Explain the technical parts of the Walkthrough somewhere

[spastorino]

Was reading this https://rust-lang.github.io/rustc-guide/walkthrough.html and I thought that it could be very nice if we can explain the actual code of the PR. It seems like a feature that touches a lot of parts of the compiler so it may be a good idea to explain things there or later in the guide but using this same example.

cc @mark-i-m

[spastorino]

Well I'm having second thoughts if this is a good example for what I'm thinking right now. But would be nice to explain a PR that touches a lot of the different parts of the compiler.

[mark-i-m]

I think it mostly goes through parts of the front-end... I guess that chapter was aimed more at being about the processes of contribution, rather than technical content...