String/str should not support sliced indexing

[tomjakubowski]

A common question from Rust newbies is why String/str don't allow simple indexing, like this:

let foo = "foobar";
let x = foo[0];

The answer has always been that foo[0] is both ambiguous and dangerous given that str is a UTF-8 string: it could be a byte offset returning a char, a byte offset returning a u8, a codepoint index returning a char, etc., each having its own potential pitfalls of panicking or slowness.

For the same reason that foo[0] is not allowed, foo[m..n] and foo[..n] and similar such "sliced" indexing operations shouldn't be allowed.

(It's also a bit strange that both String and str impl these traits; since String is Deref<str>, shouldn't it only be needed on str?)

[steveklabnik]

Please open issues like these on the RFC repo instead, changes like this go through the RFC process.