memoffset

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.

Usage

Add the following dependency to your Cargo.toml:

[dependencies]
memoffset = "0.8"

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

Examples

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);
}

Usage in constants

memoffset has support for compile-time offset_of! on rust>=1.65, or on older nightly compilers.

Usage on stable Rust

Constant evaluation is automatically enabled and avilable on stable compilers starting with rustc 1.65.

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.

This means that if need to get the offset of a cell, you'll have to remain on nightly for now.

Usage on recent nightlies

If you're using a new-enough nightly and you require the ability to get the offset of a Cell, you'll have to enable the unstable_const cargo feature, as well as enabling const_refs_to_cell in your crate root.

Do note that unstable_const is an unstable feature that is set to be removed in a future version of memoffset.

Cargo.toml:

[dependencies.memoffset]
version = "0.8"
features = ["unstable_const"]

Your crate root: (lib.rs/main.rs)

#![feature(const_refs_to_cell)]

Usage on older nightlies

In order to use it on an older nightly compiler, you must enable the unstable_const crate feature and several compiler features.