std::str::StrSlice needs documentation

[luisbg]

Reporting the issue here to assign it to myself and soon push a Pull Request against it.

[luisbg]

This is a bug in the language.

There are two rules on implementing a trait. Either one of the rules must be met, always. Or both. Never none.

1. The trait must be defined in your own crate
2. The type you're implementing the trait for must be in your own crate

This is so that you can always combine any two crates and not get conflicting implementations (coherency).
libstd has a special position in that it defines all of the methods on the builtins, but it doesn't define the types

You can't just say "impl int", or "impl ~str", you need to have a "dummy" trait like StrSlice
So that you meet one of the rules, specifically rule 1

The thing to fix is to be able to add an attribute that says "pretend that this crate defines this builtin type so I can give it methods directly", a 3rd rule to the coherency rules that only applies ot builtins

[luisbg]

So the issue reported is something completely different and out of scope. Closing this issue then.