| <!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" type="text/css" href="../normalize.css"><link rel="stylesheet" type="text/css" href="../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" type="text/css" href="../ayu.css" disabled><link rel="stylesheet" type="text/css" href="../dark.css" disabled><link rel="stylesheet" type="text/css" href="../light.css" id="themeStyle"><script id="default-settings" ></script><script src="../storage.js"></script><script src="../crates.js"></script><script defer src="../main.js"></script> |
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| </a><div class="sidebar-elems"><h2 class="location">Other items in<br><a href="index.html">quote</a></h2><div id="sidebar-vars" data-name="quote" data-ty="macro" data-relpath=""></div><script defer src="sidebar-items.js"></script></div></nav><main><div class="width-limiter"><div class="sub-container"><a class="sub-logo-container" href="../quote/index.html"><img class="rust-logo" src="../rust-logo.png" alt="logo"></a><nav class="sub"><div class="theme-picker"><button id="theme-picker" aria-label="Pick another theme!" aria-haspopup="menu" title="themes"><img width="18" height="18" alt="Pick another theme!" src="../brush.svg"></button><div id="theme-choices" role="menu"></div></div><form class="search-form"><div class="search-container"><div> |
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| <h1 class="fqn"><span class="in-band">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></span></h1><span class="out-of-band"> |
| <a class="srclink" href="../src/quote/lib.rs.html#480-489" title="goto source code">source</a> · |
| <a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">[<span class="inner">−</span>]</a></span></div> |
| <div class="docblock item-decl"><div class="example-wrap"><pre class="rust macro"><code><span class="macro">macro_rules!</span> <span class="ident">quote</span> { |
| () => { ... }; |
| ($(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">tt</span> : <span class="ident">tt</span>) <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" class="section-header"><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" class="section-header"><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" class="section-header"><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" class="section-header"><a href="#examples">Examples</a></h2><h4 id="procedural-macro" class="section-header"><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</span> <span class="kw">crate</span> <span class="ident">proc_macro</span>; |
| |
| <span class="kw">use</span> <span class="ident">proc_macro::TokenStream</span>; |
| <span class="kw">use</span> <span class="ident">quote::quote</span>; |
| |
| <span class="attribute">#[<span class="ident">proc_macro_derive</span>(<span class="ident">HeapSize</span>)]</span> |
| <span class="kw">pub</span> <span class="kw">fn</span> <span class="ident">derive_heap_size</span>(<span class="ident">input</span>: <span class="ident">TokenStream</span>) -> <span class="ident">TokenStream</span> { |
| <span class="comment">// Parse the input and figure out what implementation to generate...</span> |
| <span class="kw">let</span> <span class="ident">name</span> <span class="op">=</span> <span class="comment">/* ... */</span>; |
| <span class="kw">let</span> <span class="ident">expr</span> <span class="op">=</span> <span class="comment">/* ... */</span>; |
| |
| <span class="kw">let</span> <span class="ident">expanded</span> <span class="op">=</span> <span class="macro">quote!</span> { |
| <span class="comment">// The generated impl.</span> |
| <span class="kw">impl</span> <span class="ident">heapsize::HeapSize</span> <span class="kw">for</span> #<span class="ident">name</span> { |
| <span class="kw">fn</span> <span class="ident">heap_size_of_children</span>(<span class="kw-2">&</span><span class="self">self</span>) -> <span class="ident">usize</span> { |
| #<span class="ident">expr</span> |
| } |
| } |
| }; |
| |
| <span class="comment">// Hand the output tokens back to the compiler.</span> |
| <span class="ident">TokenStream::from</span>(<span class="ident">expanded</span>) |
| }</code></pre></div> |
| <p><br></p> |
| <h4 id="combining-quoted-fragments" class="section-header"><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> <span class="ident">type_definition</span> <span class="op">=</span> <span class="macro">quote!</span> {...}; |
| <span class="kw">let</span> <span class="ident">methods</span> <span class="op">=</span> <span class="macro">quote!</span> {...}; |
| |
| <span class="kw">let</span> <span class="ident">tokens</span> <span class="op">=</span> <span class="macro">quote!</span> { |
| #<span class="ident">type_definition</span> |
| #<span class="ident">methods</span> |
| };</code></pre></div> |
| <p><br></p> |
| <h4 id="constructing-identifiers" class="section-header"><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> <span class="ident">_</span>#<span class="ident">ident</span> <span class="op">=</span> <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> <span class="ident">varname</span> <span class="op">=</span> <span class="macro">format_ident!</span>(<span class="string">"_{}"</span>, <span class="ident">ident</span>); |
| <span class="macro">quote!</span> { |
| <span class="kw">let</span> <span class="kw-2">mut</span> #<span class="ident">varname</span> <span class="op">=</span> <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> <span class="ident">concatenated</span> <span class="op">=</span> <span class="macro">format!</span>(<span class="string">"_{}"</span>, <span class="ident">ident</span>); |
| <span class="kw">let</span> <span class="ident">varname</span> <span class="op">=</span> <span class="ident">syn::Ident::new</span>(<span class="kw-2">&</span><span class="ident">concatenated</span>, <span class="ident">ident</span>.<span class="ident">span</span>()); |
| <span class="macro">quote!</span> { |
| <span class="kw">let</span> <span class="kw-2">mut</span> #<span class="ident">varname</span> <span class="op">=</span> <span class="number">0</span>; |
| }</code></pre></div> |
| <p><br></p> |
| <h4 id="making-method-calls" class="section-header"><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> <span class="ident">value</span> <span class="op">=</span> #<span class="ident">field_type::new</span>(); |
| }</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> <span class="ident">value</span> <span class="op">=</span> <span class="op"><</span>#<span class="ident">field_type</span><span class="op">></span><span class="ident">::new</span>(); |
| }</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> <span class="ident">value</span> <span class="op">=</span> <span class="op"><</span>#<span class="ident">field_type</span> <span class="kw">as</span> <span class="ident">core::default::Default</span><span class="op">></span><span class="ident">::default</span>(); |
| }</code></pre></div> |
| <p><br></p> |
| <h4 id="interpolating-text-inside-of-doc-comments" class="section-header"><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='information'><div class='tooltip compile_fail'>ⓘ</div></div><div class="example-wrap"><pre class="rust rust-example-rendered compile_fail"><code><span class="macro">quote!</span> { |
| <span class="doccomment">/// try to interpolate: #ident</span> |
| <span class="doccomment">///</span> |
| <span class="doccomment">/// ...</span> |
| }</code></pre></div> |
| |
| <div class='information'><div class='tooltip compile_fail'>ⓘ</div></div><div class="example-wrap"><pre class="rust rust-example-rendered compile_fail"><code><span class="macro">quote!</span> { |
| <span class="attribute">#[<span class="ident">doc</span> <span class="op">=</span> <span class="string">"try to interpolate: #ident"</span>]</span> |
| }</code></pre></div> |
| <p>Macro calls in a doc attribute are not valid syntax:</p> |
| |
| <div class='information'><div class='tooltip compile_fail'>ⓘ</div></div><div class="example-wrap"><pre class="rust rust-example-rendered compile_fail"><code><span class="macro">quote!</span> { |
| <span class="attribute">#[<span class="ident">doc</span> <span class="op">=</span> <span class="macro">concat!</span>(<span class="string">"try to interpolate: "</span>, <span class="macro">stringify!</span>(#<span class="ident">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> <span class="ident">msg</span> <span class="op">=</span> <span class="macro">format!</span>(...); |
| <span class="macro">quote!</span> { |
| <span class="attribute">#[<span class="ident">doc</span> <span class="op">=</span> #<span class="ident">msg</span>]</span> |
| <span class="doccomment">///</span> |
| <span class="doccomment">/// ...</span> |
| }</code></pre></div> |
| <p><br></p> |
| <h4 id="indexing-into-a-tuple-struct" class="section-header"><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='information'><div class='tooltip compile_fail'>ⓘ</div></div><div class="example-wrap"><pre class="rust rust-example-rendered compile_fail"><code><span class="kw">let</span> <span class="ident">i</span> <span class="op">=</span> <span class="number">0usize</span>..<span class="self">self</span>.<span class="ident">fields</span>.<span class="ident">len</span>(); |
| |
| <span class="comment">// expands to 0 + self.0usize.heap_size() + self.1usize.heap_size() + ...</span> |
| <span class="comment">// which is not valid syntax</span> |
| <span class="macro">quote!</span> { |
| <span class="number">0</span> #( <span class="op">+</span> <span class="self">self</span>.#<span class="ident">i</span>.<span class="ident">heap_size</span>() )<span class="op">*</span> |
| }</code></pre></div> |
| |
| <div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">let</span> <span class="ident">i</span> <span class="op">=</span> (<span class="number">0</span>..<span class="self">self</span>.<span class="ident">fields</span>.<span class="ident">len</span>()).<span class="ident">map</span>(<span class="ident">syn::Index::from</span>); |
| |
| <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="op">+</span> <span class="self">self</span>.#<span class="ident">i</span>.<span class="ident">heap_size</span>() )<span class="op">*</span> |
| }</code></pre></div> |
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