soundness hole with types that are Sync + !Send

[niklasf]

Hi, I believe that the unsafe impl Sync needs to also require Send. Otherwise it is possible to transfer non-Send values between threads.

Similar issues in other crates:

• Possibly add a T: Send + Sync requirement to the Send and Sync impls indiv0/lazycell#67
• niklasf/double-checked-cell@ee6b6f3

// make a type that is Sync but not Send
// such types are arguably very rare
#![feature(optin_builtin_traits)]
#[derive(Debug)]
struct Foo(u32);
impl !Send for Foo { }

fn should_fail_to_compile() {
    let lazy = Lazy::new();

    let pool = Pool::new(1);
    pool.scoped(|scope| {
        let l = &lazy;
        scope.execute(move || {
            // we transfer a value from this thread ...
            l.get_or_create(|| Foo(42));
        });
    });

    // ... to another thread
    let result = lazy.into_inner();
    println!("{:?}", result);
}

[khuey]

I need to think about this a bit more but I believe you're correct.

[niklasf]

Any news? I see 66bbfc2, but I believe U requires the same bound. For example the snippet above still compiles.

[matklad]

+1

This is unsound, it's better to fix the issue and yank old releases ;)