| <!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="The whole point."><meta name="keywords" content="rust, rustlang, rust-lang, quote"><title>quote in quote - Rust</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="sidebar-items.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 macro"><!--[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><a class="sidebar-logo" href="../quote/index.html"><div class="logo-container"><img class="rust-logo" src="../rust-logo.svg" alt="logo"></div></a><h2></h2></nav><nav class="sidebar"><a class="sidebar-logo" href="../quote/index.html"><div class="logo-container"><img class="rust-logo" src="../rust-logo.svg" alt="logo"></div></a><div class="sidebar-elems"><h2><a href="index.html">In quote</a></h2></div></nav><main><div class="width-limiter"><nav class="sub"><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="main-heading"><h1 class="fqn">Macro <a href="index.html">quote</a>::<wbr><a class="macro" href="#">quote</a><button id="copy-path" onclick="copy_path(this)" title="Copy item path to clipboard"><img src="../clipboard.svg" width="19" height="18" alt="Copy item path"></button></h1><span class="out-of-band"><a class="srclink" href="../src/quote/lib.rs.html#481-485">source</a> · <a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">[<span class="inner">−</span>]</a></span></div><div class="item-decl"><div class="example-wrap"><pre class="rust macro"><code><span class="macro">macro_rules! </span>quote { |
| ($(<span class="macro-nonterminal">$tt</span>:tt)<span class="kw-2">*</span>) => { ... }; |
| }</code></pre></div> |
| </div><details class="rustdoc-toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>The whole point.</p> |
| <p>Performs variable interpolation against the input and produces it as |
| <a href="../proc_macro2/struct.TokenStream.html" title="proc_macro2::TokenStream"><code>proc_macro2::TokenStream</code></a>.</p> |
| <p>Note: for returning tokens to the compiler in a procedural macro, use |
| <code>.into()</code> on the result to convert to <a href="https://doc.rust-lang.org/nightly/proc_macro/struct.TokenStream.html" title="proc_macro::TokenStream"><code>proc_macro::TokenStream</code></a>.</p> |
| <br> |
| <h2 id="interpolation"><a href="#interpolation">Interpolation</a></h2> |
| <p>Variable interpolation is done with <code>#var</code> (similar to <code>$var</code> in |
| <code>macro_rules!</code> macros). This grabs the <code>var</code> variable that is currently in |
| scope and inserts it in that location in the output tokens. Any type |
| implementing the <a href="trait.ToTokens.html"><code>ToTokens</code></a> trait can be interpolated. This includes most |
| Rust primitive types as well as most of the syntax tree types from the <a href="https://github.com/dtolnay/syn">Syn</a> |
| crate.</p> |
| <p>Repetition is done using <code>#(...)*</code> or <code>#(...),*</code> again similar to |
| <code>macro_rules!</code>. This iterates through the elements of any variable |
| interpolated within the repetition and inserts a copy of the repetition body |
| for each one. The variables in an interpolation may be a <code>Vec</code>, slice, |
| <code>BTreeSet</code>, or any <code>Iterator</code>.</p> |
| <ul> |
| <li><code>#(#var)*</code> — no separators</li> |
| <li><code>#(#var),*</code> — the character before the asterisk is used as a separator</li> |
| <li><code>#( struct #var; )*</code> — the repetition can contain other tokens</li> |
| <li><code>#( #k => println!("{}", #v), )*</code> — even multiple interpolations</li> |
| </ul> |
| <br> |
| <h2 id="hygiene"><a href="#hygiene">Hygiene</a></h2> |
| <p>Any interpolated tokens preserve the <code>Span</code> information provided by their |
| <code>ToTokens</code> implementation. Tokens that originate within the <code>quote!</code> |
| invocation are spanned with <a href="https://docs.rs/proc-macro2/1.0/proc_macro2/struct.Span.html#method.call_site"><code>Span::call_site()</code></a>.</p> |
| <p>A different span can be provided through the <a href="macro.quote_spanned.html"><code>quote_spanned!</code></a> macro.</p> |
| <br> |
| <h2 id="return-type"><a href="#return-type">Return type</a></h2> |
| <p>The macro evaluates to an expression of type <code>proc_macro2::TokenStream</code>. |
| Meanwhile Rust procedural macros are expected to return the type |
| <code>proc_macro::TokenStream</code>.</p> |
| <p>The difference between the two types is that <code>proc_macro</code> types are entirely |
| specific to procedural macros and cannot ever exist in code outside of a |
| procedural macro, while <code>proc_macro2</code> types may exist anywhere including |
| tests and non-macro code like main.rs and build.rs. This is why even the |
| procedural macro ecosystem is largely built around <code>proc_macro2</code>, because |
| that ensures the libraries are unit testable and accessible in non-macro |
| contexts.</p> |
| <p>There is a <a href="https://doc.rust-lang.org/std/convert/trait.From.html"><code>From</code></a>-conversion in both directions so returning the output of |
| <code>quote!</code> from a procedural macro usually looks like <code>tokens.into()</code> or |
| <code>proc_macro::TokenStream::from(tokens)</code>.</p> |
| <br> |
| <h2 id="examples"><a href="#examples">Examples</a></h2><h4 id="procedural-macro"><a href="#procedural-macro">Procedural macro</a></h4> |
| <p>The structure of a basic procedural macro is as follows. Refer to the <a href="https://github.com/dtolnay/syn">Syn</a> |
| crate for further useful guidance on using <code>quote!</code> as part of a procedural |
| macro.</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">extern crate </span>proc_macro; |
| |
| <span class="kw">use </span>proc_macro::TokenStream; |
| <span class="kw">use </span>quote::quote; |
| |
| <span class="attribute">#[proc_macro_derive(HeapSize)] |
| </span><span class="kw">pub fn </span>derive_heap_size(input: TokenStream) -> TokenStream { |
| <span class="comment">// Parse the input and figure out what implementation to generate... |
| </span><span class="kw">let </span>name = <span class="comment">/* ... */</span>; |
| <span class="kw">let </span>expr = <span class="comment">/* ... */</span>; |
| |
| <span class="kw">let </span>expanded = <span class="macro">quote! </span>{ |
| <span class="comment">// The generated impl. |
| </span><span class="kw">impl </span>heapsize::HeapSize <span class="kw">for </span>#name { |
| <span class="kw">fn </span>heap_size_of_children(<span class="kw-2">&</span><span class="self">self</span>) -> usize { |
| #expr |
| } |
| } |
| }; |
| |
| <span class="comment">// Hand the output tokens back to the compiler. |
| </span>TokenStream::from(expanded) |
| }</code></pre></div> |
| <p><br></p> |
| <h4 id="combining-quoted-fragments"><a href="#combining-quoted-fragments">Combining quoted fragments</a></h4> |
| <p>Usually you don’t end up constructing an entire final <code>TokenStream</code> in one |
| piece. Different parts may come from different helper functions. The tokens |
| produced by <code>quote!</code> themselves implement <code>ToTokens</code> and so can be |
| interpolated into later <code>quote!</code> invocations to build up a final result.</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">let </span>type_definition = <span class="macro">quote! </span>{...}; |
| <span class="kw">let </span>methods = <span class="macro">quote! </span>{...}; |
| |
| <span class="kw">let </span>tokens = <span class="macro">quote! </span>{ |
| #type_definition |
| #methods |
| };</code></pre></div> |
| <p><br></p> |
| <h4 id="constructing-identifiers"><a href="#constructing-identifiers">Constructing identifiers</a></h4> |
| <p>Suppose we have an identifier <code>ident</code> which came from somewhere in a macro |
| input and we need to modify it in some way for the macro output. Let’s |
| consider prepending the identifier with an underscore.</p> |
| <p>Simply interpolating the identifier next to an underscore will not have the |
| behavior of concatenating them. The underscore and the identifier will |
| continue to be two separate tokens as if you had written <code>_ x</code>.</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="comment">// incorrect |
| </span><span class="macro">quote! </span>{ |
| <span class="kw">let </span><span class="kw-2">mut </span>_#ident = <span class="number">0</span>; |
| }</code></pre></div> |
| <p>The solution is to build a new identifier token with the correct value. As |
| this is such a common case, the <a href="macro.format_ident.html" title="format_ident!"><code>format_ident!</code></a> macro provides a |
| convenient utility for doing so correctly.</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">let </span>varname = <span class="macro">format_ident!</span>(<span class="string">"_{}"</span>, ident); |
| <span class="macro">quote! </span>{ |
| <span class="kw">let </span><span class="kw-2">mut </span>#varname = <span class="number">0</span>; |
| }</code></pre></div> |
| <p>Alternatively, the APIs provided by Syn and proc-macro2 can be used to |
| directly build the identifier. This is roughly equivalent to the above, but |
| will not handle <code>ident</code> being a raw identifier.</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">let </span>concatenated = <span class="macro">format!</span>(<span class="string">"_{}"</span>, ident); |
| <span class="kw">let </span>varname = syn::Ident::new(<span class="kw-2">&</span>concatenated, ident.span()); |
| <span class="macro">quote! </span>{ |
| <span class="kw">let </span><span class="kw-2">mut </span>#varname = <span class="number">0</span>; |
| }</code></pre></div> |
| <p><br></p> |
| <h4 id="making-method-calls"><a href="#making-method-calls">Making method calls</a></h4> |
| <p>Let’s say our macro requires some type specified in the macro input to have |
| a constructor called <code>new</code>. We have the type in a variable called |
| <code>field_type</code> of type <code>syn::Type</code> and want to invoke the constructor.</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="comment">// incorrect |
| </span><span class="macro">quote! </span>{ |
| <span class="kw">let </span>value = #field_type::new(); |
| }</code></pre></div> |
| <p>This works only sometimes. If <code>field_type</code> is <code>String</code>, the expanded code |
| contains <code>String::new()</code> which is fine. But if <code>field_type</code> is something |
| like <code>Vec<i32></code> then the expanded code is <code>Vec<i32>::new()</code> which is invalid |
| syntax. Ordinarily in handwritten Rust we would write <code>Vec::<i32>::new()</code> |
| but for macros often the following is more convenient.</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="macro">quote! </span>{ |
| <span class="kw">let </span>value = <#field_type>::new(); |
| }</code></pre></div> |
| <p>This expands to <code><Vec<i32>>::new()</code> which behaves correctly.</p> |
| <p>A similar pattern is appropriate for trait methods.</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="macro">quote! </span>{ |
| <span class="kw">let </span>value = <#field_type <span class="kw">as </span>core::default::Default>::default(); |
| }</code></pre></div> |
| <p><br></p> |
| <h4 id="interpolating-text-inside-of-doc-comments"><a href="#interpolating-text-inside-of-doc-comments">Interpolating text inside of doc comments</a></h4> |
| <p>Neither doc comments nor string literals get interpolation behavior in |
| quote:</p> |
| |
| <div class="example-wrap compile_fail"><div class='tooltip'>ⓘ</div><pre class="rust rust-example-rendered"><code><span class="macro">quote! </span>{ |
| <span class="doccomment">/// try to interpolate: #ident |
| /// |
| /// ... |
| </span>}</code></pre></div> |
| |
| <div class="example-wrap compile_fail"><div class='tooltip'>ⓘ</div><pre class="rust rust-example-rendered"><code><span class="macro">quote! </span>{ |
| <span class="attribute">#[doc = <span class="string">"try to interpolate: #ident"</span>] |
| </span>}</code></pre></div> |
| <p>Instead the best way to build doc comments that involve variables is by |
| formatting the doc string literal outside of quote.</p> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">let </span>msg = <span class="macro">format!</span>(...); |
| <span class="macro">quote! </span>{ |
| <span class="attribute">#[doc = #msg] |
| </span><span class="doccomment">/// |
| /// ... |
| </span>}</code></pre></div> |
| <p><br></p> |
| <h4 id="indexing-into-a-tuple-struct"><a href="#indexing-into-a-tuple-struct">Indexing into a tuple struct</a></h4> |
| <p>When interpolating indices of a tuple or tuple struct, we need them not to |
| appears suffixed as integer literals by interpolating them as <a href="https://docs.rs/syn/1.0/syn/struct.Index.html"><code>syn::Index</code></a> |
| instead.</p> |
| |
| <div class="example-wrap compile_fail"><div class='tooltip'>ⓘ</div><pre class="rust rust-example-rendered"><code><span class="kw">let </span>i = <span class="number">0usize</span>..<span class="self">self</span>.fields.len(); |
| |
| <span class="comment">// expands to 0 + self.0usize.heap_size() + self.1usize.heap_size() + ... |
| // which is not valid syntax |
| </span><span class="macro">quote! </span>{ |
| <span class="number">0 </span>#( + <span class="self">self</span>.#i.heap_size() )* |
| }</code></pre></div> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">let </span>i = (<span class="number">0</span>..<span class="self">self</span>.fields.len()).map(syn::Index::from); |
| |
| <span class="comment">// expands to 0 + self.0.heap_size() + self.1.heap_size() + ... |
| </span><span class="macro">quote! </span>{ |
| <span class="number">0 </span>#( + <span class="self">self</span>.#i.heap_size() )* |
| }</code></pre></div> |
| </div></details></section></div></main><div id="rustdoc-vars" data-root-path="../" data-current-crate="quote" data-themes="ayu,dark,light" data-resource-suffix="" data-rustdoc-version="1.66.0-nightly (5c8bff74b 2022-10-21)" ></div></body></html> |
