[derefr]

> They're inherently imperative; they don't return anything because they have no one to return it to.

If your think of your whole program as being a wrapped in an implicit State monad, holding a POSIXProcessState (e.g. exit code, registered signal handlers, file descriptors, etc.), then destructors are (POSIXProcessState -> POSIXProcessState) functions.

> You can't create, destroy, or manipulate a doubly linked list or a tree with backpointers in safe Rust. ... Maybe forward pointer/backpointer pairs need to be a language level concept.

You can define abstractions like this using unsafe code just fine. It doesn't need to be part of the language. (Think about how C++ "smart pointers" work: it's just a library.)