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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 `quote` macro in crate `quote`."><meta name="keywords" content="rust, rustlang, rust-lang, quote"><title>quote::quote - 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="../dark.css"><link rel="stylesheet" type="text/css" href="../light.css" id="themeStyle"><script src="../storage.js"></script><noscript><link rel="stylesheet" href="../noscript.css"></noscript><link rel="shortcut icon" href="../favicon.ico"><style type="text/css">#crate-search{background-image:url("../down-arrow.svg");}</style></head><body class="rustdoc macro"><!--[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">&#9776;</div><a href='../quote/index.html'><div class='logo-container'><img src='../rust-logo.png' alt='logo'></div></a><div class="sidebar-elems"><p class='location'><a href='index.html'>quote</a></p><script>window.sidebarCurrent = {name: 'quote', ty: 'macro', relpath: ''};</script><script defer src="sidebar-items.js"></script></div></nav><div class="theme-picker"><button id="theme-picker" aria-label="Pick another theme!"><img src="../brush.svg" width="18" alt="Pick another theme!"></button><div id="theme-choices"></div></div><script src="../theme.js"></script><nav class="sub"><form class="search-form js-only"><div class="search-container"><div><select id="crate-search"><option value="All crates">All crates</option></select><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"></div><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='out-of-band'><span id='render-detail'><a id="toggle-all-docs" href="javascript:void(0)" title="collapse all docs">[<span class='inner'>&#x2212;</span>]</a></span><a class='srclink' href='../src/quote/lib.rs.html#335-339' title='goto source code'>[src]</a></span><span class='in-band'>Macro <a href='index.html'>quote</a>::<wbr><a class="macro" href=''>quote</a></span></h1><div class="docblock type-decl hidden-by-usual-hider"><div class="example-wrap"><pre class="rust macro">
<span class="macro">macro_rules</span><span class="macro">!</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>) <span class="op">=</span><span class="op">&gt;</span> { ... };
}</pre></div>
</div><div class='docblock'><p>The whole point.</p>
<p>Performs variable interpolation against the input and produces it as
<a href="https://docs.rs/proc-macro2/0.4/proc_macro2/struct.TokenStream.html"><code>TokenStream</code></a>. For returning tokens to the compiler in a procedural macro, use
<code>into()</code> to build a <code>TokenStream</code>.</p>
<h1 id="interpolation" class="section-header"><a href="#interpolation">Interpolation</a></h1>
<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 anything that
implements <code>IntoIterator</code>, including <code>Vec</code> or a pre-existing iterator.</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 =&gt; println!(&quot;{}&quot;, #v), )*</code> — even multiple interpolations</li>
</ul>
<p>There are two limitations around interpolations in a repetition:</p>
<ul>
<li>
<p>Every interpolation inside of a repetition must be a distinct variable.
That is, <code>#(#a #a)*</code> is not allowed. Work around this by collecting <code>a</code>
into a vector and taking references <code>a1 = &amp;a</code> and <code>a2 = &amp;a</code> which you use
inside the repetition: <code>#(#a1 #a2)*</code>. Where possible, use meaningful names
that indicate the distinct role of each copy.</p>
</li>
<li>
<p>Every interpolation inside of a repetition must be iterable. If we have
<code>vec</code> which is a vector and <code>ident</code> which is a single identifier,
<code>#(#ident #vec)*</code> is not allowed. Work around this by using
<code>std::iter::repeat(ident)</code> to produce an iterable that can be used from
within the repetition.</p>
</li>
</ul>
<h1 id="hygiene" class="section-header"><a href="#hygiene">Hygiene</a></h1>
<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-macro2/0.4/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>
<h1 id="return-type" class="section-header"><a href="#return-type">Return type</a></h1>
<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>
<h1 id="examples" class="section-header"><a href="#examples">Examples</a></h1><h2 id="procedural-macro" class="section-header"><a href="#procedural-macro">Procedural macro</a></h2>
<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>
<pre><code class="language-edition2018"># #[cfg(any())]
extern crate proc_macro;
# use proc_macro2 as proc_macro;
use proc_macro::TokenStream;
use quote::quote;
# const IGNORE_TOKENS: &amp;'static str = stringify! {
#[proc_macro_derive(HeapSize)]
# };
pub fn derive_heap_size(input: TokenStream) -&gt; TokenStream {
// Parse the input and figure out what implementation to generate...
# const IGNORE_TOKENS: &amp;'static str = stringify! {
let name = /* ... */;
let expr = /* ... */;
# };
#
# let name = 0;
# let expr = 0;
let expanded = quote! {
// The generated impl.
impl heapsize::HeapSize for #name {
fn heap_size_of_children(&amp;self) -&gt; usize {
#expr
}
}
};
// Hand the output tokens back to the compiler.
TokenStream::from(expanded)
}
</code></pre>
<h2 id="combining-quoted-fragments" class="section-header"><a href="#combining-quoted-fragments">Combining quoted fragments</a></h2>
<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>
<pre><code class="language-edition2018"># use quote::quote;
#
let type_definition = quote! {...};
let methods = quote! {...};
let tokens = quote! {
#type_definition
#methods
};
</code></pre>
<h2 id="constructing-identifiers" class="section-header"><a href="#constructing-identifiers">Constructing identifiers</a></h2>
<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>
<pre><code class="language-edition2018"># use proc_macro2::{self as syn, Span};
# use quote::quote;
#
# let ident = syn::Ident::new(&quot;i&quot;, Span::call_site());
#
// incorrect
quote! {
let mut _#ident = 0;
}
# ;
</code></pre>
<p>The solution is to perform token-level manipulations using the APIs provided
by Syn and proc-macro2.</p>
<pre><code class="language-edition2018"># use proc_macro2::{self as syn, Span};
# use quote::quote;
#
# let ident = syn::Ident::new(&quot;i&quot;, Span::call_site());
#
let concatenated = format!(&quot;_{}&quot;, ident);
let varname = syn::Ident::new(&amp;concatenated, ident.span());
quote! {
let mut #varname = 0;
}
# ;
</code></pre>
<h2 id="making-method-calls" class="section-header"><a href="#making-method-calls">Making method calls</a></h2>
<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>
<pre><code class="language-edition2018"># use quote::quote;
#
# let field_type = quote!(...);
#
// incorrect
quote! {
let value = #field_type::new();
}
# ;
</code></pre>
<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&lt;i32&gt;</code> then the expanded code is <code>Vec&lt;i32&gt;::new()</code> which is invalid
syntax. Ordinarily in handwritten Rust we would write <code>Vec::&lt;i32&gt;::new()</code>
but for macros often the following is more convenient.</p>
<pre><code class="language-edition2018"># use quote::quote;
#
# let field_type = quote!(...);
#
quote! {
let value = &lt;#field_type&gt;::new();
}
# ;
</code></pre>
<p>This expands to <code>&lt;Vec&lt;i32&gt;&gt;::new()</code> which behaves correctly.</p>
<p>A similar pattern is appropriate for trait methods.</p>
<pre><code class="language-edition2018"># use quote::quote;
#
# let field_type = quote!(...);
#
quote! {
let value = &lt;#field_type as core::default::Default&gt;::default();
}
# ;
</code></pre>
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