Implement slice::Vector for Option<T> and CVec<T>

[donkopotamus]

The std::slice::Vector trait effectively provides an interface for saying an object can give a slice view in to itself. Both Option<T> and CVec<T> expose as_slice but do so by implementing them directly rather than implementing them via the Vector trait.

This patch alters both Option<T> and CVec<T> to instead expose it by implementing Vector.

[lilyball]

I'm not convinced it really makes sense to think of Option as a vector, but I guess it does already have the method.

[donkopotamus]

I think, in truth, that the Vector trait is misnamed. Really it is a trait that says "you can get a readable array view into me".

If I can take it further ... currently it feels like there is an inconsistency in std::slice between the vector-like traits. There is:

• Vector - an object can provide a slice view into itself (as_slice).
• ImmutableVector - an object provides methods for slicing and dicing by integral index (and it doesn't inherit from Vector btw).
• MutableVector - an object provides both a slice view into itself (as_mut_slice) and methods for directly manipulating that object.

In practice, this means I can write a generic function fn needs_slice<T: Vector>(x: &T) that manipulates x only via its as_slice representation. And I can easily declare that some new funky object can masquerade as a slice by simply implementing a single method. Good.

In contrast, I can also write a function fn needs_mut_slice<T: MutableVector>(x: &T) that manipulates x only via its slice representation as_mut_slice. But I can't declare that some new funky object can masquerade as a mutable slice without adding a bunch of other methods that I don't actually want exposed on that object ... when all I really want to do is implement just as_mut_slice. Less Good.

So it feels like there should be four traits:

• SliceAccessor - an object can provide a slice view into itself (as_slice) (the existing Vector)
• MutableSliceAccessor - an object can provide a mutable slice view into itself (as_mut_slice) (NEW)
• ImmutableVector - an object provides a large number of methods for slicing and dicing itself.
• MutableVector - an object provides methods for directly manipulating the object.

Sorry, all a bit long ...

[lilyball]

ImmutableVector and MutableVector exist only because libcore cannot provide an impl on &[T] directly. This wasn't some grand plan for documenting functionality with traits.

[donkopotamus]

@kballard Sure ... I just think that it might still prove useful if as_mut_slice was split out of MutableVector so it would be possible to write functions that would operate on anything that can provide a writable slice view in to themselves (happy to implement that)

Not very important!

[huonw]

FWIW, the Vector names are just hold-overs from history, they should all theoretically be called ...Slice... now.

[bluss]

So what do you think about implementing .as_slice() for all homogenous slices (T, ) and (T, T) and (T, T, T) .. etc?

[lilyball]

@bluss AFAIK tuples don't guarantee that they are laid out in memory the same way an array would be, which would be necessary in order to turn them into slices.

[huonw]

So what do you think about implementing .as_slice() for all homogenous slices (T, ) and (T, T) and (T, T, T) .. etc?

The compiler actually already does this for it's own internal use.