So it re-adds manual lifetime checking. Got it.

It might or might not be a toy project, I'm not sure, but one advantage of subtracting the borrow checking is that the compiler avoids a lot of complex machinery.

Borrow checking in Rust isn't sound AFAIK, even after all these years, so some of the problems with designing and implementing lifetimes, region checking, and borrow checking algorithms, aren't trivial.