Module doctest_tlcell

This tests the TLCell implementation.

It’s not possible to have two simultaneous owners for the same marker type:

struct Marker;
type ACellOwner = TLCellOwner<Marker>;
let mut owner1 = ACellOwner::new();
let mut owner2 = ACellOwner::new();  // Panics here

It should be impossible to copy a TLCellOwner:

type ACell<T> = TLCell<Marker, T>;
type ACellOwner = TLCellOwner<Marker>;
let mut owner1 = ACellOwner::new();
let mut owner2 = owner1;
let rc = Rc::new(owner1.cell(100u32));  // Compile fail

It should be impossible to clone a TLCellOwner:

let mut owner1 = ACellOwner::new();
let owner2 = owner1.clone();  // Compile fail

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

struct MarkerA;
type ACellOwner = TLCellOwner<MarkerA>;
type ACell<T> = TLCell<MarkerA, T>;
struct MarkerB;
type BCellOwner = TLCellOwner<MarkerB>;
type BCell<T> = TLCell<MarkerB, T>;

let mut owner_a = ACellOwner::new();
let mut owner_b = BCellOwner::new();
let c1 = Rc::new(ACell::new(100u32));

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

Or mutably:

let mut owner_a = ACellOwner::new();
let mut owner_b = BCellOwner::new();
let c1 = Rc::new(ACell::new(100u32));

let c1mutref = owner_b.rw(&*c1);    // Compile error
println!("{}", *c1mutref);

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

let mut owner = ACellOwner::new();
let c1 = Rc::new(ACell::new(100u32));
let c2 = Rc::new(ACell::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:

let c1 = Rc::new(ACell::new(100u32));
let c2 = Rc::new(ACell::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:

let c1 = Rc::new(ACell::new(100u32));
let c2 = Rc::new(ACell::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:

let c1 = Rc::new(ACell::new(100u32));
let c2 = Rc::new(ACell::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:

let c1 = Rc::new(ACell::new(100u32));
let c2 = Rc::new(ACell::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:

let c1 = Rc::new(ACell::new(100u32));
let c2 = Rc::new(ACell::new(200u32));

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:

let c1 = Rc::new(ACell::new(100u32));
let c2 = Rc::new(ACell::new(200u32));

fn test(o: &mut ACellOwner) {}

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

Or & when there’s a &mut reference:

let c1 = Rc::new(ACell::new(100u32));
let c2 = Rc::new(ACell::new(200u32));

fn test(o: &ACellOwner) {}

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

TLCellOwner should be neither Send nor Sync, because it must not escape the thread in which it was created:

struct Marker;
fn is_send<T: Send>() {}
is_send::<TLCellOwner<Marker>>();  // Compile fail

struct Marker;
fn is_sync<T: Sync>() {}
is_sync::<TLCellOwner<Marker>>();  // Compile fail

TLCell should be Send by default, but never Sync:

struct Marker;
fn is_send<T: Send>() {}
is_send::<TLCell<Marker, ()>>();

struct Marker;
fn is_sync<T: Sync>() {}
is_sync::<TLCell<Marker, ()>>(); // Compile fail

A practical example is sending a TLCell to another thread for modification and receiving it back again:

type ACellOwner = TLCellOwner<Marker>;
type ACell = TLCell<Marker, i32>;

let mut owner = ACellOwner::new();
let cell = ACell::new(100);

*owner.rw(&cell) += 1;
let cell = std::thread::spawn(move || {
    let mut owner = ACellOwner::new();  // A different owner
    *owner.rw(&cell) += 2;
    cell
}).join().unwrap();
*owner.rw(&cell) += 4;
assert_eq!(*owner.ro(&cell), 107);

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

let owner = ACellOwner::new();
let cell = ACell::new(100);
let val_ref = owner.ro(&cell);
std::thread::spawn(move || {
    assert_eq!(*owner.ro(&cell), 100);
}).join();
assert_eq!(*val_ref, 100);

If the contained type isn’t Send, the TLCell shouldn’t be Send either:

fn is_send<T: Send>() {}
is_send::<TLCell<Marker, Rc<()>>>();  // Compile fail

type ACellOwner = TLCellOwner<Marker>;
type ACell = TLCell<Marker, Rc<i32>>;

let owner = ACellOwner::new();
let cell = ACell::new(Rc::new(100));

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

Covariant subtypes can’t be used to cheat the owner singleton check. (This code incorrectly succeeds before qcell version 0.4.3.)

type MarkerA = fn(&());
type MarkerB = fn(&'static ());

let mut owner1 = TLCellOwner::<MarkerA>::new() as TLCellOwner<MarkerB>;  // Compile fail
let mut owner2 = TLCellOwner::<MarkerB>::new();
let cell = TLCell::<MarkerB, u32>::new(1234);
let ref1 = owner1.rw(&cell);
let ref2 = owner2.rw(&cell);
*ref1 = 1;  // Two mutable refs at the same time!  Unsound!
*ref2 = 2;

A reference obtained using get_mut should exclude any other kind of borrowing.

let owner = ACellOwner::new();
let mut cell = ACell::new(100);
let cell_ref = cell.get_mut();
assert_eq!(100, *owner.ro(&cell)); // Compile fail
*cell_ref = 50;

let mut owner = ACellOwner::new();
let mut cell = ACell::new(100);
let cell_ref = cell.get_mut();
assert_eq!(100, *owner.rw(&cell)); // Compile fail
*cell_ref = 50;

let owner = ACellOwner::new();
let mut cell = ACell::new(100);
let cell_ref = owner.ro(&cell);
*cell.get_mut() = 50; // Compile fail
assert_eq!(100, *cell_ref);

let mut owner = ACellOwner::new();
let mut cell = ACell::new(100);
let cell_ref = owner.rw(&cell);
*cell.get_mut() = 50; // Compile fail
assert_eq!(100, *cell_ref);

Default is implemented, but only if the enclosed type has a default:

let mut owner = ACellOwner::new();
let mut cell: ACell<i32> = ACell::default();
assert_eq!(0, *owner.ro(&cell));

struct NoDefault(i32);
let mut owner = ACellOwner::new();
let mut cell: ACell<NoDefault> = ACell::default(); // Compile fail
assert_eq!(0, owner.ro(&cell).0);