@Azerupi With the current set of crates being considered for submodulification it won't help much, since they are all stable-compatible crates that work on cargo, and are already maintained outside of tree. The frustrating parts of in-tree development (hacking on the compiler especially), will remain frustrating.
It may be possible to move rustbook out of tree and re-add it as a submodule since it's a pretty simple project, but it currently has dependencies on compiler internals that complicate that prospect. In fact, I believe when we merged rustbook that was part of the plan: we would dump it in-tree to get the book building, but because we aren't sure about the wisdom of distributing rustbook with rust, would also package it out-of-tree for others to use.
It would be great to fill out rustbook to make it a viable tool for everybody.