2.6 KiB
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.
Your crate root: (lib.rs/main.rs)
#![feature(const_ptr_offset_from, const_refs_to_cell)]