You want to map an immutable `Ref` guard to a borrow of one struct field so the borrow stays tracked. Which function produces a `Ref` that points to a component of the originally borrowed data?
use std::cell::{RefCell, Ref};
struct P { name: String, age: u32 }
let c = RefCell::new(P { name: "x".into(), age: 1 });
let name: Ref<String> = Ref::map(c.borrow(), |p| /* ??? */);