working_scoped_take_mut_lifetimes.rs

use std::marker::PhantomData;
use std::cell::Cell;

struct Scope<'s> {
    num: Cell<usize>,
    phantom: PhantomData<Cell<&'s mut ()>>,
    //ms: Vec<&'s mut i32>, <-- useful trick for understanding lifetimes
}

impl<'s> Scope<'s> {
    fn take<'a: 's>(&self, m: &'a mut i32) -> &Cell<usize> {
        //self.ms.push(m); <-- useful trick for understanding lifetimes
        // ...
        &self.num
    }
}

fn scope<'s, R, F: FnOnce(&Scope<'s>) -> R>(f: F) -> R {
    let r;
    {
        let scope = Scope {
            num: Cell::new(0),
            phantom: PhantomData,
            //ms: Vec::new(), <-- useful trick for understanding lifetimes
        };
        r = f(&scope);
    }
    r
}

fn main() {
    let mut a: i32 = 0;
    let mut c: i32 = 0;
    scope(|scope| {
        let ca = scope.take(&mut a);
        let cc = scope.take(&mut c);
        let mut b: i32 = 0;
        //scope.take(&mut b);
        // ^^ uncomment for compilation failure
    });
    
}