gecko-dev/third_party/rust/memoffset/README.md

# memoffset #

[![](http://meritbadge.herokuapp.com/memoffset)](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.
 * `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`:

```toml
[dependencies]
memoffset = "0.3"
```

Versions ">= 0.3" can be used in a constant expression context (though not in a `const fn`),
but require a rust version greater than or equal to 1.33.
These versions will compile fine with rustc versions greater or equal to 1.19, but will
lack support for constant expression.

If you wish to use an older rustc version, lock your dependency to "0.2"

Add the following lines at the top of your `main.rs` or `lib.rs` files.

```rust
#[macro_use]
extern crate memoffset;
```

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

assert_eq!(offset_of!(Foo, b), 4);
assert_eq!(offset_of!(Foo, c[3]), 11);

assert_eq!(span_of!(Foo, a),          0..4);
assert_eq!(span_of!(Foo, a ..  c),    0..8);
assert_eq!(span_of!(Foo, a ..  c[1]), 0..9);
assert_eq!(span_of!(Foo, a ..= c[1]), 0..10);
assert_eq!(span_of!(Foo, ..= d),      0..14);
assert_eq!(span_of!(Foo, b ..),       4..17);
```