Tracking issue: https://github.com/rust-lang/rust/issues/39588
After some work and some bugs, nightly docs now have the bookshelf mostly enabled. See the tracking issue for more details.
However, I want to point out some cool things:
rustbook -> mdbook
We have killed rustbook, and moved to mdbook for all book-related things. Technically speaking, rustbook is now a tiny wrapper over mdbook, so that we don't have to pull in all of mdbook's optional dependencies.
What this means is a few things:
- book generation is a bit nicer, generally
- themes!
- hidden code blocks now can expand to show the whole output
There are two bugs that were fixed that will render in today's docs, but hopefully not tomorrow's: clicking some heading links won't work, and you might see # in some code blocks. https://github.com/rust-lang/rust/pull/40151 should fix those.
In general, if you see issues with this, please file bugs on mdBook, and we'll try to knock them out! This is a huge migration, and so it's likely some small things have fallen through the cracks.
Oh, and finally, this was the first crates.io package to land in-tree. It helped shake out bugs that will make it easier to depend on crates.io stuff in the future 
Stuff with the book
Check it out here: The Rust Programming Language - The Rust Programming Language
This is mostly 'woo it's in mdbook now', but also, the nightly-only section has moved out into the unstable book. See below.
Eventually it will move out into the repo with the second edition, and we'll render both. That's yet to be done.
Stuff with the reference
Check it out here: Introduction - The Rust Reference
The reference is now ported to mdbook. Eventually, it will be in its own repository. Also, some work on the TOC would be nice; it was very deeply nested, and some of that is obscured now.
Stuff with the nomicon
Check it out here: Introduction - The Rustonomicon
Same as the book basically: ported to mdbook, will be in its own repo someday.
Stuff with the Unstable Book
Check it out here: The Unstable Book - The Rust Unstable Book
The unstable book is a place for unstable docs to live. This is organized by feature flag. Largely, it's empty, but hopefully that won't be true forever! This solves a couple of different issues with the docs, but if you want some nightly-only documentation, here's the place to go. It also keeps track of each tracking issue, and I'm hoping to eventually have some sort of "how close is this to being stable" marker....
Oh, and I have a PR to oragnize this alphabetically: https://github.com/rust-lang/rust/pull/40153
Please kick the tires and file bugs with issues you find! Thanks!
, but THIS IS AWESOME.