1# memoffset #
2
3[![](https://img.shields.io/crates/v/memoffset.svg)](https://crates.io/crates/memoffset)
4
5C-Like `offset_of` functionality for Rust structs.
6
7Introduces the following macros:
8 * `offset_of!` for obtaining the offset of a member of a struct.
9 * `offset_of_tuple!` for obtaining the offset of a member of a tuple. (Requires Rust 1.20+)
10 * `offset_of_union!` for obtaining the offset of a member of a union.
11 * `span_of!` for obtaining the range that a field, or fields, span.
12
13`memoffset` works under `no_std` environments.
14
15## Usage ##
16Add the following dependency to your `Cargo.toml`:
17
18```toml
19[dependencies]
20memoffset = "0.8"
21```
22
23These versions will compile fine with rustc versions greater or equal to 1.19.
24
25## Examples ##
26```rust
27use memoffset::{offset_of, span_of};
28
29#[repr(C, packed)]
30struct Foo {
31    a: u32,
32    b: u32,
33    c: [u8; 5],
34    d: u32,
35}
36
37fn main() {
38    assert_eq!(offset_of!(Foo, b), 4);
39    assert_eq!(offset_of!(Foo, d), 4+4+5);
40
41    assert_eq!(span_of!(Foo, a),        0..4);
42    assert_eq!(span_of!(Foo, a ..  c),  0..8);
43    assert_eq!(span_of!(Foo, a ..= c),  0..13);
44    assert_eq!(span_of!(Foo, ..= d),    0..17);
45    assert_eq!(span_of!(Foo, b ..),     4..17);
46}
47```
48
49## Usage in constants ##
50`memoffset` has support for compile-time `offset_of!` on rust>=1.65, or on older nightly compilers.
51
52### Usage on stable Rust ###
53Constant evaluation is automatically enabled and avilable on stable compilers starting with rustc 1.65.
54
55This is an incomplete implementation with one caveat:
56Due to dependence on [`#![feature(const_refs_to_cell)]`](https://github.com/rust-lang/rust/issues/80384), you cannot get the offset of a `Cell` field in a const-context.
57
58This means that if need to get the offset of a cell, you'll have to remain on nightly for now.
59
60### Usage on recent nightlies ###
61
62If you're using a new-enough nightly and you require the ability to get the offset of a `Cell`,
63you'll have to enable the `unstable_const` cargo feature, as well as enabling `const_refs_to_cell` in your crate root.
64
65Do note that `unstable_const` is an unstable feature that is set to be removed in a future version of `memoffset`.
66
67Cargo.toml:
68```toml
69[dependencies.memoffset]
70version = "0.8"
71features = ["unstable_const"]
72```
73
74Your crate root: (`lib.rs`/`main.rs`)
75```rust,ignore
76#![feature(const_refs_to_cell)]
77```
78
79### Usage on older nightlies ###
80In order to use it on an older nightly compiler, you must enable the `unstable_const` crate feature and several compiler features.
81
82Your crate root: (`lib.rs`/`main.rs`)
83```rust,ignore
84#![feature(const_ptr_offset_from, const_refs_to_cell)]
85```
86