C-Like offset_of functionality for Rust structs.

Introduces the following macros:

• offset_of! for obtaining the offset of a member of a struct.
• offset_of_tuple! for obtaining the offset of a member of a tuple. (Requires Rust 1.20+)
• offset_of_union! for obtaining the offset of a member of a union.
• span_of! for obtaining the range that a field, or fields, span.

memoffset works under no_std environments.

If you're using a rustc version greater or equal to 1.77, this crate's offset_of!() macro simply forwards to core::mem::offset_of!().

Add the following dependency to your Cargo.toml:

[dependencies]
memoffset = "0.9"

These versions will compile fine with rustc versions greater or equal to 1.19.

use memoffset::{offset_of, span_of};

#[repr(C, packed)]
struct Foo {
    a: u32,
    b: u32,
    c: [u8; 5],
    d: u32,
}

fn main() {
    assert_eq!(offset_of!(Foo, b), 4);
    assert_eq!(offset_of!(Foo, d), 4+4+5);

    assert_eq!(span_of!(Foo, a),        0..4);
    assert_eq!(span_of!(Foo, a ..  c),  0..8);
    assert_eq!(span_of!(Foo, a ..= c),  0..13);
    assert_eq!(span_of!(Foo, ..= d),    0..17);
    assert_eq!(span_of!(Foo, b ..),     4..17);
}

memoffset has support for compile-time offset_of! on rust>=1.65.

On versions below 1.77, this is an incomplete implementation with one caveat: Due to dependence on #![feature(const_refs_to_cell)], you cannot get the offset of a Cell field in a const-context.