Implement IzZero for zeroable default types

[Pzixel]

As discussed in #54628 there are some critical optimizations based on fact that type default value can be represented with mem::zeroed(), such as this vec allocation code.

There is a trait describing this behavior. I propose implementing it for some standard types, such as std::num::Wrapping or Option, or even making it public.

[jonas-schievink]

That trait is only used by internal Windows things. It probably does not have the semantics you're looking for.

[jonas-schievink]

Ah, did you mean to link this trait instead?

rust/src/liballoc/vec.rs

Lines 1594 to 1597 in 372be4f

	unsafe trait IsZero {
	/// Whether this value is zero
	fn is_zero(&self) -> bool;
	}

[Pzixel]

@jonas-schievink yes, I linked the wrong trait, fixed. Thanks.

[hellow554]

Is Option::None really always 0? I doubt that, nor have I examples for that :)

[sinkuu]

cc #58628 (comment)

[the8472]

Is Option::None really always 0? I doubt that, nor have I examples for that :)

let foo : Option<bool> = Option::None;
println!("size:{} val:{:08b}",
          std::mem::size_of::<Option<bool>>(),
          unsafe {std::mem::transmute::<Option<bool>, u8>(foo)});
// size:1 val:00000010