Module qcell::doctest_lcell_generativity

This tests the LCell implementation when using generativity.

It should be impossible to copy a &mut LCellOwner:

make_guard!(guard1);
let mut owner1 = LCellOwner::new(guard1);
let owner2 = owner1;
let rc = Rc::new(owner1.cell(100u32)); // Compile fail

It should be impossible to clone an LCellOwner:

make_guard!(guard1);
let mut owner1 = LCellOwner::new(guard1);
let owner2 = owner1.clone(); // Compile fail

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

make_guard!(guard1);
make_guard!(guard2);
let mut owner1 = LCellOwner::new(guard1);
let mut owner2 = LCellOwner::new(guard2);
let c1 = Rc::new(LCell::new(100u32));
let c1ref1 = owner1.ro(&c1);
let c1ref2 = owner2.ro(&c1);   // Compile error
println!("{}", *c1ref2);

Or mutably:

make_guard!(guard1);
make_guard!(guard2);
let mut owner1 = LCellOwner::new(guard1);
let mut owner2 = LCellOwner::new(guard2);
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:

make_guard!(guard);
let mut owner = LCellOwner::new(guard);
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:

make_guard!(guard);
let mut owner = LCellOwner::new(guard);
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:

make_guard!(guard);
let mut owner = LCellOwner::new(guard);
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:

make_guard!(guard);
let mut owner = LCellOwner::new(guard);
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:

make_guard!(guard);
let mut owner = LCellOwner::new(guard);
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:

make_guard!(guard);
let mut owner = LCellOwner::new(guard);
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:

make_guard!(guard);
let mut owner = LCellOwner::new(guard);
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:

make_guard!(guard);
let mut owner = LCellOwner::new(guard);
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, generativity::make_guard};
use std::rc::Rc;
make_guard!(guard);
let mut owner = LCellOwner::new(guard);
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, generativity::make_guard};
use std::rc::Rc;
make_guard!(guard);
let mut owner = LCellOwner::new(guard);
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):

make_guard!(guard);
let mut owner = LCellOwner::new(guard);
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:

make_guard!(guard);
let mut owner = LCellOwner::new(guard);
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

make_guard!(guard);
let mut owner = LCellOwner::new(guard);
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

make_guard!(guard);
let mut owner = LCellOwner::new(guard);
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();