mapにするとうまくいかないやつがわかったとき

test.rs

struct Nanika {
    v: V,
}

struct V(u8);

impl V {
    fn incr(&mut self) {
        self.0 += 1
    }
}

impl Nanika {
    fn do_nanika(&mut self) -> Option<&V> {
        self.v.incr();
        Some(&V(1))
    }

    fn get_nanika(&self) -> Option<&V> {
        Some(&self.v)
    }
}

fn main() {
    let mut n = Nanika { v: V(0) };
    let v = n.v.0;
    
    // これはOKだけどNoneのケースが冗長なので書きたくない
    let _ = match n.do_nanika() {
        Some(nanika) => Some(nanika.0 * n.v.0),
        None => None
    };
    
    let _ = n.do_nanika().map(|nanika| {
        // nanika.0 * n.v.0
        // ^ これはエラー。
        nanika.0 * v
    });
    
    // これは不変なのでOK
    let _ = n.get_nanika().map(|nanika| {
        nanika.0 * n.v.0
    });
}

error[E0502]: cannot borrow `n` as immutable because it is also borrowed as mutable
  --> .\scratch.rs:27:40
   |
27 |     let _ = Option::map(n.do_nanika(), |nanika| {
   |             ----------- -              ^^^^^^^^ immutable borrow occurs here
   |             |           |
   |             |           mutable borrow occurs here
   |             mutable borrow later used by call
28 |         nanika.0 * n.v.0
   |                    - second borrow occurs due to use of `n` in closure