Code shared from the Rust Playground

playground.rs

fn check_safe_transmute<T: Copy>(buf: &[u8]) {
    if buf.len() < ::std::mem::size_of::<T>() {
        panic!("Slice has insufficient capacity!");
    }
    let p = buf.as_ptr() as usize;
    let align_of = ::std::mem::align_of::<T>();
    if align_of > 0 && (p & (align_of - 1)) != 0 {
        panic!("Slice has bad alignment!");
    }
}

trait ViewExt {
    fn view_as<T>(&self) -> &T where T: Copy;
    
    fn view_mut<T>(&mut self) -> &mut T where T: Copy;
}

impl ViewExt for [u8] {
    fn view_as<T>(&self) -> &T where T: Copy {
        check_safe_transmute::<T>(self);
        unsafe { 
            &*::std::mem::transmute::<*const u8, *const T>(self.as_ptr())
        }
    }
    
    fn view_mut<T>(&mut self) -> &mut T where T: Copy {
        check_safe_transmute::<T>(self);
        unsafe { 
            &mut *::std::mem::transmute::<*mut u8, *mut T>(self.as_mut_ptr())
        }
    }
}

#[repr(packed)]
#[derive(Clone, Copy, Debug)]
struct Ipv4Addr {
    addr: [u8; 4],
}

fn main() {
    let mut buf = [0; 8];
    buf[0] = 1;
    
    {
        let addr = buf.view_mut::<Ipv4Addr>();
        assert!(addr.addr[0] == 1);
        addr.addr[1] = 2;
        println!("{:?}", addr);
    }
    
    assert!(buf[1] == 2);
    println!("{:?}", buf);
}