I admit I'm quite confused by the part of the book about Salsa.
Not the content, it feels relatively clear after the previous sections explains in details the idea of the algoritm.
But why it is mentioned.

First, in incremental compilation, I read

> 1 I have long wanted to rename it to the Salsa algorithm, but it never caught on. -@nikomatsakis

Then I realize in how salsa works that something is called Salsa.
So I guess the book should clarify somehow how the algorithm wannabe-name and the library are related. In particular, that the Salsa library is not an implementation of what nikomatsakis calls the Salsa algorithm, even if the talk is by the same person. I admit that it's relatively confusing here.

More importantly, I think, is, why is this section in the rustc-dev-guide if rustc does not use it. The actual implementation was linked to in the "in detail" section. So I guess the point is not to tell us the red-green algorithm is really used.
If the point is only to state "by the way, this is a clearly related topic you might be interested", I suspect you should explicitly state that, while interesting, one won't learn anything about rustc specifically in this section