Introduce DST-enabled Rc as unstable under a feature flag

[bluss]

It's irresponsible not to do it.

See #25515.

[Thiez]

Must we really have feature flags for everything? If the bug turns out to have an easy fix, surely just fixing the bug would be preferable to introducing yet another feature flag. Let's wait for a day so the bug can get discussed, and if it turns out to be a very hard problem the feature flag can always be introduced then. Alternatively, temporarily change the line

impl<T: ?Sized+Unsize<U>, U: ?Sized> CoerceUnsized<Rc<U>> for Rc<T> {}

to

impl<T: ?Sized+Copy+Unsize<U>, U: ?Sized> CoerceUnsized<Rc<U>> for Rc<T> {}

because the Copy bound will prevent types with a Drop implementation from being coerced for now (why don't we have a Pod trait anymore?), and removing the bound is probably backwards compatible.

[bluss]

We have feature flags for everything that's unstable. That's not really the problem, I just want it to be unstable while in development. We only have 6-week cycles now and why rush out something that's broken?

I think the default should be that big features are introduced under feature flag.

cc @nikomatsakis since you introduced it, what do you think? If you have 100% belief in the API that's fine, but sometimes we need to adjust APIs to fix soundness bugs.

[alexcrichton]

The current system of stability attributes is known to not be bullet proof and doesn't cover the stability of impl blocks, for example, so I don't think we need to make sure that everything comes in under a feature gate, adding functionality like this is fine to land without it. Although there may be bugs no one's going to rely on the bug of double-drop, for example.

[bluss]

I think I'm overreacting a bit, I'm sorry for that. Especially given that the bug is not in Rc-DST at all!

@alexcrichton Stabilizing precludes changes to API, if those are needed to provide a sound interface for say for example Rc-DST. See my question to niko.

[bluss]

Another data point: see bug #26352.

The feature is broken in the beta, which is unfortunate. With a feature flag we could have quality assured it before we stabilized it.

[apasel422]

Rc is DST-enabled:

use std::rc::Rc;

fn main() {
    let x: Rc<[i32]> = Rc::new([1, 2, 3]);
}

I think this can be closed.

[steveklabnik]

Yup, this should have been okay as of 1.2.