Shared via Rust Playground

playground.rs

use std::borrow::Cow;
use std::mem;
use std::ptr;
use std::slice;

#[derive(Clone)]
struct S {
    pub x: u32,
}

fn get_s(bytes: &[u8]) -> Cow<S> {
    if bytes.len() < mem::size_of::<S>() {
        println!("too small");
        Cow::Owned(S { x: 0 })
    } else {
        let ptr = bytes.as_ptr();
        println!("ptr: {:?}", ptr);
        if (ptr as usize) % mem::align_of::<S>() == 0 {
            println!("aligned: transmuting");
            Cow::Borrowed(unsafe { mem::transmute(ptr) })
        } else {
            println!("unaligned: copying");
            unsafe {
                Cow::Owned(ptr::read_unaligned(mem::transmute(ptr)))
            }
        }
    }
}

fn main() {
    let s = get_s(&[]);
    println!("{}", s.x);
    let b = &[1, 0, 0, 0];
    let s = get_s(b);
    println!("{}", s.x);
    let sb = S { x: 2 };
    let b: &[u8] = unsafe { slice::from_raw_parts(mem::transmute(&sb), mem::size_of::<S>()) };
    let s = get_s(b);
    println!("{}", s.x);
}