Summary

Index should be split into Index and IndexMut.

Motivation

Currently, the Index trait is not suitable for most array indexing tasks. The slice functionality cannot be replicated using it, and as a result the new Vec has to use .get() and .get_mut() methods.

Additionally, this simply follows the Deref/DerefMut split that has been implemented for a while.

Detailed design

We split Index into two traits (borrowed from @nikomatsakis):

// self[element] -- if used as rvalue, implicitly a deref of the result
trait Index<E,R> {
    fn index<'a>(&'a self, element: &E) -> &'a R;
}

// &mut self[element] -- when used as a mutable lvalue
trait IndexMut<E,R> {
    fn index_mut<'a>(&'a mut self, element: &E) -> &'a mut R;
}

Drawbacks

The number of lang. items increases.
This design doesn’t support moving out of a vector-like object. This can be added backwards compatibly.
This design doesn’t support hash tables because there is no assignment operator. This can be added backwards compatibly.

Alternatives

The impact of not doing this is that the [] notation will not be available to Vec.

Unresolved questions