ; ++r)
     if (!rules[r].useful)
+      {
+        if (feature_flag & feature_caret)
+          warn_at (rules[r].location, "%s", message);
+        else
           {
             warn_at (rules[r].location, "%s: ", message);
             if (warnings_flag & warnings_other)
@@ -315,6 +319,7 @@ grammar_rules_useless_report (const char *message)
                 fflush (stderr);
               }
           }
+      }
 }
 
 void

src/location.c

@@ -138,6 +138,89 @@ location_print (FILE *out, location loc)
   return res;
 }
 
+
+/* Persistant data used by location_caret to avoid reopening and rereading the
+   same file all over for each error.  */
+struct caret_info
+{
+  FILE* source;
+  size_t line;
+  size_t offset;
+};
+
+static struct caret_info caret_info = { NULL, 1, 0 };
+
+/* Free any allocated ressources and close any open file handles that are
+   left-over by the usage of location_caret.  */
+void
+cleanup_caret ()
+{
+  if (caret_info.source)
+    fclose (caret_info.source);
+}
+
+/* Output to OUT the line and caret corresponding to location LOC.  */
+void
+location_caret (FILE *out, location loc)
+{
+  /* FIXME: find a way to support X-file locations, and only open once each
+     file. That would make the procedure future-proof.  */
+  if (! (caret_info.source
+         || (caret_info.source = fopen (loc.start.file, "r")))
+      || loc.start.column == -1 || loc.start.line == -1)
+    return;
+
+  /* If the line we want to quote is seekable (the same line as the previous
+     location), just seek it. If it was before, we lost track of it, so
+     return to the start of file.  */
+  if (caret_info.line <= loc.start.line)
+    fseek (caret_info.source, caret_info.offset, SEEK_SET);
+  else
+    {
+      caret_info.line = 1;
+      caret_info.offset = 0;
+      fseek (caret_info.source, caret_info.offset, SEEK_SET);
+    }
+
+  /* Advance to the line's position, keeping track of the offset.  */
+  {
+    int i;
+    for (i = caret_info.line; i < loc.start.line; caret_info.offset++)
+      if (fgetc (caret_info.source) == '\n')
+        ++i;
+  }
+  caret_info.line = loc.start.line;
+
+  /* Read the actual line.  Don't update the offset, so that we keep a pointer
+     to the start of the line.  */
+  {
+    ssize_t len = 0;
+    char *buf = NULL;
+    if ((len = getline (&buf, (size_t*) &len, caret_info.source)) != -1)
+      {
+        /* The caret of a multiline location ends with the first line.  */
+        int end = loc.start.line != loc.end.line ? len : loc.end.column;
+
+        if (len)
+          {
+            int i = loc.start.column;
+            /* Quote the file, indent by a single column.  */
+            fputc (' ', out);
+            fwrite (buf, 1, len, out);
+
+            /* Print the caret, with the same indent as above.  */
+            fputc (' ', out);
+            fprintf (out, "%*s", loc.start.column - 1, "");
+            do {
+              fputc ('^', out);
+            } while (++i < end);
+          }
+        fputc ('\n', out);
+        free (buf);
+      }
+  }
+}
+
 void
 boundary_set_from_string (boundary *bound, char *loc_str)
 {

src/location.h

@@ -102,6 +102,13 @@ void location_compute (location *loc,
    characters.  */
 unsigned location_print (FILE *out, location loc);
 
+/* Free any allocated ressources and close any open file handles that are
+   left-over by the usage of location_caret.  */
+void cleanup_caret (void);
+
+/* Output to OUT the line and caret corresponding to location LOC.  */
+void location_caret (FILE *out, location loc);
+
 /* Return -1, 0, 1, depending whether a is before, equal, or
    after b.  */
 static inline int

src/main.c

@@ -216,5 +216,7 @@ main (int argc, char *argv[])
   timevar_stop (TV_TOTAL);
   timevar_print (stderr);
 
+  cleanup_caret ();
+
   return complaint_issued ? EXIT_FAILURE : EXIT_SUCCESS;
 }