# memoffset #

[![](https://img.shields.io/crates/v/memoffset.svg)](https://crates.io/crates/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.

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!()`.

## Usage ##
Add the following dependency to your `Cargo.toml`:

```toml
[dependencies]
memoffset = "0.9"
```

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

## Examples ##
```rust
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.

On versions below 1.77, this is an incomplete implementation with one caveat:
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.