Surprising type inference failure around "as"

[RalfJung]

The following code works fine:

fn test(x: usize) {
    let v: [u32; 4] = [[0, 1, 2, 3], [4, 5, 6, 7]][x];
}

Hence I would expect that this also builds:

fn test(x: usize) {
    let v = [[0, 1, 2, 3], [4, 5, 6, 7]][x] as [u32; 4];
}

However, it does not:

error[[E0605]](https://doc.rust-lang.org/stable/error-index.html#E0605): non-primitive cast: `[i32; 4]` as `[u32; 4]`
 --> src/lib.rs:2:13
  |
2 |     let v = [[0, 1, 2, 3], [4, 5, 6, 7]][x] as [u32; 4];
  |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ an `as` expression can only be used to convert between primitive types or to coerce to a specific trait object

For more information about this error, try `rustc --explain E0605`.
error: could not compile `playground` due to previous error

Looks like it inferred the integers to be i32. Proper use of the type information provided by as should enable it to infer u32 instead. This already happens successfully in simpler cases such as [0, 1, 2, 3] as [u32; 4].