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// lifetimes4.rs
//
// Sometimes, we have structs which hold on to data temporarily. A use-case of
// this could be a routing component which accepts data and returns it to
// another recipient. To avoid copying the data, we just accept a reference with
// lifetime and return this reference later.
//
// In the example below, we create a `Router` instance in a limited scope. It
// accepts a number reference created in the enclosing scope and returns it.
// In theory, this should be possible given that the number reference outlives
// the scope from which it is returned. However the borrow checker does not
// seem to understand it. What can we do about that?
//
// Execute `rustlings hint lifetimes4` or use the `hint` watch subcommand for a
// hint.
// I AM NOT DONE
struct Router<'a> {
number_ref: Option<&'a u64>,
}
impl<'a> Router<'a> {
fn new() -> Self {
Self { number_ref: None }
}
fn take_number_ref(&mut self, number_ref: &'a u64) {
self.number_ref = Some(number_ref);
}
fn return_number_ref(&mut self) -> Option<&u64> {
self.number_ref.take().take()
}
}
fn main() {
let number_ref = &123;
let returned_ref = {
// Create router within scope.
let mut router = Router::new();
// Accept number ref which lives longer than the router.
router.take_number_ref(number_ref);
// Return number ref which **should** live longer than the router.
router.return_number_ref()
};
if let Some(number) = returned_ref {
println!("The number is {number}");
}
}

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@ -879,6 +879,25 @@ hint = """
If you use a lifetime annotation in a struct's fields, where else does it need
to be added?"""
[[exercises]]
name = "lifetimes4"
path = "exercises/lifetimes/lifetimes4.rs"
mode = "compile"
hint = """
The compiler complains that the `Router` is dropped at the end of the smaller
scope while still being borrowed. However, we never intended to return a borrow
to the `Router`. What are we really returning from
`Router::return_number_ref`?
The method `Router::return_number_ref` only takes `&mut self`
and returns `Option<&u64>`. No explicit lifetimes are specified. What lifetime
will the borrow checker assume for the `&u64` in the `Option`? You may want to
re-read the chapter on lifetime elision:
https://doc.rust-lang.org/reference/lifetime-elision.html
What lifetime do we really want the `Option<&u64>` to have? Can we make that
explicit?"""
# TESTS
[[exercises]]