Publicize the GC untracking optimization
diff --git a/Doc/whatsnew/2.7.rst b/Doc/whatsnew/2.7.rst
index e44a42b..e035eb0 100644
--- a/Doc/whatsnew/2.7.rst
+++ b/Doc/whatsnew/2.7.rst
@@ -129,6 +129,14 @@
(Suggested by Martin von Loewis and implemented by Antoine Pitrou;
:issue:`4074`.)
+* The garbage collector tries to avoid tracking simple containers which
+ can't be part of a cycle. As of now, this is true for tuples and dicts
+ containing atomic types (such as ints, strings, etc.). Transitively, a dict
+ containing tuples of atomic types won't be tracked either. This helps brings
+ down the cost of full garbage collections, since it decreases the number of
+ objects to be considered and traversed by the collector.
+ (Contributed by Antoine Pitrou; :issue:`4688`.)
+
.. ======================================================================