The rustc guide has gone quite a long way. However, there are still a couple missing key chapters:
Monomorphization, codegen, LLVM, linkage https://github.com/rust-lang-nursery/rustc-guide/issues/89. There is nothing written about these important topics at all. This is probably the biggest gap in the guide at this point.
MIR construction, MIR Optimizations. There is nothing written about these at all.
EDIT: also, macro expansion and hygiene. We have a little here, but the set of people knowledgable on these topics is extremely small…
Unfortunately, both of these chapters require some more in-depth knowledge to write (and I don’t have it). Is there anyone willing to write up something? It would be fine to take a small portion of either chapter (e.g. only linkage), but it would be good to have something on these topics.