#![feature(asm)]
pub struct Task {
    sp: *mut (),
}

pub unsafe fn switch(next: &Task, prev: &mut Task) { /* &mut is probably invalid in real code and would need to be *mut or something? */
    asm!("mov $0, rsp; mov rsp, $1": "=*m"(&mut prev.sp) : "*m"(&next.sp) :
        "rcx", "rbx", "rdx", "r8", "r9", "r10", "r11",
	  "r12", "r13", "r14", "r15" : "intel", "volatile")
}