1 # memoffset #
2 
3 [![](https://img.shields.io/crates/v/memoffset.svg)](https://crates.io/crates/memoffset)
4 
5 C-Like `offset_of` functionality for Rust structs.
6 
7 Introduces 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 ##
16 Add the following dependency to your `Cargo.toml`:
17 
18 ```toml
19 [dependencies]
20 memoffset = "0.8"
21 ```
22 
23 These versions will compile fine with rustc versions greater or equal to 1.19.
24 
25 ## Examples ##
26 ```rust
27 use memoffset::{offset_of, span_of};
28 
29 #[repr(C, packed)]
30 struct Foo {
31     a: u32,
32     b: u32,
33     c: [u8; 5],
34     d: u32,
35 }
36 
37 fn 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 ###
53 Constant evaluation is automatically enabled and avilable on stable compilers starting with rustc 1.65.
54 
55 This is an incomplete implementation with one caveat:
56 Due 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 
58 This 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 
62 If you're using a new-enough nightly and you require the ability to get the offset of a `Cell`,
63 you'll have to enable the `unstable_const` cargo feature, as well as enabling `const_refs_to_cell` in your crate root.
64 
65 Do note that `unstable_const` is an unstable feature that is set to be removed in a future version of `memoffset`.
66 
67 Cargo.toml:
68 ```toml
69 [dependencies.memoffset]
70 version = "0.8"
71 features = ["unstable_const"]
72 ```
73 
74 Your crate root: (`lib.rs`/`main.rs`)
75 ```rust,ignore
76 #![feature(const_refs_to_cell)]
77 ```
78 
79 ### Usage on older nightlies ###
80 In order to use it on an older nightly compiler, you must enable the `unstable_const` crate feature and several compiler features.
81 
82 Your crate root: (`lib.rs`/`main.rs`)
83 ```rust,ignore
84 #![feature(const_ptr_offset_from, const_refs_to_cell)]
85 ```
86