mozilla/rust — pull request #12367 by darnuria (2014-02-18 12:58:15+00:00)
Title: Removing '15.3 Do syntax'since it is deprecated.

The 'do' keyword was deprecated in 0.10, and is keep as
reserved keyword.

So the tutorial part about it doesn't make sense.
The spawning explanation was move into '15.2 Closure compatibility'.