RFC for index traits

active/0034-index-traits.md

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+- Start Date: 2014-06-09
+- RFC PR #: #111
+- Rust Issue #: #6515
+
+# 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
+
+None that I'm aware of.