Summary: as discussed in another diff, this might help with NRVO

Test Plan: unit

Reviewed By: hans@fb.com

Subscribers: trunkagent, fugalh, folly-diffs@

FB internal diff: D1764921

Signature: t1:1764921:1420677311:c6b63d7bf04c0e17283117b3edef798c288e5fbb