Error code E0620

A cast to an unsized type was attempted.

Erroneous code example:

#![allow(unused)]
fn main() {
let x = &[1_usize, 2] as [usize]; // error: cast to unsized type: `&[usize; 2]`
                                  //        as `[usize]`
}

In Rust, some types don't have a known size at compile-time. For example, in a slice type like [u32], the number of elements is not known at compile-time and hence the overall size cannot be computed. As a result, such types can only be manipulated through a reference (e.g., &T or &mut T) or other pointer-type (e.g., Box or Rc). Try casting to a reference instead:

#![allow(unused)]
fn main() {
let x = &[1_usize, 2] as &[usize]; // ok!
}