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214 lines
20 KiB
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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="../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-16x16.png">
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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 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">☰</div><a href='../quote/index.html'><div class='logo-container rust-logo'><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!" 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><span class="help-button">?</span>
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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="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/quote/lib.rs.html#473-482" 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">
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<span class="macro">macro_rules</span><span class="macro">!</span> <span class="ident">quote</span> {
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() <span class="op">=</span><span class="op">></span> { ... };
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($(<span class="macro-nonterminal">$</span><span class="macro-nonterminal">tt</span>:<span class="ident">tt</span>)<span class="kw-2">*</span>) <span class="op">=</span><span class="op">></span> { ... };
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}</pre></div>
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</div><div class="docblock"><p>The whole point.</p>
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<p>Performs variable interpolation against the input and produces it as
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<a href="../proc_macro2/struct.TokenStream.html" title="proc_macro2::TokenStream"><code>proc_macro2::TokenStream</code></a>.</p>
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<p>Note: for returning tokens to the compiler in a procedural macro, use
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<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>
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<br>
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<h1 id="interpolation" class="section-header"><a href="#interpolation">Interpolation</a></h1>
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<p>Variable interpolation is done with <code>#var</code> (similar to <code>$var</code> in
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<code>macro_rules!</code> macros). This grabs the <code>var</code> variable that is currently in
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scope and inserts it in that location in the output tokens. Any type
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implementing the <a href="trait.ToTokens.html"><code>ToTokens</code></a> trait can be interpolated. This includes most
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Rust primitive types as well as most of the syntax tree types from the <a href="https://github.com/dtolnay/syn">Syn</a>
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crate.</p>
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<p>Repetition is done using <code>#(...)*</code> or <code>#(...),*</code> again similar to
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<code>macro_rules!</code>. This iterates through the elements of any variable
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interpolated within the repetition and inserts a copy of the repetition body
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for each one. The variables in an interpolation may be a <code>Vec</code>, slice,
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<code>BTreeSet</code>, or any <code>Iterator</code>.</p>
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<ul>
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<li><code>#(#var)*</code> — no separators</li>
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<li><code>#(#var),*</code> — the character before the asterisk is used as a separator</li>
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<li><code>#( struct #var; )*</code> — the repetition can contain other tokens</li>
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<li><code>#( #k => println!("{}", #v), )*</code> — even multiple interpolations</li>
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</ul>
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<br>
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<h1 id="hygiene" class="section-header"><a href="#hygiene">Hygiene</a></h1>
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<p>Any interpolated tokens preserve the <code>Span</code> information provided by their
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<code>ToTokens</code> implementation. Tokens that originate within the <code>quote!</code>
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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>
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<p>A different span can be provided through the <a href="macro.quote_spanned.html"><code>quote_spanned!</code></a> macro.</p>
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<br>
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<h1 id="return-type" class="section-header"><a href="#return-type">Return type</a></h1>
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<p>The macro evaluates to an expression of type <code>proc_macro2::TokenStream</code>.
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Meanwhile Rust procedural macros are expected to return the type
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<code>proc_macro::TokenStream</code>.</p>
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<p>The difference between the two types is that <code>proc_macro</code> types are entirely
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specific to procedural macros and cannot ever exist in code outside of a
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procedural macro, while <code>proc_macro2</code> types may exist anywhere including
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tests and non-macro code like main.rs and build.rs. This is why even the
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procedural macro ecosystem is largely built around <code>proc_macro2</code>, because
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that ensures the libraries are unit testable and accessible in non-macro
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contexts.</p>
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<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
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<code>quote!</code> from a procedural macro usually looks like <code>tokens.into()</code> or
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<code>proc_macro::TokenStream::from(tokens)</code>.</p>
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<br>
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<h1 id="examples" class="section-header"><a href="#examples">Examples</a></h1><h3 id="procedural-macro" class="section-header"><a href="#procedural-macro">Procedural macro</a></h3>
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<p>The structure of a basic procedural macro is as follows. Refer to the <a href="https://github.com/dtolnay/syn">Syn</a>
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crate for further useful guidance on using <code>quote!</code> as part of a procedural
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macro.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">extern</span> <span class="kw">crate</span> <span class="ident">proc_macro</span>;
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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">quote</span>::<span class="ident">quote</span>;
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<span class="attribute">#[<span class="ident">proc_macro_derive</span>(<span class="ident">HeapSize</span>)]</span>
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<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="op">-</span><span class="op">></span> <span class="ident">TokenStream</span> {
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<span class="comment">// Parse the input and figure out what implementation to generate...</span>
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<span class="kw">let</span> <span class="ident">name</span> <span class="op">=</span> <span class="comment">/* ... */</span>;
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<span class="kw">let</span> <span class="ident">expr</span> <span class="op">=</span> <span class="comment">/* ... */</span>;
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<span class="kw">let</span> <span class="ident">expanded</span> <span class="op">=</span> <span class="macro">quote</span><span class="macro">!</span> {
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<span class="comment">// The generated impl.</span>
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<span class="kw">impl</span> <span class="ident">heapsize</span>::<span class="ident">HeapSize</span> <span class="kw">for</span> #<span class="ident">name</span> {
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<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="op">-</span><span class="op">></span> <span class="ident">usize</span> {
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#<span class="ident">expr</span>
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}
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}
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};
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<span class="comment">// Hand the output tokens back to the compiler.</span>
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<span class="ident">TokenStream</span>::<span class="ident">from</span>(<span class="ident">expanded</span>)
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}</pre></div>
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<p><br></p>
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<h3 id="combining-quoted-fragments" class="section-header"><a href="#combining-quoted-fragments">Combining quoted fragments</a></h3>
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<p>Usually you don't end up constructing an entire final <code>TokenStream</code> in one
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piece. Different parts may come from different helper functions. The tokens
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produced by <code>quote!</code> themselves implement <code>ToTokens</code> and so can be
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interpolated into later <code>quote!</code> invocations to build up a final result.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">let</span> <span class="ident">type_definition</span> <span class="op">=</span> <span class="macro">quote</span><span class="macro">!</span> {...};
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<span class="kw">let</span> <span class="ident">methods</span> <span class="op">=</span> <span class="macro">quote</span><span class="macro">!</span> {...};
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<span class="kw">let</span> <span class="ident">tokens</span> <span class="op">=</span> <span class="macro">quote</span><span class="macro">!</span> {
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#<span class="ident">type_definition</span>
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#<span class="ident">methods</span>
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};</pre></div>
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<p><br></p>
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<h3 id="constructing-identifiers" class="section-header"><a href="#constructing-identifiers">Constructing identifiers</a></h3>
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<p>Suppose we have an identifier <code>ident</code> which came from somewhere in a macro
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input and we need to modify it in some way for the macro output. Let's
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consider prepending the identifier with an underscore.</p>
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<p>Simply interpolating the identifier next to an underscore will not have the
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behavior of concatenating them. The underscore and the identifier will
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continue to be two separate tokens as if you had written <code>_ x</code>.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="comment">// incorrect</span>
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<span class="macro">quote</span><span class="macro">!</span> {
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<span class="kw">let</span> <span class="kw-2">mut</span> <span class="kw">_</span>#<span class="ident">ident</span> <span class="op">=</span> <span class="number">0</span>;
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}</pre></div>
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<p>The solution is to build a new identifier token with the correct value. As
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this is such a common case, the <a href="../quote/macro.format_ident.html" title="format_ident!"><code>format_ident!</code></a> macro provides a
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convenient utility for doing so correctly.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">let</span> <span class="ident">varname</span> <span class="op">=</span> <span class="macro">format_ident</span><span class="macro">!</span>(<span class="string">"_{}"</span>, <span class="ident">ident</span>);
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<span class="macro">quote</span><span class="macro">!</span> {
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<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>;
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}</pre></div>
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<p>Alternatively, the APIs provided by Syn and proc-macro2 can be used to
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directly build the identifier. This is roughly equivalent to the above, but
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will not handle <code>ident</code> being a raw identifier.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">let</span> <span class="ident">concatenated</span> <span class="op">=</span> <span class="macro">format</span><span class="macro">!</span>(<span class="string">"_{}"</span>, <span class="ident">ident</span>);
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<span class="kw">let</span> <span class="ident">varname</span> <span class="op">=</span> <span class="ident">syn</span>::<span class="ident">Ident</span>::<span class="ident">new</span>(<span class="kw-2">&</span><span class="ident">concatenated</span>, <span class="ident">ident</span>.<span class="ident">span</span>());
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<span class="macro">quote</span><span class="macro">!</span> {
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<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>;
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}</pre></div>
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<p><br></p>
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<h3 id="making-method-calls" class="section-header"><a href="#making-method-calls">Making method calls</a></h3>
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<p>Let's say our macro requires some type specified in the macro input to have
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a constructor called <code>new</code>. We have the type in a variable called
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<code>field_type</code> of type <code>syn::Type</code> and want to invoke the constructor.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="comment">// incorrect</span>
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<span class="macro">quote</span><span class="macro">!</span> {
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<span class="kw">let</span> <span class="ident">value</span> <span class="op">=</span> #<span class="ident">field_type</span>::<span class="ident">new</span>();
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}</pre></div>
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<p>This works only sometimes. If <code>field_type</code> is <code>String</code>, the expanded code
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contains <code>String::new()</code> which is fine. But if <code>field_type</code> is something
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like <code>Vec<i32></code> then the expanded code is <code>Vec<i32>::new()</code> which is invalid
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syntax. Ordinarily in handwritten Rust we would write <code>Vec::<i32>::new()</code>
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but for macros often the following is more convenient.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="macro">quote</span><span class="macro">!</span> {
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<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>();
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}</pre></div>
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<p>This expands to <code><Vec<i32>>::new()</code> which behaves correctly.</p>
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<p>A similar pattern is appropriate for trait methods.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="macro">quote</span><span class="macro">!</span> {
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<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</span>::<span class="ident">default</span>::<span class="ident">Default</span><span class="op">></span>::<span class="ident">default</span>();
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}</pre></div>
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<p><br></p>
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<h3 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></h3>
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<p>Neither doc comments nor string literals get interpolation behavior in
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quote:</p>
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<div class='information'><div class='tooltip compile_fail'>ⓘ<span class='tooltiptext'>This example deliberately fails to compile</span></div></div><div class="example-wrap"><pre class="rust rust-example-rendered compile_fail">
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<span class="macro">quote</span><span class="macro">!</span> {
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<span class="doccomment">/// try to interpolate: #ident</span>
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<span class="doccomment">///</span>
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<span class="doccomment">/// ...</span>
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}</pre></div>
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<div class='information'><div class='tooltip compile_fail'>ⓘ<span class='tooltiptext'>This example deliberately fails to compile</span></div></div><div class="example-wrap"><pre class="rust rust-example-rendered compile_fail">
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<span class="macro">quote</span><span class="macro">!</span> {
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<span class="attribute">#[<span class="ident">doc</span> <span class="op">=</span> <span class="string">"try to interpolate: #ident"</span>]</span>
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}</pre></div>
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<p>Macro calls in a doc attribute are not valid syntax:</p>
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<div class='information'><div class='tooltip compile_fail'>ⓘ<span class='tooltiptext'>This example deliberately fails to compile</span></div></div><div class="example-wrap"><pre class="rust rust-example-rendered compile_fail">
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<span class="macro">quote</span><span class="macro">!</span> {
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<span class="attribute">#[<span class="ident">doc</span> <span class="op">=</span> <span class="macro">concat</span><span class="macro">!</span>(<span class="string">"try to interpolate: "</span>, <span class="macro">stringify</span><span class="macro">!</span>(#<span class="ident">ident</span>))]</span>
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}</pre></div>
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<p>Instead the best way to build doc comments that involve variables is by
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formatting the doc string literal outside of quote.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered">
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<span class="kw">let</span> <span class="ident">msg</span> <span class="op">=</span> <span class="macro">format</span><span class="macro">!</span>(...);
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<span class="macro">quote</span><span class="macro">!</span> {
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<span class="attribute">#[<span class="ident">doc</span> <span class="op">=</span> #<span class="ident">msg</span>]</span>
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<span class="doccomment">///</span>
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<span class="doccomment">/// ...</span>
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}</pre></div>
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<p><br></p>
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<h3 id="indexing-into-a-tuple-struct" class="section-header"><a href="#indexing-into-a-tuple-struct">Indexing into a tuple struct</a></h3>
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<p>When interpolating indices of a tuple or tuple struct, we need them not to
|
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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>
|
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instead.</p>
|
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|
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<div class='information'><div class='tooltip compile_fail'>ⓘ<span class='tooltiptext'>This example deliberately fails to compile</span></div></div><div class="example-wrap"><pre class="rust rust-example-rendered compile_fail">
|
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<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>();
|
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<span class="comment">// expands to 0 + self.0usize.heap_size() + self.1usize.heap_size() + ...</span>
|
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|
<span class="comment">// which is not valid syntax</span>
|
|||
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<span class="macro">quote</span><span class="macro">!</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>
|
|||
|
|
}</pre></div>
|
|||
|
|
|
|||
|
|
<div class="example-wrap"><pre class="rust rust-example-rendered">
|
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|
|
<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</span>::<span class="ident">Index</span>::<span class="ident">from</span>);
|
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|
|
|
|||
|
|
<span class="comment">// expands to 0 + self.0.heap_size() + self.1.heap_size() + ...</span>
|
|||
|
|
<span class="macro">quote</span><span class="macro">!</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>
|
|||
|
|
}</pre></div>
|
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|
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