Question to the borrow checker

borrow_checker.rs

pub struct One<'a> {
    foo: &'a f64,
    bar: f64,
}

pub struct Two {
    oof: f64,
    rab: f64,
}

impl<'a> One<'a> {
    pub fn inc(&'a mut self, x:f64) {
        *self.foo += x;
    }
    
    pub fn dec(&'a mut self, x:f64) {
        *self.foo -= x;
    }
    
    pub fn inc_dec(&'a mut self, x:f64) {
        self.inc(x);
        self.dec(x);
    }
}

pub fn main() {
    let mut one = One {
        foo: &0.0,
        bar: 0.0,
    };
    
    one.inc(1.0);
    one.dec(1.0);
}