| <!DOCTYPE html><html lang="en"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><meta name="generator" content="rustdoc"><meta name="description" content="Source of the Rust file `/root/.cargo/registry/src/github.com-1ecc6299db9ec823/bytemuck-1.13.1/src/offset_of.rs`."><meta name="keywords" content="rust, rustlang, rust-lang"><title>offset_of.rs - source</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceSerif4-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../FiraSans-Regular.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../FiraSans-Medium.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceCodePro-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceSerif4-Bold.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../SourceCodePro-Semibold.ttf.woff2"><link rel="stylesheet" href="../../normalize.css"><link rel="stylesheet" href="../../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" href="../../ayu.css" disabled><link rel="stylesheet" href="../../dark.css" disabled><link rel="stylesheet" href="../../light.css" id="themeStyle"><script id="default-settings" ></script><script src="../../storage.js"></script><script defer src="../../source-script.js"></script><script defer src="../../source-files.js"></script><script defer src="../../main.js"></script><noscript><link rel="stylesheet" href="../../noscript.css"></noscript><link rel="alternate icon" type="image/png" href="../../favicon-16x16.png"><link rel="alternate icon" type="image/png" href="../../favicon-32x32.png"><link rel="icon" type="image/svg+xml" href="../../favicon.svg"></head><body class="rustdoc source"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="sidebar"><a class="sidebar-logo" href="../../bytemuck/index.html"><div class="logo-container"><img class="rust-logo" src="../../rust-logo.svg" alt="logo"></div></a></nav><main><div class="width-limiter"><nav class="sub"><a class="sub-logo-container" href="../../bytemuck/index.html"><img class="rust-logo" src="../../rust-logo.svg" alt="logo"></a><form class="search-form"><div class="search-container"><span></span><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><div id="help-button" title="help" tabindex="-1"><a href="../../help.html">?</a></div><div id="settings-menu" tabindex="-1"><a href="../../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../../wheel.svg"></a></div></div></form></nav><section id="main-content" class="content"><div class="example-wrap"><pre class="src-line-numbers"><span id="1">1</span> |
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| </pre><pre class="rust"><code><span class="attribute">#![forbid(unsafe_code)] |
| |
| </span><span class="doccomment">/// Find the offset in bytes of the given `$field` of `$Type`. Requires an |
| /// already initialized `$instance` value to work with. |
| /// |
| /// This is similar to the macro from [`memoffset`](https://docs.rs/memoffset), |
| /// however it uses no `unsafe` code. |
| /// |
| /// This macro has a 3-argument and 2-argument version. |
| /// * In the 3-arg version you specify an instance of the type, the type itself, |
| /// and the field name. |
| /// * In the 2-arg version the macro will call the [`default`](Default::default) |
| /// method to make a temporary instance of the type for you. |
| /// |
| /// The output of this macro is the byte offset of the field (as a `usize`). The |
| /// calculations of the macro are fixed across the entire program, but if the |
| /// type used is `repr(Rust)` then they're *not* fixed across compilations or |
| /// compilers. |
| /// |
| /// ## Examples |
| /// |
| /// ### 3-arg Usage |
| /// |
| /// ```rust |
| /// # use bytemuck::offset_of; |
| /// // enums can't derive default, and for this example we don't pick one |
| /// enum MyExampleEnum { |
| /// A, |
| /// B, |
| /// C, |
| /// } |
| /// |
| /// // so now our struct here doesn't have Default |
| /// #[repr(C)] |
| /// struct MyNotDefaultType { |
| /// pub counter: i32, |
| /// pub some_field: MyExampleEnum, |
| /// } |
| /// |
| /// // but we provide an instance of the type and it's all good. |
| /// let val = MyNotDefaultType { counter: 5, some_field: MyExampleEnum::A }; |
| /// assert_eq!(offset_of!(val, MyNotDefaultType, some_field), 4); |
| /// ``` |
| /// |
| /// ### 2-arg Usage |
| /// |
| /// ```rust |
| /// # use bytemuck::offset_of; |
| /// #[derive(Default)] |
| /// #[repr(C)] |
| /// struct Vertex { |
| /// pub loc: [f32; 3], |
| /// pub color: [f32; 3], |
| /// } |
| /// // if the type impls Default the macro can make its own default instance. |
| /// assert_eq!(offset_of!(Vertex, loc), 0); |
| /// assert_eq!(offset_of!(Vertex, color), 12); |
| /// ``` |
| /// |
| /// # Usage with `#[repr(packed)]` structs |
| /// |
| /// Attempting to compute the offset of a `#[repr(packed)]` struct with |
| /// `bytemuck::offset_of!` requires an `unsafe` block. We hope to relax this in |
| /// the future, but currently it is required to work around a soundness hole in |
| /// Rust (See [rust-lang/rust#27060]). |
| /// |
| /// [rust-lang/rust#27060]: https://github.com/rust-lang/rust/issues/27060 |
| /// |
| /// <p style="background:rgba(255,181,77,0.16);padding:0.75em;"> |
| /// <strong>Warning:</strong> This is only true for versions of bytemuck > |
| /// 1.4.0. Previous versions of |
| /// <code style="background:rgba(41,24,0,0.1);">bytemuck::offset_of!</code> |
| /// will only emit a warning when used on the field of a packed struct in safe |
| /// code, which can lead to unsoundness. |
| /// </p> |
| /// |
| /// For example, the following will fail to compile: |
| /// |
| /// ```compile_fail |
| /// #[repr(C, packed)] |
| /// #[derive(Default)] |
| /// struct Example { |
| /// field: u32, |
| /// } |
| /// // Doesn't compile: |
| /// let _offset = bytemuck::offset_of!(Example, field); |
| /// ``` |
| /// |
| /// While the error message this generates will mention the |
| /// `safe_packed_borrows` lint, the macro will still fail to compile even if |
| /// that lint is `#[allow]`ed: |
| /// |
| /// ```compile_fail |
| /// # #[repr(C, packed)] #[derive(Default)] struct Example { field: u32 } |
| /// // Still doesn't compile: |
| /// #[allow(safe_packed_borrows)] |
| /// { |
| /// let _offset = bytemuck::offset_of!(Example, field); |
| /// } |
| /// ``` |
| /// |
| /// This *can* be worked around by using `unsafe`, but it is only sound to do so |
| /// if you can guarantee that taking a reference to the field is sound. |
| /// |
| /// In practice, this means it only works for fields of align(1) types, or if |
| /// you know the field's offset in advance (defeating the point of `offset_of`) |
| /// and can prove that the struct's alignment and the field's offset are enough |
| /// to prove the field's alignment. |
| /// |
| /// Once the `raw_ref` macros are available, a future version of this crate will |
| /// use them to lift the limitations of packed structs. For the duration of the |
| /// `1.x` version of this crate that will be behind an on-by-default cargo |
| /// feature (to maintain minimum rust version support). |
| </span><span class="attribute">#[macro_export] |
| </span><span class="macro">macro_rules! </span>offset_of { |
| (<span class="macro-nonterminal">$instance</span>:expr, <span class="macro-nonterminal">$Type</span>:path, <span class="macro-nonterminal">$field</span>:tt) => {{ |
| <span class="attribute">#[forbid(safe_packed_borrows)] |
| </span>{ |
| <span class="comment">// This helps us guard against field access going through a Deref impl. |
| </span><span class="attribute">#[allow(clippy::unneeded_field_pattern)] |
| </span><span class="kw">let </span><span class="macro-nonterminal">$Type </span>{ <span class="macro-nonterminal">$field</span>: <span class="kw">_</span>, .. }; |
| <span class="kw">let </span>reference: <span class="kw-2">&</span><span class="macro-nonterminal">$Type </span>= <span class="kw-2">&</span><span class="macro-nonterminal">$instance</span>; |
| <span class="kw">let </span>address = reference <span class="kw">as </span><span class="kw-2">*const </span><span class="kw">_ as </span>usize; |
| <span class="kw">let </span>field_pointer = <span class="kw-2">&</span>reference.<span class="macro-nonterminal">$field </span><span class="kw">as </span><span class="kw-2">*const </span><span class="kw">_ as </span>usize; |
| <span class="comment">// These asserts/unwraps are compiled away at release, and defend against |
| // the case where somehow a deref impl is still invoked. |
| </span><span class="kw">let </span>result = field_pointer.checked_sub(address).unwrap(); |
| <span class="macro">assert!</span>(result <= <span class="macro-nonterminal">$crate::__core::mem::size_of</span>::<<span class="macro-nonterminal">$Type</span>>()); |
| result |
| } |
| }}; |
| (<span class="macro-nonterminal">$Type</span>:path, <span class="macro-nonterminal">$field</span>:tt) => {{ |
| <span class="macro-nonterminal">$</span><span class="macro">crate::offset_of!</span>(<<span class="macro-nonterminal">$Type </span><span class="kw">as </span>Default>::default(), <span class="macro-nonterminal">$Type</span>, <span class="macro-nonterminal">$field</span>) |
| }}; |
| } |
| </code></pre></div> |
| </section></div></main><div id="rustdoc-vars" data-root-path="../../" data-current-crate="bytemuck" data-themes="ayu,dark,light" data-resource-suffix="" data-rustdoc-version="1.66.0-nightly (5c8bff74b 2022-10-21)" ></div></body></html> |
