Try add covariance for containers

[zetanumbers]

Enables a small(?) but valid set of cases. May come with a small penalty, until we have Unique built-in. Tho it seems like even in std Unique<T> does not enable appropriate optimizations currently, based on my experiments with std::vec::Vec. On the other hand Box enables "ownership optimizations", maybe due to #[lang = "..."] attribute.

Make sure to benchmark difference.

[fitzgen]

I doubt that lifetime variance has any effect on optimizations. AFAIK that's all erased by the time code goes to LLVM, and I'd expect that any rustc optimizations for Box are looking at the lang attributes.

[zetanumbers]

I doubt that lifetime variance has any effect on optimizations.

Of course, it's just that there's currently no nonaliased covariant pointer for rust, so the only option is maybe aliased covariant NonNull<T>, vs. nonaliased but invariant &mut ManuallyDrop<T>

[douglas-raillard-arm]

I just bumped into this issue while replacing a std::boxed::Box inside a recursive enum like this one:

struct Value<'a> {
    Str(&'a str),
    Wrapped(Box<Value<'a>>)
} 

The consuming code rightfully expects Value<'a> to be covariant in 'a, but that breaks with bumpalo currently. For example the code relies in multiple places on Value<'static> to always be a valid value for a Value<'a> parameter.

EDIT: Is there any workaround for that issue ?
EDIT2: So I ended up with turning the Box into std::ptr::NonNull and turning it back into a Box when required.