Part of the code is:

    unsafe {
        std::ptr::write(stack_ptr.offset(SSIZE - 16) as *mut u64, hello as u64);
        ctx.rsp = stack_ptr.offset(SSIZE - 16) as u64;
        gt_switch(&mut ctx, &mut n);
    }

Only the last line should be unsafe — the first line appears to be writing in bounds to a Vec, which is easy (and much more readable) in safe Rust.

Linux used to do its actual context switch in C with inline asm, and it changed to being in straight asm a while back. This was absolutely a win. Rather than trying to make everything work out behind the compiler’s back, it’s much more straightforward to write a function in asm that does the stack switch.