Obtain a reference from a RefCell<Option<Rc<T>>> in Rust - rust

Question

I am having problem getting a reference out of a RefCell<Option<Rc>>.
Any suggestion?
struct Node<T> {
value: T
}
struct Consumer3<T> {
tail: RefCell<Option<Rc<Node<T>>>>,
}
impl<T> Consumer3<T> {
fn read<'s>(&'s self) -> Ref<Option<T>> {
Ref::map(self.tail.borrow(), |f| {
f.map(|s| {
let v = s.as_ref();
v.value
})
})
}
}
Gives:
error[E0308]: mismatched types
--> src/lib.rs:15:13
|
15 | / f.map(|s| {
16 | | let v = s.as_ref();
17 | | v.value
18 | | })
| |______________^ expected reference, found enum `Option`
|
= note: expected reference `&_`
found enum `Option<T>`
help: consider borrowing here
|
15 | &f.map(|s| {
16 | let v = s.as_ref();
17 | v.value
18 | })
|
error: aborting due to previous error
Playground

Answer 1

Mapping from one Ref to another requires that the target already exist in memory somewhere. So you can't get a Ref<Option<T>> from a RefCell<Option<Rc<Node<T>>>>, because there's no Option<T> anywhere in memory.
However, if the Option is Some, then there will be a T in memory from which you can obtain a Ref<T>; if the Option is None, obviously you can't. So returning Option<Ref<T>> may be a viable alternative for you:
use std::{cell::{Ref, RefCell}, rc::Rc};
struct Node<T> {
value: T
}
struct Consumer3<T> {
tail: RefCell<Option<Rc<Node<T>>>>,
}
impl<T> Consumer3<T> {
fn read(&self) -> Option<Ref<T>> {
let tail = self.tail.borrow();
if tail.is_some() {
Some(Ref::map(tail, |tail| {
let node = tail.as_deref().unwrap();
&node.value
}))
} else {
None
}
}
}
Playground.