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<!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="API documentation for the Rust `proc_macro_hack` crate."><meta name="keywords" content="rust, rustlang, rust-lang, proc_macro_hack"><title>proc_macro_hack - Rust</title><link rel="stylesheet" type="text/css" href="../normalize.css"><link rel="stylesheet" type="text/css" href="../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" type="text/css" href="../light.css" id="themeStyle"><link rel="stylesheet" type="text/css" href="../dark.css" disabled ><link rel="stylesheet" type="text/css" href="../ayu.css" disabled ><script id="default-settings"></script><script src="../storage.js"></script><noscript><link rel="stylesheet" href="../noscript.css"></noscript><link rel="icon" type="image/svg+xml" href="../favicon.svg">
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<link rel="alternate icon" type="image/png" href="../favicon-32x32.png"><style type="text/css">#crate-search{background-image:url("../down-arrow.svg");}</style></head><body class="rustdoc mod"><!--[if lte IE 8]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="sidebar"><div class="sidebar-menu">☰</div><a href='../proc_macro_hack/index.html'><div class='logo-container rust-logo'><img src='../rust-logo.png' alt='logo'></div></a><p class="location">Crate proc_macro_hack</p><div class="block version"><p>Version 0.5.19</p></div><div class="sidebar-elems"><a id="all-types" href="all.html"><p>See all proc_macro_hack's items</p></a><p class="location"></p><div id="sidebar-vars" data-name="proc_macro_hack" data-ty="mod" data-relpath="../"></div></div></nav><div class="theme-picker"><button id="theme-picker" aria-label="Pick another theme!" aria-haspopup="menu"><img src="../brush.svg" width="18" alt="Pick another theme!"></button><div id="theme-choices" role="menu"></div></div><script src="../theme.js"></script><nav class="sub"><form class="search-form"><div class="search-container"><div><select id="crate-search"><option value="All crates">All crates</option></select><input class="search-input" name="search" disabled autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"></div><button type="button" class="help-button">?</button>
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<a id="settings-menu" href="../settings.html"><img src="../wheel.svg" width="18" alt="Change settings"></a></div></form></nav><section id="main" class="content"><h1 class="fqn"><span class="in-band">Crate <a class="mod" href="">proc_macro_hack</a></span><span class="out-of-band"><span id="render-detail"><a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">[<span class="inner">−</span>]</a></span><a class="srclink" href="../src/proc_macro_hack/lib.rs.html#1-591" title="goto source code">[src]</a></span></h1><div class="docblock"><p><a href="https://github.com/dtolnay/proc-macro-hack"><img src="https://img.shields.io/badge/github-8da0cb?style=for-the-badge&labelColor=555555&logo=github" alt="github" /></a> <a href="https://crates.io/crates/proc-macro-hack"><img src="https://img.shields.io/badge/crates.io-fc8d62?style=for-the-badge&labelColor=555555&logo=rust" alt="crates-io" /></a> <a href="https://docs.rs/proc-macro-hack"><img src="https://img.shields.io/badge/docs.rs-66c2a5?style=for-the-badge&labelColor=555555&logoColor=white&logo=data:image/svg+xml;base64,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" alt="docs-rs" /></a></p>
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<br>
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<table><tr><td><hr>
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<b>Note:</b> <i>As of Rust 1.45 this crate is superseded by native support
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for #[proc_macro] in expression position. Only consider using this crate if
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you care about supporting compilers between 1.31 and 1.45.</i>
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<hr></td></tr></table>
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<p>Since Rust 1.30, the language supports user-defined function-like procedural
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macros. However these can only be invoked in item position, not in
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statements or expressions.</p>
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<p>This crate implements an alternative type of procedural macro that can be
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invoked in statement or expression position.</p>
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<h1 id="defining-procedural-macros" class="section-header"><a href="#defining-procedural-macros">Defining procedural macros</a></h1>
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<p>Two crates are required to define a procedural macro.</p>
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<h2 id="the-implementation-crate" class="section-header"><a href="#the-implementation-crate">The implementation crate</a></h2>
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<p>This crate must contain nothing but procedural macros. Private helper
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functions and private modules are fine but nothing can be public.</p>
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<p><a href="https://github.com/dtolnay/proc-macro-hack/tree/master/demo-hack-impl">» example of an implementation crate</a></p>
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<p>Just like you would use a #[proc_macro] attribute to define a natively
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supported procedural macro, use proc-macro-hack's #[proc_macro_hack]
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attribute to define a procedural macro that works in expression position.
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The function signature is the same as for ordinary function-like procedural
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macros.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">use</span> <span class="ident">proc_macro</span>::<span class="ident">TokenStream</span>;
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<span class="kw">use</span> <span class="ident">proc_macro_hack</span>::<span class="ident">proc_macro_hack</span>;
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<span class="kw">use</span> <span class="ident">quote</span>::<span class="ident">quote</span>;
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<span class="kw">use</span> <span class="ident">syn</span>::{<span class="ident">parse_macro_input</span>, <span class="ident">Expr</span>};
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<span class="attribute">#[<span class="ident">proc_macro_hack</span>]</span>
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<span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">add_one</span>(<span class="ident">input</span>: <span class="ident">TokenStream</span>) <span class="op">-</span><span class="op">></span> <span class="ident">TokenStream</span> {
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<span class="kw">let</span> <span class="ident">expr</span> <span class="op">=</span> <span class="macro">parse_macro_input</span><span class="macro">!</span>(<span class="ident">input</span> <span class="kw">as</span> <span class="ident">Expr</span>);
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<span class="ident">TokenStream</span>::<span class="ident">from</span>(<span class="macro">quote</span><span class="macro">!</span> {
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<span class="number">1</span> <span class="op">+</span> (#<span class="ident">expr</span>)
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})
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}</pre></div>
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<h2 id="the-declaration-crate" class="section-header"><a href="#the-declaration-crate">The declaration crate</a></h2>
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<p>This crate is allowed to contain other public things if you need, for
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example traits or functions or ordinary macros.</p>
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<p><a href="https://github.com/dtolnay/proc-macro-hack/tree/master/demo-hack">» example of a declaration crate</a></p>
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<p>Within the declaration crate there needs to be a re-export of your
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procedural macro from the implementation crate. The re-export also carries a
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#[proc_macro_hack] attribute.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">use</span> <span class="ident">proc_macro_hack</span>::<span class="ident">proc_macro_hack</span>;
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<span class="doccomment">/// Add one to an expression.</span>
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<span class="doccomment">///</span>
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<span class="doccomment">/// (Documentation goes here on the re-export, not in the other crate.)</span>
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<span class="attribute">#[<span class="ident">proc_macro_hack</span>]</span>
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<span class="kw">pub</span> <span class="kw">use</span> <span class="ident">demo_hack_impl</span>::<span class="ident">add_one</span>;</pre></div>
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<p>Both crates depend on <code>proc-macro-hack</code>:</p>
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<pre><code class="language-toml">[dependencies]
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proc-macro-hack = "0.5"
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</code></pre>
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<p>Additionally, your implementation crate (but not your declaration crate) is
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a proc macro crate:</p>
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<pre><code class="language-toml">[lib]
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proc-macro = true
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</code></pre>
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<h1 id="using-procedural-macros" class="section-header"><a href="#using-procedural-macros">Using procedural macros</a></h1>
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<p>Users of your crate depend on your declaration crate (not your
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implementation crate), then use your procedural macros as usual.</p>
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<p><a href="https://github.com/dtolnay/proc-macro-hack/tree/master/example">» example of a downstream crate</a></p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">use</span> <span class="ident">demo_hack</span>::<span class="ident">add_one</span>;
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<span class="kw">fn</span> <span class="ident">main</span>() {
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<span class="kw">let</span> <span class="ident">two</span> <span class="op">=</span> <span class="number">2</span>;
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<span class="kw">let</span> <span class="ident">nine</span> <span class="op">=</span> <span class="macro">add_one</span><span class="macro">!</span>(<span class="ident">two</span>) <span class="op">+</span> <span class="macro">add_one</span><span class="macro">!</span>(<span class="number">2</span> <span class="op">+</span> <span class="number">3</span>);
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<span class="macro">println</span><span class="macro">!</span>(<span class="string">"nine = {}"</span>, <span class="ident">nine</span>);
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}</pre></div>
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<h1 id="limitations" class="section-header"><a href="#limitations">Limitations</a></h1>
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<ul>
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<li>
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<p>Only proc macros in expression position are supported. Proc macros in
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pattern position (<a href="https://github.com/dtolnay/proc-macro-hack/issues/20">#20</a>) are not supported.</p>
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</li>
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<li>
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<p>By default, nested invocations are not supported i.e. the code emitted by
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a proc-macro-hack macro invocation cannot contain recursive calls to the
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same proc-macro-hack macro nor calls to any other proc-macro-hack macros.
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Use <a href="https://docs.rs/proc-macro-nested"><code>proc-macro-nested</code></a> if you require support for nested invocations.</p>
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</li>
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<li>
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<p>By default, hygiene is structured such that the expanded code can't refer
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to local variables other than those passed by name somewhere in the macro
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input. If your macro must refer to <em>local</em> variables that don't get named
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in the macro input, use <code>#[proc_macro_hack(fake_call_site)]</code> on the
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re-export in your declaration crate. <em>Most macros won't need this.</em></p>
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</li>
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<li>
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<p>On compilers that are new enough to natively support proc macros in
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expression position, proc-macro-hack does not automatically use that
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support, since the hygiene can be subtly different between the two
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implementations. To opt in to compiling your macro to native
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<code>#[proc_macro]</code> on sufficiently new compilers, use
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<code>#[proc_macro_hack(only_hack_old_rustc)]</code> on the re-export in your
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declaration crate.</p>
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</li>
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</ul>
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</div><h2 id="attributes" class="section-header"><a href="#attributes">Attribute Macros</a></h2>
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