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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="github crates-io docs-rs"><title>proc_macro2 - Rust</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/SourceSerif4-Regular-46f98efaafac5295.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/FiraSans-Regular-018c141bf0843ffd.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/FiraSans-Medium-8f9a781e4970d388.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/SourceCodePro-Regular-562dcc5011b6de7d.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../static.files/SourceCodePro-Semibold-d899c5a5c4aeb14a.ttf.woff2"><link rel="stylesheet" href="../static.files/normalize-76eba96aa4d2e634.css"><link rel="stylesheet" href="../static.files/rustdoc-ac92e1bbe349e143.css"><meta name="rustdoc-vars" data-root-path="../" data-static-root-path="../static.files/" data-current-crate="proc_macro2" data-themes="" data-resource-suffix="" data-rustdoc-version="1.76.0 (07dca489a 2024-02-04)" data-channel="1.76.0" data-search-js="search-2b6ce74ff89ae146.js" data-settings-js="settings-4313503d2e1961c2.js" ><script src="../static.files/storage-f2adc0d6ca4d09fb.js"></script><script defer src="../crates.js"></script><script defer src="../static.files/main-305769736d49e732.js"></script><noscript><link rel="stylesheet" href="../static.files/noscript-feafe1bb7466e4bd.css"></noscript><link rel="alternate icon" type="image/png" href="../static.files/favicon-16x16-8b506e7a72182f1c.png"><link rel="alternate icon" type="image/png" href="../static.files/favicon-32x32-422f7d1d52889060.png"><link rel="icon" type="image/svg+xml" href="../static.files/favicon-2c020d218678b618.svg"></head><body class="rustdoc mod crate"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="mobile-topbar"><button class="sidebar-menu-toggle">☰</button></nav><nav class="sidebar"><div class="sidebar-crate"><h2><a href="../proc_macro2/index.html">proc_macro2</a><span class="version">1.0.78</span></h2></div><div class="sidebar-elems"><ul class="block">
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<li><a id="all-types" href="all.html">All Items</a></li></ul><section><ul class="block"><li><a href="#modules">Modules</a></li><li><a href="#structs">Structs</a></li><li><a href="#enums">Enums</a></li></ul></section></div></nav><div class="sidebar-resizer"></div>
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<main><div class="width-limiter"><nav class="sub"><form class="search-form"><span></span><div id="sidebar-button" tabindex="-1"><a href="../proc_macro2/all.html" title="show sidebar"></a></div><input class="search-input" name="search" aria-label="Run search in the documentation" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><div id="help-button" tabindex="-1"><a href="../help.html" title="help">?</a></div><div id="settings-menu" tabindex="-1"><a href="../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../static.files/wheel-7b819b6101059cd0.svg"></a></div></form></nav><section id="main-content" class="content"><div class="main-heading"><h1>Crate <a class="mod" href="#">proc_macro2</a><button id="copy-path" title="Copy item path to clipboard"><img src="../static.files/clipboard-7571035ce49a181d.svg" width="19" height="18" alt="Copy item path"></button></h1><span class="out-of-band"><a class="src" href="../src/proc_macro2/lib.rs.html#1-1345">source</a> · <button id="toggle-all-docs" title="collapse all docs">[<span>−</span>]</button></span></div><details class="toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p><a href="https://github.com/dtolnay/proc-macro2"><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-macro2"><img src="https://img.shields.io/badge/crates.io-fc8d62?style=for-the-badge&labelColor=555555&logo=rust" alt="crates-io" /></a> <a href="index.html" title="mod proc_macro2"><img src="https://img.shields.io/badge/docs.rs-66c2a5?style=for-the-badge&labelColor=555555&logo=docs.rs" alt="docs-rs" /></a></p>
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<br>
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<p>A wrapper around the procedural macro API of the compiler’s <a href="https://doc.rust-lang.org/proc_macro/"><code>proc_macro</code></a>
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crate. This library serves two purposes:</p>
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<ul>
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<li>
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<p><strong>Bring proc-macro-like functionality to other contexts like build.rs and
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main.rs.</strong> Types from <code>proc_macro</code> are entirely specific to procedural
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macros and cannot ever exist in code outside of a procedural macro.
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Meanwhile <code>proc_macro2</code> types may exist anywhere including non-macro code.
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By developing foundational libraries like <a href="https://github.com/dtolnay/syn">syn</a> and <a href="https://github.com/dtolnay/quote">quote</a> against
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<code>proc_macro2</code> rather than <code>proc_macro</code>, the procedural macro ecosystem
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becomes easily applicable to many other use cases and we avoid
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reimplementing non-macro equivalents of those libraries.</p>
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</li>
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<li>
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<p><strong>Make procedural macros unit testable.</strong> As a consequence of being
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specific to procedural macros, nothing that uses <code>proc_macro</code> can be
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executed from a unit test. In order for helper libraries or components of
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a macro to be testable in isolation, they must be implemented using
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<code>proc_macro2</code>.</p>
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</li>
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</ul>
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<h2 id="usage"><a href="#usage">Usage</a></h2>
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<p>The skeleton of a typical procedural macro typically looks like this:</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">extern crate </span>proc_macro;
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<span class="attr">#[proc_macro_derive(MyDerive)]
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</span><span class="kw">pub fn </span>my_derive(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
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<span class="kw">let </span>input = proc_macro2::TokenStream::from(input);
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<span class="kw">let </span>output: proc_macro2::TokenStream = {
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<span class="comment">/* transform input */
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</span>};
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proc_macro::TokenStream::from(output)
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}</code></pre></div>
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<p>If parsing with <a href="https://github.com/dtolnay/syn">Syn</a>, you’ll use <a href="https://docs.rs/syn/2.0/syn/macro.parse_macro_input.html"><code>parse_macro_input!</code></a> instead to
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propagate parse errors correctly back to the compiler when parsing fails.</p>
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<h2 id="unstable-features"><a href="#unstable-features">Unstable features</a></h2>
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<p>The default feature set of proc-macro2 tracks the most recent stable
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compiler API. Functionality in <code>proc_macro</code> that is not yet stable is not
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exposed by proc-macro2 by default.</p>
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<p>To opt into the additional APIs available in the most recent nightly
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compiler, the <code>procmacro2_semver_exempt</code> config flag must be passed to
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rustc. We will polyfill those nightly-only APIs back to Rust 1.56.0. As
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these are unstable APIs that track the nightly compiler, minor versions of
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proc-macro2 may make breaking changes to them at any time.</p>
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<div class="example-wrap"><pre class="language-sh"><code>RUSTFLAGS='--cfg procmacro2_semver_exempt' cargo build
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</code></pre></div>
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<p>Note that this must not only be done for your crate, but for any crate that
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depends on your crate. This infectious nature is intentional, as it serves
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as a reminder that you are outside of the normal semver guarantees.</p>
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<p>Semver exempt methods are marked as such in the proc-macro2 documentation.</p>
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<h2 id="thread-safety"><a href="#thread-safety">Thread-Safety</a></h2>
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<p>Most types in this crate are <code>!Sync</code> because the underlying compiler
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types make use of thread-local memory, meaning they cannot be accessed from
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a different thread.</p>
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</div></details><h2 id="modules" class="section-header"><a href="#modules">Modules</a></h2><ul class="item-table"><li><div class="item-name"><a class="mod" href="extra/index.html" title="mod proc_macro2::extra">extra</a></div><div class="desc docblock-short">Items which do not have a correspondence to any API in the proc_macro crate,
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but are necessary to include in proc-macro2.</div></li><li><div class="item-name"><a class="mod" href="token_stream/index.html" title="mod proc_macro2::token_stream">token_stream</a></div><div class="desc docblock-short">Public implementation details for the <code>TokenStream</code> type, such as iterators.</div></li></ul><h2 id="structs" class="section-header"><a href="#structs">Structs</a></h2><ul class="item-table"><li><div class="item-name"><a class="struct" href="struct.Group.html" title="struct proc_macro2::Group">Group</a></div><div class="desc docblock-short">A delimited token stream.</div></li><li><div class="item-name"><a class="struct" href="struct.Ident.html" title="struct proc_macro2::Ident">Ident</a></div><div class="desc docblock-short">A word of Rust code, which may be a keyword or legal variable name.</div></li><li><div class="item-name"><a class="struct" href="struct.LexError.html" title="struct proc_macro2::LexError">LexError</a></div><div class="desc docblock-short">Error returned from <code>TokenStream::from_str</code>.</div></li><li><div class="item-name"><a class="struct" href="struct.Literal.html" title="struct proc_macro2::Literal">Literal</a></div><div class="desc docblock-short">A literal string (<code>"hello"</code>), byte string (<code>b"hello"</code>), character (<code>'a'</code>),
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byte character (<code>b'a'</code>), an integer or floating point number with or without
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a suffix (<code>1</code>, <code>1u8</code>, <code>2.3</code>, <code>2.3f32</code>).</div></li><li><div class="item-name"><a class="struct" href="struct.Punct.html" title="struct proc_macro2::Punct">Punct</a></div><div class="desc docblock-short">A <code>Punct</code> is a single punctuation character like <code>+</code>, <code>-</code> or <code>#</code>.</div></li><li><div class="item-name"><a class="struct" href="struct.Span.html" title="struct proc_macro2::Span">Span</a></div><div class="desc docblock-short">A region of source code, along with macro expansion information.</div></li><li><div class="item-name"><a class="struct" href="struct.TokenStream.html" title="struct proc_macro2::TokenStream">TokenStream</a></div><div class="desc docblock-short">An abstract stream of tokens, or more concretely a sequence of token trees.</div></li></ul><h2 id="enums" class="section-header"><a href="#enums">Enums</a></h2><ul class="item-table"><li><div class="item-name"><a class="enum" href="enum.Delimiter.html" title="enum proc_macro2::Delimiter">Delimiter</a></div><div class="desc docblock-short">Describes how a sequence of token trees is delimited.</div></li><li><div class="item-name"><a class="enum" href="enum.Spacing.html" title="enum proc_macro2::Spacing">Spacing</a></div><div class="desc docblock-short">Whether a <code>Punct</code> is followed immediately by another <code>Punct</code> or followed by
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another token or whitespace.</div></li><li><div class="item-name"><a class="enum" href="enum.TokenTree.html" title="enum proc_macro2::TokenTree">TokenTree</a></div><div class="desc docblock-short">A single token or a delimited sequence of token trees (e.g. <code>[1, (), ..]</code>).</div></li></ul></section></div></main></body></html>
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