What about the Servo project updates? I really like those, to watch it's progress.

Didn't jemalloc get reintegrated as well?

Any idea how they keep up with the breaking changes in Servo? Do they have some kind of rule so that whoever introduces them in Rust needs to patch Servo?

This is not a direct answer to your question, but here's a look at the historical lag before servo updates it's version of rust...

http://people.mozilla.org/~mbrubeck/servo-rust-updates.svg

http://www.reddit.com/r/rust/comments/25irjc/rust_update_his...

Last month they introduced rules that one should mark breaking changes in the git log and that removed functions should have a clearer fail message. At least I think that's what #[deprecated] does?

https://mail.mozilla.org/pipermail/rust-dev/2014-April/00954...

Yes, #[deprecated="some text"] causes the compiler to emit a warning message (that includes the specified text) whenever, for example, a function marked with it is called.

We batch up the changes and every so often we do a Rust upgrade in Servo, making all the changes at once.