Returning a RWLockReadGuard independently from a method

Question

I have an object of type

Arc>

And I have a method that is supposed to take some kind of reference to

Answer 1

As described in Why can't I store a value and a reference to that value in the same struct?, the Rental crate allows for self-referential structs in certain cases.

#[macro_use]
extern crate rental;

use std::sync::{Arc, RwLock};

struct SessionData;
impl SessionData {
    fn hello(&self) -> u8 { 42 }
}

rental! {
    mod owning_lock {
        use std::sync::{Arc, RwLock, RwLockReadGuard};

        #[rental(deref_suffix)]
        pub struct OwningReadGuard<T>
        where
            T: 'static,
        {
            lock: Arc<RwLock<T>>,
            guard: RwLockReadGuard<'lock, T>,
        }
    }
}

use owning_lock::OwningReadGuard;

fn owning_lock(session: Arc<RwLock<SessionData>>) -> OwningReadGuard<SessionData> {
    OwningReadGuard::new(session, |s| s.read().unwrap())
}

fn main() {
    let session = Arc::new(RwLock::new(SessionData));

    let lock = owning_lock(session.clone());
    println!("{}", lock.hello());

    assert!(session.try_read().is_ok());
    assert!(session.try_write().is_err());

    drop(lock);

    assert!(session.try_write().is_ok());
}

See also:

• How to return a reference to a sub-value of a value that is under a mutex?
• Returning the T borrowed from RefCell<T>
• Why can't I store a value and a reference to that value in the same struct?