mozilla/rust — pull request #11230 by csherratt (2013-12-31 02:33:11+00:00)
Title: libextra: Add a copy-on-write arc container.

This allows patch adds a new arc type that allows for creation of copy-on-write data structures. The idea is that it is safe to mutate any data structure as long as it has only one reference to it. If there are multiple, it requires cloning of the data structure before mutation is possible.