"borrow checker invariants" section of the "leveraging the type system" chapter

[tall-vase]

Adds materials on the "leveraging the type system/borrow checker invariants" subject.

I'm still calibrating what's expected subject-and-style wise, so do spell out things where I've drifted off mark.

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[djmitche]

I apologize for the enormous delay here -- it's been a busy couple of months!

This looks great, and I appreciate seeing previous comments included here.

[djmitche]

It's unclear what purpose T serves here, aside from illustrating PhantomData<T>?

[tall-vase]

It's largely to prevent there being a gap in understanding what PhantomData can do. PhantomData has not been introduced until this point, and one might imagine it is "just" for lifetimes without this nudge towards "you can put anything here."

[tall-vase]

Which is to say, you're correct that it is just illustrative of the broader possible uses of PhantomData.