Macro out_field

macro_rules! out_field {
    ($out:ident.$field:tt) => { ... };
}

Returns a tuple of (field_pos, field_out), where field_pos is the “position”, i.e. offset in bytes, of the field relative to the base address of the struct and field_out is a *mut that points to the field directly.

This is essentially a convenience wrapper around core::ptr::addr_of_mut! that also gives back the relative offset of the field, as these are often needed together. You will often see the return values named (fp, fo) in internal use of this macro, which stand for field_pos and field_out respectively as discussed above.

Example

use core::mem::MaybeUninit;
use rkyv::out_field;

// The macro works on repr(Rust) structs as well, but for the purposes of asserting
// an exact guaranteed position of each field in this example, we'll use repr(C)
#[repr(C)]
struct Example {
    a: i32,
    b: bool,
}

let mut result = MaybeUninit::<Example>::zeroed();
let out = result.as_mut_ptr();

let (a_pos, a_out) = out_field!(out.a);
assert_eq!(a_pos, 0); // guaranteed by repr(C) layout, repr(Rust) has free reign
unsafe { a_out.write(42); }

let (b_pos, b_out) = out_field!(out.b);
assert_eq!(b_pos, 4); // guaranteed by repr(C) layout, repr(Rust) has free reign
unsafe { b_out.write(true); }

let result = unsafe { result.assume_init() };
assert_eq!(result.a, 42);
assert_eq!(result.b, true);