Stack

Whenever we hold a value (includes function arguments) we must know its size, so that the stack pointer can be moved properly.

#![allow(unused)]
fn main() {
struct S {
    data: [u8; 0x1000],
}
println!("0x{:x}", std::mem::size_of::<S>());
}

Knowing the size of a value of a type is indicated by the marker trait Sized.

fn f<T: Sized>(t: T) {}