Support closures as array initializer

Currently the way to initialize an array is a bit unwieldy:

use std::mem;

#[derive(Debug)]
struct Codebook<T> {
    x: T,
    y: T
}

fn fill_thing_array(a : &mut [[Codebook<u8>; 4]; 4]) {
    for (i, line) in a.iter_mut().enumerate() {
        for (j, el) in line.iter_mut().enumerate() {
            *el = Codebook { x: i as u8, y: j as u8};
        }
    }
}

... 

    let mut a = unsafe { mem::uninitialized() };
    fill_thing_array(&mut a);
}

Would be nicer to provide a syntax to have something along the lines of

let a = [[|x, y| [Codebook {x: i as u8, y: j as u8}]; 4]; 4];

Start with the issue's current and proposed Rust snippets, then research existing array-initialization and closure syntax to define the intended semantics. No files or tests are named; done would require a settled, reviewed design for closure-based array initialization and the corresponding RFC decision.