Confusing error when using unsized types without indirection

``` rust
fn foo(arg: Foo) {} //Foo is a trait
fn bar (arg: [i32]) {}
fn quz (arg: str) {}
```

All those functions return the following error: **the trait `core::marker::Sized` is not implemented for the type...**
This is confusing for a beginner, because it makes you think you need to implement a trait to make it work. The message could suggest using `&Foo/&[i32]/&str` instead.
