Cannot do `x as Box<[T]>` for `x: Box<[T; k]>`

[pnkfelix]

This explicit cast does not work:

fn main() {
    let x = Box::new([1, 2, 3]);
    let y = x as Box<[i32]>;

    println!("y: {:?}", y);
}

However, this implicit coercion does work:

fn main() {
    let x = Box::new([1, 2, 3]);
    let y: Box<[i32]> = x;

    println!("y: {:?}", y);
}

It is unfortunate, because I would like the overloaded-box inference to work for expr as Box<[T]> in the same manner that it does for expr as Box<Trait>.

[pnkfelix]

cc #22181