New name UI_UPDATE for interpret_move's return "".

Now midend.c directly tests the returned pointer for equality to this
value, instead of checking whether it's the empty string.

A minor effect of this is that games may now return a dynamically
allocated empty string from interpret_move() and treat it as just
another legal move description. But I don't expect anyone to be
perverse enough to actually do that! The main purpose is that it
avoids returning a string literal from a function whose return type is
a pointer to _non-const_ char, i.e. we are now one step closer to
being able to make this code base clean under -Wwrite-strings.

1 files changed, 4 insertions, 4 deletions

diff --git a/rect.c b/rect.c
index 465e143..67cc30a 100644
--- a/rect.c
+++ b/rect.c
@@ -2407,7 +2407,7 @@ static char *interpret_move(const game_state *from, game_ui *ui,
         move_cursor(button, &ui->cur_x, &ui->cur_y, from->w, from->h, 0);
         ui->cur_visible = TRUE;
         active = TRUE;
-        if (!ui->cur_dragging) return "";
+        if (!ui->cur_dragging) return UI_UPDATE;
         coord_round((float)ui->cur_x + 0.5F, (float)ui->cur_y + 0.5F, &xc, &yc);
     } else if (IS_CURSOR_SELECT(button)) {
         if (ui->drag_start_x >= 0 && !ui->cur_dragging) {
@@ -2420,7 +2420,7 @@ static char *interpret_move(const game_state *from, game_ui *ui,
         if (!ui->cur_visible) {
             assert(!ui->cur_dragging);
             ui->cur_visible = TRUE;
-            return "";
+            return UI_UPDATE;
         }
         coord_round((float)ui->cur_x + 0.5F, (float)ui->cur_y + 0.5F, &xc, &yc);
         erasing = (button == CURSOR_SELECT2);
@@ -2441,7 +2441,7 @@ static char *interpret_move(const game_state *from, game_ui *ui,
             reset_ui(ui); /* cancel keyboard dragging */
             ui->cur_dragging = FALSE;
         }
-        return "";
+        return UI_UPDATE;
     } else if (button != LEFT_DRAG && button != RIGHT_DRAG) {
         return NULL;
     }
@@ -2525,7 +2525,7 @@ static char *interpret_move(const game_state *from, game_ui *ui,
     if (ret)
 	return ret;		       /* a move has been made */
     else if (active)
-        return "";		       /* UI activity has occurred */
+        return UI_UPDATE;
     else
 	return NULL;
 }