diff --git a/src/bytes/buffer.go b/src/bytes/buffer.go
index 4176d670ec8..f90d9eca0fe 100644
--- a/src/bytes/buffer.go
+++ b/src/bytes/buffer.go
@@ -247,8 +247,8 @@ func growSlice(b []byte, n int) []byte {
 		c = 2 * cap(b)
 	}
 	b2 := append([]byte(nil), make([]byte, c)...)
-	copy(b2, b)
-	return b2[:len(b)]
+	i := copy(b2, b)
+	return b2[:i]
 }
 
 // WriteTo writes data to w until the buffer is drained or an error occurs.
diff --git a/src/runtime/string.go b/src/runtime/string.go
index 640ee02a3cb..e43f4cca51c 100644
--- a/src/runtime/string.go
+++ b/src/runtime/string.go
@@ -51,8 +51,8 @@ func concatstrings(buf *tmpBuf, a []string) string {
 	}
 	s, b := rawstringtmp(buf, l)
 	for _, x := range a {
-		copy(b, x)
-		b = b[len(x):]
+		n := copy(b, x)
+		b = b[n:]
 	}
 	return s
 }