Can't return mutable borrow of self

Question

In the main method, I first create an owned builder which is then mutably borrowed by the number function which tries to return a reference to itself:

struct Builder {
    string: Option<String>,
    number: Option<usize>,
}

impl Builder {
    fn default() -> Builder {
        Builder {
            string: None,
            number: None,
        }
    }

    fn number(&mut self, n: usize) -> &mut Self {
        self.number = Some(n);
        &mut self
    }

    fn to_string(&self) -> &'static str {
        "tte"
    }
}

// Do not modify this function.
fn main() {
    let _test = Builder::default().number(456).to_string();
}

I don't see why the borrowed value wouldn't "live long enough":

error[E0597]: `self` does not live long enough
  --> src/main.rs:16:14
   |
16 |         &mut self
   |              ^^^^ borrowed value does not live long enough
17 |     }
   |     - borrowed value only lives until here
   |
note: borrowed value must be valid for the anonymous lifetime #1 defined on the method body at 14:5...
  --> src/main.rs:14:5
   |
14 | /     fn number(&mut self, n: usize) -> &mut Self {
15 | |         self.number = Some(n);
16 | |         &mut self
17 | |     }
   | |_____^

What is exactly going on here and how do I fix it? I tried to add lifetimes to the number method but that seems to be the wrong approach.