format symbols under shared frames

Author: KodrAus

library/std/src/sys_common/backtrace.rs

@@ -71,6 +71,7 @@ unsafe fn _print_fmt(fmt: &mut fmt::Formatter<'_>, print_fmt: PrintFmt) -> fmt::
 
         let mut hit = false;
         let mut stop = false;
+        let mut frame_fmt = bt_fmt.frame();
         backtrace_rs::resolve_frame_unsynchronized(frame, |symbol| {
             hit = true;
             if print_fmt == PrintFmt::Short {
@@ -87,15 +88,15 @@ unsafe fn _print_fmt(fmt: &mut fmt::Formatter<'_>, print_fmt: PrintFmt) -> fmt::
             }
 
             if start {
-                res = bt_fmt.frame().symbol(frame, symbol);
+                res = frame_fmt.symbol(frame, symbol);
             }
         });
         if stop {
             return false;
         }
         if !hit {
             if start {
-                res = bt_fmt.frame().print_raw(frame.ip(), None, None, None);
+                res = frame_fmt.print_raw(frame.ip(), None, None, None);
             }
         }