The Rust RFC Book

• Start Date: 2014-06-09
• RFC PR: rust-lang/rfcs#111
• Rust Issue: rust-lang/rust#6515

摘要

Index should be split into Index and IndexMut.

動機

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.

詳細設計

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.

替代方案

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

未解決的問題

None that I’m aware of.