diagnostics: simplify location handling

[Akim Demaille]

Taken from my branch playing with fuzzing.

commit fa9871a2fb6adaca6aa72b75959bac865af81cc8
Author: Akim Demaille <address@hidden>
Date:   Mon Oct 21 17:33:32 2019 +0200

    diagnostics: simplify location handling
    
    Locations start at line 1.  Don't accept line 0.
    
    * src/location.c (location_print): Don't print locations with line 0.
    (location_caret): Simplify.

diff --git a/src/location.c b/src/location.c
index 4d545472..357cce7c 100644
--- a/src/location.c
+++ b/src/location.c
@@ -178,10 +178,10 @@ location_print (location loc, FILE *out)
       int end_col = 0 != loc.end.column ? loc.end.column - 1 : 0;
       res += fprintf (out, "%s",
                       quotearg_n_style (3, escape_quoting_style, 
loc.start.file));
-      if (0 <= loc.start.line)
+      if (0 < loc.start.line)
         {
           res += fprintf (out, ":%d", loc.start.line);
-          if (0 <= loc.start.column)
+          if (0 < loc.start.column)
             res += fprintf (out, ".%d", loc.start.column);
         }
       if (loc.start.file != loc.end.file)
@@ -189,14 +189,14 @@ location_print (location loc, FILE *out)
           res += fprintf (out, "-%s",
                           quotearg_n_style (3, escape_quoting_style,
                                             loc.end.file));
-          if (0 <= loc.end.line)
+          if (0 < loc.end.line)
             {
               res += fprintf (out, ":%d", loc.end.line);
               if (0 <= end_col)
                 res += fprintf (out, ".%d", end_col);
             }
         }
-      else if (0 <= loc.end.line)
+      else if (0 < loc.end.line)
         {
           if (loc.start.line < loc.end.line)
             {
@@ -400,7 +400,7 @@ caret_set_column (int col)
 void
 location_caret (location loc, const char *style, FILE *out)
 {
-  if (loc.start.column == -1 || loc.start.line == -1)
+  if (!loc.start.line)
     return;
   if (!caret_set_file (loc.start.file))
     return;
diff --git a/src/location.h b/src/location.h
index cbd6f482..24fba608 100644
--- a/src/location.h
+++ b/src/location.h
@@ -33,25 +33,25 @@ typedef struct
   /* The name of the file that contains the boundary.  */
   uniqstr file;
 
-  /* If nonnegative, the (origin-1) line that contains the boundary.
+  /* If positive, the line (starting at 1) that contains the boundary.
      If this is INT_MAX, the line number has overflowed.
 
-     Meaningless and not displayed if negative.
+     Meaningless and not displayed if nonpositive.
   */
   int line;
 
-  /* If nonnegative, the (origin-1) column just after the boundary.
+  /* If positive, the column (starting at 1) just after the boundary.
      This is neither a byte count, nor a character count; it is a
      column count.  If this is INT_MAX, the column number has
      overflowed.
 
-     Meaningless and not displayed if negative.
+     Meaningless and not displayed if nonpositive.
   */
   int column;
 
-  /* If nonnegative, (origin-0) bytes number in the current line.
+  /* If nonnegative, the byte number (starting at 0) in the current line.
      Never displayed, used when printing error messages with colors to
-     know where colors start and ends.  */
+     know where colors start and end.  */
   int byte;
 
 } boundary;