Add std types for Rocket (and uuid)

[the10thWiz]

Adds Static implementations for a number of std error types, and adds an optional dependency on uuid, with appropriate Static impls.

[JRRudy1]

The one problem here is that the unsafe impl<S: Static> Transient for S { ... } impl will not apply to this since it has an implicit S: Sized bound, and in fact the Transient trait has Sized as a super-type. The reason for this is kinda funky, and I was torn on whether to add it, so I am curious on your take.

Let's pretend that the Transient trait did not have the Sized requirement, and implement it for Vec<T: Transient>:

unsafe impl<T: Transient> Transient for Vec<T> {
    type Static = Vec<T::Static>;
    type Transience = T::Transience;
}

This will not work, since Vec requires its parameter to be Sized, which is implied for T, but not for T::Static. So you would need to add a where clause:

unsafe impl<T: Transient> Transient for Vec<T> 
where
    T::Static: Sized
{
    type Static = Vec<T::Static>;
    type Transience = T::Transience;
}

This wouldn't be too bad on its own, but then consider using it:

fn erase_vec<T: Transient>(x: &Vec<T>) -> &dyn Any<Co> {
    x
}

Again this won't work, since the compiler needs you to tell it again that T::Static: Sized with another where clause.

So to avoid this awkwardness in usage, I just decided to restrict Transient to Sized types so that the Transient::Static associated type also have a Sized bound. But this reminds me that I should probably do the same to the Static trait.

[the10thWiz]

Although it would be nice to support unsized types, it looks like the std any doesn't support unsized types either.

I'm pretty sure the issue is that you can't upcast an unsized type into a trait object, since it already has metadata. E.g. casting &str to &dyn Any isn't possible, even though str: Any.

[JRRudy1]

Oh you're totally right. I noticed that std::any::Any is indeed implemented for T: ?Sized and allows you to get a TypeId, but didn't look far enough to realize that all the useful dyn Any methods are implicitly T: Sized. So I'm happy to learn that we don't have a limitation beyond the stdlib version :)

[the10thWiz]

At least for now. Technically, something like std::boxed::ThinBox could eventually make it possible to upcast unsized types, but it's still nightly only for now.

I don't think it's worth even trying to make unsized types work until (and unless) std does.

[JRRudy1]

Agreed, I am perfectly happy matching the stdlib's limitation 👍

[JRRudy1]

Awesome, thanks for adding these! I think we should remove the impl for str as explained above unless you have any thoughts on getting around it, but otherwise looks good to merge.

[JRRudy1]

I removed the Static impl for str, and added the Static: Sized requirement, as mentioned in my comment above. If we want to investigate removing this restriction we can do that in another Issue or PR.