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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 `DeserializeSeed` trait in crate `serde`."><meta name="keywords" content="rust, rustlang, rust-lang, DeserializeSeed"><title>serde::de::DeserializeSeed - 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 trait"><!--[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='../../serde/index.html'><div class='logo-container'><img src='../../rust-logo.png' alt='logo'></div></a><p class='location'>Trait DeserializeSeed</p><div class="sidebar-elems"><div class="block items"><a class="sidebar-title" href="#associated-types">Associated Types</a><div class="sidebar-links"><a href="#associatedtype.Value">Value</a></div><a class="sidebar-title" href="#required-methods">Required Methods</a><div class="sidebar-links"><a href="#tymethod.deserialize">deserialize</a></div><a class="sidebar-title" href="#foreign-impls">Implementations on Foreign Types</a><div class="sidebar-links"><a href="#impl-DeserializeSeed%3C%27de%3E-for-PhantomData%3CT%3E">PhantomData<T></a></div><a class="sidebar-title" href="#implementors">Implementors</a></div><p class='location'><a href='../index.html'>serde</a>::<wbr><a href='index.html'>de</a></p><script>window.sidebarCurrent = {name: 'DeserializeSeed', ty: 'trait', 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'>−</span>]</a></span><a class='srclink' href='../../src/serde/de/mod.rs.html#757-766' title='goto source code'>[src]</a></span><span class='in-band'>Trait <a href='../index.html'>serde</a>::<wbr><a href='index.html'>de</a>::<wbr><a class="trait" href=''>DeserializeSeed</a></span></h1><div class="docblock type-decl hidden-by-usual-hider"><pre class='rust trait'>pub trait DeserializeSeed<'de>: <a class="trait" href="https://doc.rust-lang.org/nightly/core/marker/trait.Sized.html" title="trait core::marker::Sized">Sized</a> {
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type <a href='#associatedtype.Value' class="type">Value</a>;
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fn <a href='#tymethod.deserialize' class='fnname'>deserialize</a><D>(self, deserializer: D) -> <a class="enum" href="https://doc.rust-lang.org/nightly/core/result/enum.Result.html" title="enum core::result::Result">Result</a><Self::<a class="type" href="../../serde/de/trait.DeserializeSeed.html#associatedtype.Value" title="type serde::de::DeserializeSeed::Value">Value</a>, D::<a class="type" href="../../serde/trait.Deserializer.html#associatedtype.Error" title="type serde::Deserializer::Error">Error</a>><br> <span class="where">where<br> D: <a class="trait" href="../../serde/trait.Deserializer.html" title="trait serde::Deserializer">Deserializer</a><'de></span>;
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}</pre></div><div class='docblock'><p><code>DeserializeSeed</code> is the stateful form of the <code>Deserialize</code> trait. If you
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ever find yourself looking for a way to pass data into a <code>Deserialize</code> impl,
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this trait is the way to do it.</p>
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<p>As one example of stateful deserialization consider deserializing a JSON
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array into an existing buffer. Using the <code>Deserialize</code> trait we could
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deserialize a JSON array into a <code>Vec<T></code> but it would be a freshly allocated
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<code>Vec<T></code>; there is no way for <code>Deserialize</code> to reuse a previously allocated
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buffer. Using <code>DeserializeSeed</code> instead makes this possible as in the
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example code below.</p>
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<p>The canonical API for stateless deserialization looks like this:</p>
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<pre><code class="language-edition2018"># use serde::Deserialize;
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#
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# enum Error {}
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#
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fn func<'de, T: Deserialize<'de>>() -> Result<T, Error>
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# {
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# unimplemented!()
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# }
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</code></pre>
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<p>Adjusting an API like this to support stateful deserialization is a matter
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of accepting a seed as input:</p>
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<pre><code class="language-edition2018"># use serde::de::DeserializeSeed;
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#
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# enum Error {}
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#
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fn func_seed<'de, T: DeserializeSeed<'de>>(seed: T) -> Result<T::Value, Error>
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# {
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# let _ = seed;
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# unimplemented!()
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# }
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</code></pre>
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<p>In practice the majority of deserialization is stateless. An API expecting a
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seed can be appeased by passing <code>std::marker::PhantomData</code> as a seed in the
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case of stateless deserialization.</p>
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<h1 id="lifetime" class="section-header"><a href="#lifetime">Lifetime</a></h1>
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<p>The <code>'de</code> lifetime of this trait is the lifetime of data that may be
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borrowed by <code>Self::Value</code> when deserialized. See the page <a href="https://serde.rs/lifetimes.html">Understanding
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deserializer lifetimes</a> for a more detailed explanation of these lifetimes.</p>
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<h1 id="example" class="section-header"><a href="#example">Example</a></h1>
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<p>Suppose we have JSON that looks like <code>[[1, 2], [3, 4, 5], [6]]</code> and we need
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to deserialize it into a flat representation like <code>vec![1, 2, 3, 4, 5, 6]</code>.
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Allocating a brand new <code>Vec<T></code> for each subarray would be slow. Instead we
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would like to allocate a single <code>Vec<T></code> and then deserialize each subarray
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into it. This requires stateful deserialization using the <code>DeserializeSeed</code>
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trait.</p>
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<pre><code class="language-edition2018">use std::fmt;
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use std::marker::PhantomData;
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use serde::de::{Deserialize, DeserializeSeed, Deserializer, SeqAccess, Visitor};
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// A DeserializeSeed implementation that uses stateful deserialization to
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// append array elements onto the end of an existing vector. The preexisting
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// state ("seed") in this case is the Vec<T>. The `deserialize` method of
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// `ExtendVec` will be traversing the inner arrays of the JSON input and
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// appending each integer into the existing Vec.
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struct ExtendVec<'a, T: 'a>(&'a mut Vec<T>);
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impl<'de, 'a, T> DeserializeSeed<'de> for ExtendVec<'a, T>
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where
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T: Deserialize<'de>,
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{
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// The return type of the `deserialize` method. This implementation
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// appends onto an existing vector but does not create any new data
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// structure, so the return type is ().
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type Value = ();
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fn deserialize<D>(self, deserializer: D) -> Result<Self::Value, D::Error>
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where
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D: Deserializer<'de>,
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{
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// Visitor implementation that will walk an inner array of the JSON
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// input.
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struct ExtendVecVisitor<'a, T: 'a>(&'a mut Vec<T>);
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impl<'de, 'a, T> Visitor<'de> for ExtendVecVisitor<'a, T>
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where
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T: Deserialize<'de>,
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{
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type Value = ();
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fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
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write!(formatter, "an array of integers")
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}
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fn visit_seq<A>(self, mut seq: A) -> Result<(), A::Error>
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where
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A: SeqAccess<'de>,
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{
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// Visit each element in the inner array and push it onto
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// the existing vector.
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while let Some(elem) = seq.next_element()? {
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self.0.push(elem);
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}
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Ok(())
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}
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}
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deserializer.deserialize_seq(ExtendVecVisitor(self.0))
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}
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}
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// Visitor implementation that will walk the outer array of the JSON input.
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struct FlattenedVecVisitor<T>(PhantomData<T>);
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impl<'de, T> Visitor<'de> for FlattenedVecVisitor<T>
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where
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T: Deserialize<'de>,
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{
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// This Visitor constructs a single Vec<T> to hold the flattened
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// contents of the inner arrays.
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type Value = Vec<T>;
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fn expecting(&self, formatter: &mut fmt::Formatter) -> fmt::Result {
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write!(formatter, "an array of arrays")
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}
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fn visit_seq<A>(self, mut seq: A) -> Result<Vec<T>, A::Error>
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where
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A: SeqAccess<'de>,
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{
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// Create a single Vec to hold the flattened contents.
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let mut vec = Vec::new();
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// Each iteration through this loop is one inner array.
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while let Some(()) = seq.next_element_seed(ExtendVec(&mut vec))? {
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// Nothing to do; inner array has been appended into `vec`.
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}
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// Return the finished vec.
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Ok(vec)
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}
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}
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# fn example<'de, D>(deserializer: D) -> Result<(), D::Error>
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# where
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# D: Deserializer<'de>,
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# {
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let visitor = FlattenedVecVisitor(PhantomData);
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let flattened: Vec<u64> = deserializer.deserialize_seq(visitor)?;
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# Ok(())
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# }
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</code></pre>
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</div>
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<h2 id='associated-types' class='small-section-header'>Associated Types<a href='#associated-types' class='anchor'></a></h2><div class='methods'><h3 id='associatedtype.Value' class='method'><code id='Value.t'>type <a href='#associatedtype.Value' class="type">Value</a></code></h3><div class='docblock'><p>The type produced by using this seed.</p>
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</div></div><span class='loading-content'>Loading content...</span>
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<h2 id='required-methods' class='small-section-header'>Required methods<a href='#required-methods' class='anchor'></a></h2><div class='methods'><h3 id='tymethod.deserialize' class='method'><code id='deserialize.v'>fn <a href='#tymethod.deserialize' class='fnname'>deserialize</a><D>(self, deserializer: D) -> <a class="enum" href="https://doc.rust-lang.org/nightly/core/result/enum.Result.html" title="enum core::result::Result">Result</a><Self::<a class="type" href="../../serde/de/trait.DeserializeSeed.html#associatedtype.Value" title="type serde::de::DeserializeSeed::Value">Value</a>, D::<a class="type" href="../../serde/trait.Deserializer.html#associatedtype.Error" title="type serde::Deserializer::Error">Error</a>> <span class="where fmt-newline">where<br> D: <a class="trait" href="../../serde/trait.Deserializer.html" title="trait serde::Deserializer">Deserializer</a><'de>, </span></code></h3><div class='docblock'><p>Equivalent to the more common <code>Deserialize::deserialize</code> method, except
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with some initial piece of data (the seed) passed in.</p>
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</div></div><span class='loading-content'>Loading content...</span>
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<h2 id='foreign-impls' class='small-section-header'>Implementations on Foreign Types<a href='#foreign-impls' class='anchor'></a></h2><h3 id='impl-DeserializeSeed%3C%27de%3E-for-PhantomData%3CT%3E' class='impl'><code class='in-band'>impl<'de, T> <a class="trait" href="../../serde/de/trait.DeserializeSeed.html" title="trait serde::de::DeserializeSeed">DeserializeSeed</a><'de> for <a class="struct" href="https://doc.rust-lang.org/nightly/core/marker/struct.PhantomData.html" title="struct core::marker::PhantomData">PhantomData</a><T> <span class="where fmt-newline">where<br> T: <a class="trait" href="../../serde/trait.Deserialize.html" title="trait serde::Deserialize">Deserialize</a><'de>, </span></code><a href='#impl-DeserializeSeed%3C%27de%3E-for-PhantomData%3CT%3E' class='anchor'></a><a class='srclink' href='../../src/serde/de/mod.rs.html#768-781' title='goto source code'>[src]</a></h3><div class='impl-items'><h4 id='associatedtype.Value-1' class="type"><code id='Value.t-1'>type <a href='#associatedtype.Value' class="type">Value</a> = T</code></h4><h4 id='method.deserialize' class="method hidden"><code id='deserialize.v-1'>fn <a href='#method.deserialize' class='fnname'>deserialize</a><D>(self, deserializer: D) -> <a class="enum" href="https://doc.rust-lang.org/nightly/core/result/enum.Result.html" title="enum core::result::Result">Result</a><T, D::<a class="type" href="../../serde/trait.Deserializer.html#associatedtype.Error" title="type serde::Deserializer::Error">Error</a>> <span class="where fmt-newline">where<br> D: <a class="trait" href="../../serde/trait.Deserializer.html" title="trait serde::Deserializer">Deserializer</a><'de>, </span></code><a class='srclink' href='../../src/serde/de/mod.rs.html#775-780' title='goto source code'>[src]</a></h4></div><span class='loading-content'>Loading content...</span>
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<h2 id='implementors' class='small-section-header'>Implementors<a href='#implementors' class='anchor'></a></h2><div class='item-list' id='implementors-list'></div><span class='loading-content'>Loading content...</span><script type="text/javascript">window.inlined_types=new Set([]);</script><script type="text/javascript" async
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src="../../implementors/serde/de/trait.DeserializeSeed.js">
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</script></section><section id="search" class="content hidden"></section><section class="footer"></section><aside id="help" class="hidden"><div><h1 class="hidden">Help</h1><div class="shortcuts"><h2>Keyboard Shortcuts</h2><dl><dt><kbd>?</kbd></dt><dd>Show this help dialog</dd><dt><kbd>S</kbd></dt><dd>Focus the search field</dd><dt><kbd>↑</kbd></dt><dd>Move up in search results</dd><dt><kbd>↓</kbd></dt><dd>Move down in search results</dd><dt><kbd>↹</kbd></dt><dd>Switch tab</dd><dt><kbd>⏎</kbd></dt><dd>Go to active search result</dd><dt><kbd>+</kbd></dt><dd>Expand all sections</dd><dt><kbd>-</kbd></dt><dd>Collapse all sections</dd></dl></div><div class="infos"><h2>Search Tricks</h2><p>Prefix searches with a type followed by a colon (e.g., <code>fn:</code>) to restrict the search to a given type.</p><p>Accepted types are: <code>fn</code>, <code>mod</code>, <code>struct</code>, <code>enum</code>, <code>trait</code>, <code>type</code>, <code>macro</code>, and <code>const</code>.</p><p>Search functions by type signature (e.g., <code>vec -> usize</code> or <code>* -> vec</code>)</p><p>Search multiple things at once by splitting your query with comma (e.g., <code>str,u8</code> or <code>String,struct:Vec,test</code>)</p></div></div></aside><script>window.rootPath = "../../";window.currentCrate = "serde";</script><script src="../../aliases.js"></script><script src="../../main.js"></script><script defer src="../../search-index.js"></script></body></html>
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