Module qcell::doctest_lcell

This tests the LCell implementation.

It should be impossible to copy a &mut LCellOwner:

LCellOwner::scope(|mut owner1| {
    let owner2 = owner1;
    let rc = Rc::new(owner1.cell(100u32)); // Compile fail
});

It should be impossible to clone an LCellOwner:

LCellOwner::scope(|mut owner1| {
    let owner2 = owner1.clone(); // Compile fail
});

Two different owners can’t borrow each other’s cells immutably:

LCellOwner::scope(|mut owner1| {
    LCellOwner::scope(|mut owner2| {
        let c1 = Rc::new(LCell::new(100u32));
        let c1ref1 = owner1.ro(&c1);
        let c1ref2 = owner2.ro(&c1);   // Compile error
        println!("{}", *c1ref2);
    });
});

Or mutably:

LCellOwner::scope(|mut owner1| {
    LCellOwner::scope(|mut owner2| {
        let c1 = Rc::new(owner1.cell(100u32));
        let c1mutref2 = owner2.rw(&c1);    // Compile error
        println!("{}", *c1mutref2);
    });
});

You can’t have two separate mutable borrows active on the same owner at the same time:

LCellOwner::scope(|mut owner| {
    let c1 = Rc::new(LCell::new(100u32));
    let c2 = Rc::new(LCell::new(200u32));

    let c1mutref = owner.rw(&c1);
    let c2mutref = owner.rw(&c2);  // Compile error
    *c1mutref += 1;
    *c2mutref += 2;
});

However with rw2() you can do two mutable borrows at the same time, since this call checks at runtime that the two references don’t refer to the same memory:

LCellOwner::scope(|mut owner| {
    let c1 = Rc::new(LCell::new(100u32));
    let c2 = Rc::new(LCell::new(200u32));
    let (c1mutref, c2mutref) = owner.rw2(&c1, &c2);
    *c1mutref += 1;
    *c2mutref += 2;
    assert_eq!(303, owner.ro(&c1) + owner.ro(&c2));   // Success!
});

You can’t have a mutable borrow at the same time as an immutable borrow:

LCellOwner::scope(|mut owner| {
    let c1 = Rc::new(LCell::new(100u32));
    let c2 = Rc::new(LCell::new(200u32));
    let c1ref = owner.ro(&c1);
    let c1mutref = owner.rw(&c1);    // Compile error
    println!("{}", *c1ref);
});

Not even if it’s borrowing a different object:

LCellOwner::scope(|mut owner| {
    let c1 = Rc::new(LCell::new(100u32));
    let c2 = Rc::new(LCell::new(200u32));
    let c1mutref = owner.rw(&c1);
    let c2ref = owner.ro(&c2);    // Compile error
    *c1mutref += 1;
});

Many immutable borrows at the same time is fine:

LCellOwner::scope(|mut owner| {
    let c1 = Rc::new(LCell::new(100u32));
    let c2 = Rc::new(LCell::new(200u32));
    let c1ref = owner.ro(&c1);
    let c2ref = owner.ro(&c2);
    let c1ref2 = owner.ro(&c1);
    let c2ref2 = owner.ro(&c2);
    assert_eq!(600, *c1ref + *c2ref + *c1ref2 + *c2ref2);   // Success!
});

Whilst a reference is active, it’s impossible to drop the Rc:

LCellOwner::scope(|mut owner| {
    let c1 = Rc::new(LCell::new(100u32));
    let c1ref = owner.ro(&c1);
    drop(c1);    // Compile error
    println!("{}", *c1ref);
});

Also, whilst a reference is active, it’s impossible to call anything else that uses the owner in an incompatible way, e.g. &mut when there’s a & reference:

LCellOwner::scope(|mut owner| {
    let c1 = Rc::new(LCell::new(100u32));
    fn test(o: &mut LCellOwner) {}

    let c1ref = owner.ro(&c1);
    test(&mut owner);    // Compile error
    println!("{}", *c1ref);
});

Or & when there’s a &mut reference:

LCellOwner::scope(|mut owner| {
    let c1 = Rc::new(LCell::new(100u32));
    fn test(o: &LCellOwner) {}

    let c1mutref = owner.rw(&c1);
    test(&owner);    // Compile error
    *c1mutref += 1;
});

Two examples of passing owners and cells in function arguments. This needs lifetime annotations.

use qcell::{LCell, LCellOwner};
use std::rc::Rc;
LCellOwner::scope(|mut owner| {
    let c1 = Rc::new(LCell::new(100u32));
    fn test<'id>(o: &mut LCellOwner<'id>, rc: &Rc<LCell<'id, u32>>) {
       *o.rw(rc) += 1;
    }

    test(&mut owner, &c1);
    let c1mutref = owner.rw(&c1);
    *c1mutref += 1;
});

use qcell::{LCell, LCellOwner};
use std::rc::Rc;
LCellOwner::scope(|mut owner| {
    struct Context<'id> { owner: LCellOwner<'id>, }
    let c1 = Rc::new(LCell::new(100u32));
    let mut ct = Context { owner };
    fn test<'id>(ct: &mut Context<'id>, rc: &Rc<LCell<'id, u32>>) {
       *ct.owner.rw(rc) += 1;
    }

    test(&mut ct, &c1);
    let c1mutref = ct.owner.rw(&c1);
    *c1mutref += 2;
});

LCellOwner and LCell should be both Send and Sync by default:

fn is_send_sync<T: Send + Sync>() {}
is_send_sync::<LCellOwner<'_>>();
is_send_sync::<LCell<'_, ()>>();

So for example we can share a cell ref between threads (Sync), and pass an owner back and forth (Send):

LCellOwner::scope(|mut owner| {
    let cell = LCell::new(100);

    *owner.rw(&cell) += 1;
    let cell_ref = &cell;
    let mut owner = crossbeam::scope(move |s| {
        s.spawn(move |_| {
            *owner.rw(cell_ref) += 2;
            owner
        }).join().unwrap()
    }).unwrap();
    *owner.rw(&cell) += 4;
    assert_eq!(*owner.ro(&cell), 107);
});

However you can’t send a cell that’s still borrowed:

LCellOwner::scope(|mut owner| {
    let cell = LCell::new(100);
    let val_ref = owner.ro(&cell);
    crossbeam::scope(move |s| {
        s.spawn(move |_| assert_eq!(*owner.ro(&cell), 100)).join().unwrap(); // Compile fail
    }).unwrap();
    assert_eq!(*val_ref, 100);
});

If the contained type isn’t Sync, though, then LCell shouldn’t be Sync either:

fn is_sync<T: Sync>() {}
is_sync::<LCell<'_, Cell<i32>>>();  // Compile fail

LCellOwner::scope(|owner| {
    let cell = LCell::new(Cell::new(100));

    // This would likely be a data race if it compiled
    crossbeam::scope(|s| {   // Compile fail
        let handle = s.spawn(|_| owner.ro(&cell).set(200));
        owner.ro(&cell).set(300);
        handle.join().unwrap();
    }).unwrap();
});

If the contained type isn’t Send, the LCell should be neither Sync nor Send:

fn is_sync<T: Sync>() {}
is_sync::<LCell<'_, Rc<()>>>();  // Compile fail

fn is_send<T: Send>() {}
is_send::<LCell<'_, Rc<()>>>();  // Compile fail

LCellOwner::scope(|owner| {
    let cell = LCell::new(Rc::new(100));

    // We aren't permitted to move the Rc to another thread
    crossbeam::scope(move |s| {
        s.spawn(move |_| assert_eq!(100, **owner.ro(&cell))).join().unwrap(); // Compile fail
    }).unwrap();
});