Rust: Using two u8 struct fields as a u16 - struct

Question

For a Gameboy emulator, you have two u8 fields for registers A and F, but they can sometimes be accessed as AF, a combined u16 register.
In C, it looks like you can do something like this:
struct {
union {
struct {
unsigned char f;
unsigned char a;
};
unsigned short af;
};
};
(Taken from here)
Is there a way in Rust, ideally without unsafe, of being able to access two u8s as registers.a/registers.f, but also be able to use them as the u16 registers.af?

Answer 1

I can give you a couple of ways to do it. First is a straightforward unsafe analogue but without boilerplate, the second one is safe but explicit.
Unions in rust are very similar, so you can translate it to this:
#[repr(C)]
struct Inner {
f: u8,
a: u8,
}
#[repr(C)]
union S {
inner: Inner,
af: u16,
}
// Usage:
// Putting data is safe:
let s = S { af: 12345 };
// but retrieving is not:
let a = unsafe { s.inner.a };
Or as an alternative you may manually do all of the explicit casts wrapped in a structure:
#[repr(transparent)]
// This is optional actually but allows a chaining,
// you may remove these derives and change method
// signatures to `&self` and `&mut self`.
#[derive(Clone, Copy)]
struct T(u16);
impl T {
pub fn from_af(af: u16) -> Self {
Self(af)
}
pub fn from_a_f(a: u8, f: u8) -> Self {
Self::from_af(u16::from_le_bytes([a, f]))
}
pub fn af(self) -> u16 {
self.0
}
pub fn f(self) -> u8 {
self.0.to_le_bytes()[0]
}
pub fn set_f(self, f: u8) -> Self {
Self::from_a_f(self.a(), f)
}
pub fn a(self) -> u8 {
self.0.to_le_bytes()[1]
}
pub fn set_a(self, a: u8) -> Self {
Self::from_a_f(a, self.f())
}
}
// Usage:
let t = T::from_af(12345);
let a = t.a();
let new_af = t.set_a(12).set_f(t.f() + 1).af();