mirror of
https://gitee.com/openharmony/third_party_rust_memoffset
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86 lines
2.6 KiB
Markdown
86 lines
2.6 KiB
Markdown
# memoffset #
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[](https://crates.io/crates/memoffset)
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C-Like `offset_of` functionality for Rust structs.
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Introduces the following macros:
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* `offset_of!` for obtaining the offset of a member of a struct.
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* `offset_of_tuple!` for obtaining the offset of a member of a tuple. (Requires Rust 1.20+)
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* `offset_of_union!` for obtaining the offset of a member of a union.
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* `span_of!` for obtaining the range that a field, or fields, span.
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`memoffset` works under `no_std` environments.
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## Usage ##
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Add the following dependency to your `Cargo.toml`:
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```toml
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[dependencies]
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memoffset = "0.8"
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```
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These versions will compile fine with rustc versions greater or equal to 1.19.
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## Examples ##
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```rust
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use memoffset::{offset_of, span_of};
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#[repr(C, packed)]
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struct Foo {
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a: u32,
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b: u32,
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c: [u8; 5],
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d: u32,
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}
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fn main() {
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assert_eq!(offset_of!(Foo, b), 4);
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assert_eq!(offset_of!(Foo, d), 4+4+5);
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assert_eq!(span_of!(Foo, a), 0..4);
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assert_eq!(span_of!(Foo, a .. c), 0..8);
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assert_eq!(span_of!(Foo, a ..= c), 0..13);
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assert_eq!(span_of!(Foo, ..= d), 0..17);
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assert_eq!(span_of!(Foo, b ..), 4..17);
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}
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```
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## Usage in constants ##
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`memoffset` has support for compile-time `offset_of!` on rust>=1.65, or on older nightly compilers.
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### Usage on stable Rust ###
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Constant evaluation is automatically enabled and avilable on stable compilers starting with rustc 1.65.
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This is an incomplete implementation with one caveat:
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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.
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This means that if need to get the offset of a cell, you'll have to remain on nightly for now.
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### Usage on recent nightlies ###
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If you're using a new-enough nightly and you require the ability to get the offset of a `Cell`,
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you'll have to enable the `unstable_const` cargo feature, as well as enabling `const_refs_to_cell` in your crate root.
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Do note that `unstable_const` is an unstable feature that is set to be removed in a future version of `memoffset`.
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Cargo.toml:
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```toml
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[dependencies.memoffset]
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version = "0.8"
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features = ["unstable_const"]
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```
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Your crate root: (`lib.rs`/`main.rs`)
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```rust,ignore
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#![feature(const_refs_to_cell)]
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```
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### Usage on older nightlies ###
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In order to use it on an older nightly compiler, you must enable the `unstable_const` crate feature and several compiler features.
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Your crate root: (`lib.rs`/`main.rs`)
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```rust,ignore
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#![feature(const_ptr_offset_from, const_refs_to_cell)]
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```
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