fix: added panic if type assertions failed

run.go

@@ -72,9 +72,10 @@ func (r *runner) concurrent(rate int, formatterFn func() Formatter) (failed bool
 			}
 			if useFmtCopy {
 				copyLock.Lock()
-				if d, ok := r.fmt.(*progress); ok {
-					if s, ok := fmtCopy.(*progress); ok {
-						func(source *progress, dest *progress) {
+				dest, dOk := r.fmt.(*progress)
+				source, sOk := fmtCopy.(*progress)
+
+				if dOk && sOk {
 					for _, v := range source.features {
 						dest.features = append(dest.features, v)
 					}
@@ -94,8 +95,10 @@ func (r *runner) concurrent(rate int, formatterFn func() Formatter) (failed bool
 					for _, v := range source.pending {
 						dest.pending = append(dest.pending, v)
 					}
-						}(s, d)
-					}
+				} else if !dOk {
+					panic("cant cast dest formatter to progress-typed")
+				} else if !sOk {
+					panic("cant cast source formatter to progress-typed")
 				}
 				copyLock.Unlock()
 			}