HashMap<K,V> is always Send/Sync

[talchas]

trait Foo: Send {}
impl Foo for HashMap<Rc<()>, Rc<()>> {}

This compiles, despite Rc : !Send. I'm guessing this is because RawTable only uses K and V with marker::CovariantType, and I guess that isn't enough for the negative-impl to propagate. I haven't checked which other collections might be unsafe.

[alexcrichton]

It appears that this also isn't specifically related to trait bounds:

use std::collections::HashMap;
use std::rc::Rc;
fn foo<T: Send>() {}
fn main() {
    foo::<HashMap<Rc<()>, Rc<()>>>();
}

cc @gankro, @pczarn, @cgaebel,

[Gankra]

Ah, yeah. RawTable contains only a Unique<u8>, which is of course always Send/Sync. It should presumably just contain a plain *mut u8 and impl Send/Sync conditionally?

[Gankra]

Or we need a more rich marker type.

[alexcrichton]

rust-lang/rfcs#738 may be a good replacement for the current system as it could contain a marker of Vec<K> and Vec<V> for example. For now though a manual impl of Send and Sync will probably be required.

Should other collections with an unsafe representation also be audited?

[Gankra]

I can say confidently this is the only problem case in collections. It's the only collection that uses basically a void pointer.

[alexcrichton]

ok cool, good to know!