There is value in having unsafe parts of a program clearly annotated (not just with comments). It is similar to how in some languages you annotate pure functions and they do not compile unless they are pure.
anytime you need to implement anything that shares references (just about any data structure worth its weight in implementation time) you need to use Rc<> and friends.
i am not talking about https://doc.rust-lang.org/book/ch19-01-unsafe-rust.html. fine, this escape hatch is needed.
i am referring to https://doc.rust-lang.org/book/ch15-06-reference-cycles.html.