Add T: ?Sized to mem::align_of

[joshlf]

mem::align_of currently requires that T: Sized. This bound is unnecessary, and prevents align_of from being used in generic code that operates on unsized types. mem::align_of_val is available, but only works when an instance of T is present to be passed.

I propose that we make align_of specify where T: ?Sized.

[kennytm]

No this bound is still necessary until we've sorted out the custom DST traits.

use std::fmt::Debug;
use std::mem::*;

fn main() {
    let a: &dyn Debug = &1u32;
    let b: &dyn Debug = &1u64;
    println!("{}", align_of_val(a)); // 4
    println!("{}", align_of_val(b)); // 8
    // println!("{}", align_of::<dyn Debug>());  // ???
}

[joshlf]

Poop.