foo.rs

let mut pressure_pen = evdev::enumerate().into_iter().find(|d| d.absolute_axes_supported().contains(ABS_PRESSURE));
let get_pressure: Box<FnMut() -> i32> = match pressure_pen {
    Some(pressure_pen) => Box::new(move || -> i32 {
        let mut events = pressure_pen.events_no_sync().unwrap();
        //               ^^^^^^^^^^^^ - cannot borrow captured outer variable in FnMut closure as mutable
        let pressure = events.find(|ev| ev.code < 0xFF && 1 << ev.code == ABS_PRESSURE.bits());

        match pressure {
            Some(pressure) => pressure.value,
            _ => 0i32
        }
    }),
    _ => Box::new(move || -> i32 { 1000i32 })
};