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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="Traits and types used for tracking the usage of generic parameters through a proc-macro input."><title>darling_core::usage - Rust</title><script>if(window.location.protocol!=="file:")document.head.insertAdjacentHTML("beforeend","SourceSerif4-Regular-6b053e98.ttf.woff2,FiraSans-Italic-81dc35de.woff2,FiraSans-Regular-0fe48ade.woff2,FiraSans-MediumItalic-ccf7e434.woff2,FiraSans-Medium-e1aa3f0a.woff2,SourceCodePro-Regular-8badfe75.ttf.woff2,SourceCodePro-Semibold-aa29a496.ttf.woff2".split(",").map(f=>`<link rel="preload" as="font" type="font/woff2"href="../../static.files/${f}">`).join(""))</script><link rel="stylesheet" href="../../static.files/normalize-9960930a.css"><link rel="stylesheet" href="../../static.files/rustdoc-ca0dd0c4.css"><meta name="rustdoc-vars" data-root-path="../../" data-static-root-path="../../static.files/" data-current-crate="darling_core" data-themes="" data-resource-suffix="" data-rustdoc-version="1.93.1 (01f6ddf75 2026-02-11) (Arch Linux rust 1:1.93.1-1)" data-channel="1.93.1" data-search-js="search-9e2438ea.js" data-stringdex-js="stringdex-a3946164.js" data-settings-js="settings-c38705f0.js" ><script src="../../static.files/storage-e2aeef58.js"></script><script defer src="../sidebar-items.js"></script><script defer src="../../static.files/main-a410ff4d.js"></script><noscript><link rel="stylesheet" href="../../static.files/noscript-263c88ec.css"></noscript><link rel="alternate icon" type="image/png" href="../../static.files/favicon-32x32-eab170b8.png"><link rel="icon" type="image/svg+xml" href="../../static.files/favicon-044be391.svg"></head><body class="rustdoc mod"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><rustdoc-topbar><h2><a href="#">Module usage</a></h2></rustdoc-topbar><nav class="sidebar"><div class="sidebar-crate"><h2><a href="../../darling_core/index.html">darling_<wbr>core</a><span class="version">0.20.11</span></h2></div><div class="sidebar-elems"><section id="rustdoc-toc"><h2 class="location"><a href="#">Module usage</a></h2><h3><a href="#">Sections</a></h3><ul class="block top-toc"><li><a href="#usage" title="Usage">Usage</a><ul><li><a href="#example-1-filtering" title="Example 1: Filtering">Example 1: Filtering</a></li><li><a href="#example-2-integrating-with-fromderiveinput" title="Example 2: Integrating with `FromDeriveInput`">Example 2: Integrating with <code>FromDeriveInput</code></a></li></ul></li></ul><h3><a href="#structs">Module Items</a></h3><ul class="block"><li><a href="#structs" title="Structs">Structs</a></li><li><a href="#enums" title="Enums">Enums</a></li><li><a href="#traits" title="Traits">Traits</a></li><li><a href="#types" title="Type Aliases">Type Aliases</a></li></ul></section><div id="rustdoc-modnav"><h2 class="in-crate"><a href="../index.html">In crate darling_<wbr>core</a></h2></div></div></nav><div class="sidebar-resizer" title="Drag to resize sidebar"></div><main><div class="width-limiter"><section id="main-content" class="content"><div class="main-heading"><div class="rustdoc-breadcrumbs"><a href="../index.html">darling_core</a></div><h1>Module <span>usage</span> <button id="copy-path" title="Copy item path to clipboard">Copy item path</button></h1><rustdoc-toolbar></rustdoc-toolbar><span class="sub-heading"><a class="src" href="../../src/darling_core/usage/mod.rs.html#1-111">Source</a> </span></div><details class="toggle top-doc" open><summary class="hideme"><span>Expand description</span></summary><div class="docblock"><p>Traits and types used for tracking the usage of generic parameters through a proc-macro input.</p>
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<p>When generating trait impls, libraries often want to automatically figure out which type parameters
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are used in which fields, and then emit bounds that will produce the most permissive compilable
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code.</p>
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<h2 id="usage"><a class="doc-anchor" href="#usage">§</a>Usage</h2><h3 id="example-1-filtering"><a class="doc-anchor" href="#example-1-filtering">§</a>Example 1: Filtering</h3>
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<p>This example accepts a proc-macro input, then finds all lifetimes and type parameters used
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by private fields.</p>
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<div class="example-wrap"><pre class="rust rust-example-rendered"><code><span class="kw">fn </span>process(input: <span class="kw-2">&</span>DeriveInput) -> Generics {
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<span class="kw">let </span>type_params = input.generics.declared_type_params();
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<span class="kw">let </span>lifetimes = input.generics.declared_lifetimes();
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<span class="kw">let </span><span class="kw-2">mut </span>ret_generics = input.generics.clone();
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<span class="kw">if let </span>Data::Struct(<span class="kw-2">ref </span>body) = input.data {
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<span class="kw">let </span>internal_fields = body
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.fields
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.iter()
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.filter(|field| field.vis == Visibility::Inherited)
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.collect::<Vec<<span class="kw">_</span>>>();
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<span class="kw">let </span>int_type_params = internal_fields
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.collect_type_params(<span class="kw-2">&</span>Purpose::BoundImpl.into(), <span class="kw-2">&</span>type_params);
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<span class="comment">// We could reuse the vec from above, but here we'll instead
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// directly consume the chained iterator.
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</span><span class="kw">let </span>int_lifetimes = body
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.fields
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.iter()
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.filter(|field| field.vis == Visibility::Inherited)
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.collect_lifetimes(<span class="kw-2">&</span>Purpose::BoundImpl.into(), <span class="kw-2">&</span>lifetimes);
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ret_generics.params = ret_generics
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.params
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.into_iter()
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.filter(|gp| {
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<span class="kw">match </span><span class="kw-2">*</span>gp {
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GenericParam::Type(<span class="kw-2">ref </span>ty) => int_type_params.contains(<span class="kw-2">&</span>ty.ident),
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GenericParam::Lifetime(<span class="kw-2">ref </span>lt) => int_lifetimes.contains(<span class="kw-2">&</span>lt.lifetime),
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<span class="kw">_ </span>=> <span class="bool-val">true</span>,
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}
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})
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.collect();
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}
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ret_generics
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}
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</code></pre></div><h3 id="example-2-integrating-with-fromderiveinput"><a class="doc-anchor" href="#example-2-integrating-with-fromderiveinput">§</a>Example 2: Integrating with <code>FromDeriveInput</code></h3>
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<p>It is possible to use <code>darling</code>’s magic fields feature in tandem with the <code>usage</code> feature set.
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While there is no custom derive for <code>UsesTypeParams</code> or <code>UsesLifetimes</code>, there are macros to
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generate impls.</p>
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<div class="example-wrap ignore"><a href="#" class="tooltip" title="This example is not tested">ⓘ</a><pre class="rust rust-example-rendered"><code><span class="attr">#![allow(dead_code)]
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#[derive(FromField)]
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#[darling(attributes(speak))]
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</span><span class="kw">struct </span>SpeakerField {
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ident: <span class="prelude-ty">Option</span><syn::Ident>,
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ty: syn::Type,
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<span class="attr">#[darling(default)]
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</span>volume: <span class="prelude-ty">Option</span><u32>,
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}
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<span class="macro">uses_type_params!</span>(SpeakerField, ty);
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<span class="macro">uses_lifetimes!</span>(SpeakerField, ty);
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<span class="attr">#[derive(FromDeriveInput)]
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</span><span class="kw">struct </span>SpeakerOptions {
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generics: syn::Generics,
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data: darling::ast::Data<darling::util::Ignored, SpeakerField>,
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}</code></pre></div>
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<p>At this point, you are able to call <code>uses_type_params</code> on <code>SpeakerOptions.data</code>, or any filtered
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view of it. <code>darling</code> internally uses this in conjunction with the <code>skip</code> meta-item to determine
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which type parameters don’t require the <code>FromMeta</code> bound in generated impls.</p>
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<p><strong>Note:</strong> If you are performing operations referencing generic params in meta-items parsed by <code>darling</code>,
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you should determine if those impact the emitted code and wire up <code>UsesTypeParams</code> accordingly for
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your field/variant.</p>
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</div></details><h2 id="structs" class="section-header">Structs<a href="#structs" class="anchor">§</a></h2><dl class="item-table"><dt><a class="struct" href="struct.Options.html" title="struct darling_core::usage::Options">Options</a></dt><dd>Control struct for searching type parameters.</dd></dl><h2 id="enums" class="section-header">Enums<a href="#enums" class="anchor">§</a></h2><dl class="item-table"><dt><a class="enum" href="enum.Purpose.html" title="enum darling_core::usage::Purpose">Purpose</a></dt><dd>The goal of tracing generic parameter usage.</dd></dl><h2 id="traits" class="section-header">Traits<a href="#traits" class="anchor">§</a></h2><dl class="item-table"><dt><a class="trait" href="trait.CollectLifetimes.html" title="trait darling_core::usage::CollectLifetimes">Collect<wbr>Lifetimes</a></dt><dd>Searcher for finding lifetimes in an iterator.</dd><dt><a class="trait" href="trait.CollectTypeParams.html" title="trait darling_core::usage::CollectTypeParams">Collect<wbr>Type<wbr>Params</a></dt><dd>Searcher for finding type params in an iterator.</dd><dt><a class="trait" href="trait.GenericsExt.html" title="trait darling_core::usage::GenericsExt">Generics<wbr>Ext</a></dt><dd>Extension trait for pulling specific generics data from a generics AST representation.</dd><dt><a class="trait" href="trait.UsesLifetimes.html" title="trait darling_core::usage::UsesLifetimes">Uses<wbr>Lifetimes</a></dt><dd>Searcher for finding lifetimes in a syntax tree.
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This can be used to determine which lifetimes must be emitted in generated code.</dd><dt><a class="trait" href="trait.UsesTypeParams.html" title="trait darling_core::usage::UsesTypeParams">Uses<wbr>Type<wbr>Params</a></dt><dd>Searcher for finding type params in a syntax tree.
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This can be used to determine if a given type parameter needs to be bounded in a generated impl.</dd></dl><h2 id="types" class="section-header">Type Aliases<a href="#types" class="anchor">§</a></h2><dl class="item-table"><dt><a class="type" href="type.IdentRefSet.html" title="type darling_core::usage::IdentRefSet">Ident<wbr>RefSet</a></dt><dd>A set of references to idents.</dd><dt><a class="type" href="type.IdentSet.html" title="type darling_core::usage::IdentSet">Ident<wbr>Set</a></dt><dd>A set of idents.</dd><dt><a class="type" href="type.LifetimeRefSet.html" title="type darling_core::usage::LifetimeRefSet">Lifetime<wbr>RefSet</a></dt><dd>A set of references to lifetimes.</dd><dt><a class="type" href="type.LifetimeSet.html" title="type darling_core::usage::LifetimeSet">Lifetime<wbr>Set</a></dt><dd>A set of lifetimes.</dd></dl></section></div></main></body></html> |