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<p class='location'>Trait DeserializeSeed</p><div class="block items"><ul><li><a href="#associated-types">Associated Types</a></li><li><a href="#required-methods">Required Methods</a></li><li><a href="#implementors">Implementors</a></li></ul></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>
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<h1 class='fqn'><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><span class='out-of-band'><span id='render-detail'>
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</span><a class='srclink' href='../../src/serde/de/mod.rs.html#680-689' title='goto source code'>[src]</a></span></h1>
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<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 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 class="rust rust-example-rendered">
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<span class="kw">fn</span> <span class="ident">func</span><span class="op"><</span><span class="lifetime">'de</span>, <span class="ident">T</span>: <span class="ident">Deserialize</span><span class="op"><</span><span class="lifetime">'de</span><span class="op">>></span>() <span class="op">-></span> <span class="prelude-ty">Result</span><span class="op"><</span><span class="ident">T</span>, <span class="ident">Error</span><span class="op">></span></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 class="rust rust-example-rendered">
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<span class="kw">fn</span> <span class="ident">func_seed</span><span class="op"><</span><span class="lifetime">'de</span>, <span class="ident">T</span>: <span class="ident">DeserializeSeed</span><span class="op"><</span><span class="lifetime">'de</span><span class="op">>></span>(<span class="ident">seed</span>: <span class="ident">T</span>) <span class="op">-></span> <span class="prelude-ty">Result</span><span class="op"><</span><span class="ident">T</span>::<span class="ident">Value</span>, <span class="ident">Error</span><span class="op">></span></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='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 class="rust rust-example-rendered">
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<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">fmt</span>;
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<span class="kw">use</span> <span class="ident">std</span>::<span class="ident">marker</span>::<span class="ident">PhantomData</span>;
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<span class="kw">use</span> <span class="ident">serde</span>::<span class="ident">de</span>::{<span class="ident">Deserialize</span>, <span class="ident">DeserializeSeed</span>, <span class="ident">Deserializer</span>, <span class="ident">Visitor</span>, <span class="ident">SeqAccess</span>};
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<span class="comment">// A DeserializeSeed implementation that uses stateful deserialization to</span>
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<span class="comment">// append array elements onto the end of an existing vector. The preexisting</span>
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<span class="comment">// state ("seed") in this case is the Vec<T>. The `deserialize` method of</span>
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<span class="comment">// `ExtendVec` will be traversing the inner arrays of the JSON input and</span>
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<span class="comment">// appending each integer into the existing Vec.</span>
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<span class="kw">struct</span> <span class="ident">ExtendVec</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">T</span>: <span class="lifetime">'a</span><span class="op">></span>(<span class="kw-2">&</span><span class="lifetime">'a</span> <span class="kw-2">mut</span> <span class="ident">Vec</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>);
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<span class="kw">impl</span><span class="op"><</span><span class="lifetime">'de</span>, <span class="lifetime">'a</span>, <span class="ident">T</span><span class="op">></span> <span class="ident">DeserializeSeed</span><span class="op"><</span><span class="lifetime">'de</span><span class="op">></span> <span class="kw">for</span> <span class="ident">ExtendVec</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">T</span><span class="op">></span>
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<span class="kw">where</span> <span class="ident">T</span>: <span class="ident">Deserialize</span><span class="op"><</span><span class="lifetime">'de</span><span class="op">></span>
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{
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<span class="comment">// The return type of the `deserialize` method. This implementation</span>
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<span class="comment">// appends onto an existing vector but does not create any new data</span>
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<span class="comment">// structure, so the return type is ().</span>
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<span class="kw">type</span> <span class="ident">Value</span> <span class="op">=</span> ();
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<span class="kw">fn</span> <span class="ident">deserialize</span><span class="op"><</span><span class="ident">D</span><span class="op">></span>(<span class="self">self</span>, <span class="ident">deserializer</span>: <span class="ident">D</span>) <span class="op">-></span> <span class="prelude-ty">Result</span><span class="op"><</span><span class="self">Self</span>::<span class="ident">Value</span>, <span class="ident">D</span>::<span class="ident">Error</span><span class="op">></span>
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<span class="kw">where</span> <span class="ident">D</span>: <span class="ident">Deserializer</span><span class="op"><</span><span class="lifetime">'de</span><span class="op">></span>
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{
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<span class="comment">// Visitor implementation that will walk an inner array of the JSON</span>
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<span class="comment">// input.</span>
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<span class="kw">struct</span> <span class="ident">ExtendVecVisitor</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">T</span>: <span class="lifetime">'a</span><span class="op">></span>(<span class="kw-2">&</span><span class="lifetime">'a</span> <span class="kw-2">mut</span> <span class="ident">Vec</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>);
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<span class="kw">impl</span><span class="op"><</span><span class="lifetime">'de</span>, <span class="lifetime">'a</span>, <span class="ident">T</span><span class="op">></span> <span class="ident">Visitor</span><span class="op"><</span><span class="lifetime">'de</span><span class="op">></span> <span class="kw">for</span> <span class="ident">ExtendVecVisitor</span><span class="op"><</span><span class="lifetime">'a</span>, <span class="ident">T</span><span class="op">></span>
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<span class="kw">where</span> <span class="ident">T</span>: <span class="ident">Deserialize</span><span class="op"><</span><span class="lifetime">'de</span><span class="op">></span>
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{
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<span class="kw">type</span> <span class="ident">Value</span> <span class="op">=</span> ();
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<span class="kw">fn</span> <span class="ident">expecting</span>(<span class="kw-2">&</span><span class="self">self</span>, <span class="ident">formatter</span>: <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">fmt</span>::<span class="ident">Formatter</span>) <span class="op">-></span> <span class="ident">fmt</span>::<span class="prelude-ty">Result</span> {
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<span class="macro">write</span><span class="macro">!</span>(<span class="ident">formatter</span>, <span class="string">"an array of integers"</span>)
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}
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<span class="kw">fn</span> <span class="ident">visit_seq</span><span class="op"><</span><span class="ident">A</span><span class="op">></span>(<span class="self">self</span>, <span class="kw-2">mut</span> <span class="ident">seq</span>: <span class="ident">A</span>) <span class="op">-></span> <span class="prelude-ty">Result</span><span class="op"><</span>(), <span class="ident">A</span>::<span class="ident">Error</span><span class="op">></span>
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<span class="kw">where</span> <span class="ident">A</span>: <span class="ident">SeqAccess</span><span class="op"><</span><span class="lifetime">'de</span><span class="op">></span>
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{
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<span class="comment">// Visit each element in the inner array and push it onto</span>
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<span class="comment">// the existing vector.</span>
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<span class="kw">while</span> <span class="kw">let</span> <span class="prelude-val">Some</span>(<span class="ident">elem</span>) <span class="op">=</span> <span class="ident">seq</span>.<span class="ident">next_element</span>()<span class="question-mark">?</span> {
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<span class="self">self</span>.<span class="number">0</span>.<span class="ident">push</span>(<span class="ident">elem</span>);
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}
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<span class="prelude-val">Ok</span>(())
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}
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}
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<span class="ident">deserializer</span>.<span class="ident">deserialize_seq</span>(<span class="ident">ExtendVecVisitor</span>(<span class="self">self</span>.<span class="number">0</span>))
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}
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}
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<span class="comment">// Visitor implementation that will walk the outer array of the JSON input.</span>
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<span class="kw">struct</span> <span class="ident">FlattenedVecVisitor</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>(<span class="ident">PhantomData</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>);
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<span class="kw">impl</span><span class="op"><</span><span class="lifetime">'de</span>, <span class="ident">T</span><span class="op">></span> <span class="ident">Visitor</span><span class="op"><</span><span class="lifetime">'de</span><span class="op">></span> <span class="kw">for</span> <span class="ident">FlattenedVecVisitor</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>
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<span class="kw">where</span> <span class="ident">T</span>: <span class="ident">Deserialize</span><span class="op"><</span><span class="lifetime">'de</span><span class="op">></span>
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{
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<span class="comment">// This Visitor constructs a single Vec<T> to hold the flattened</span>
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<span class="comment">// contents of the inner arrays.</span>
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<span class="kw">type</span> <span class="ident">Value</span> <span class="op">=</span> <span class="ident">Vec</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>;
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<span class="kw">fn</span> <span class="ident">expecting</span>(<span class="kw-2">&</span><span class="self">self</span>, <span class="ident">formatter</span>: <span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">fmt</span>::<span class="ident">Formatter</span>) <span class="op">-></span> <span class="ident">fmt</span>::<span class="prelude-ty">Result</span> {
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<span class="macro">write</span><span class="macro">!</span>(<span class="ident">formatter</span>, <span class="string">"an array of arrays"</span>)
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}
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<span class="kw">fn</span> <span class="ident">visit_seq</span><span class="op"><</span><span class="ident">A</span><span class="op">></span>(<span class="self">self</span>, <span class="kw-2">mut</span> <span class="ident">seq</span>: <span class="ident">A</span>) <span class="op">-></span> <span class="prelude-ty">Result</span><span class="op"><</span><span class="ident">Vec</span><span class="op"><</span><span class="ident">T</span><span class="op">></span>, <span class="ident">A</span>::<span class="ident">Error</span><span class="op">></span>
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<span class="kw">where</span> <span class="ident">A</span>: <span class="ident">SeqAccess</span><span class="op"><</span><span class="lifetime">'de</span><span class="op">></span>
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{
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<span class="comment">// Create a single Vec to hold the flattened contents.</span>
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<span class="kw">let</span> <span class="kw-2">mut</span> <span class="ident">vec</span> <span class="op">=</span> <span class="ident">Vec</span>::<span class="ident">new</span>();
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<span class="comment">// Each iteration through this loop is one inner array.</span>
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<span class="kw">while</span> <span class="kw">let</span> <span class="prelude-val">Some</span>(()) <span class="op">=</span> <span class="ident">seq</span>.<span class="ident">next_element_seed</span>(<span class="ident">ExtendVec</span>(<span class="kw-2">&</span><span class="kw-2">mut</span> <span class="ident">vec</span>))<span class="question-mark">?</span> {
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<span class="comment">// Nothing to do; inner array has been appended into `vec`.</span>
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}
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<span class="comment">// Return the finished vec.</span>
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<span class="prelude-val">Ok</span>(<span class="ident">vec</span>)
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}
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}
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<span class="kw">let</span> <span class="ident">visitor</span> <span class="op">=</span> <span class="ident">FlattenedVecVisitor</span>(<span class="ident">PhantomData</span>);
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<span class="kw">let</span> <span class="ident">flattened</span>: <span class="ident">Vec</span><span class="op"><</span><span class="ident">u64</span><span class="op">></span> <span class="op">=</span> <span class="ident">deserializer</span>.<span class="ident">deserialize_seq</span>(<span class="ident">visitor</span>)<span class="question-mark">?</span>;</pre>
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</div>
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<h2 id='associated-types'>Associated Types</h2>
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<div class='methods'>
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<h3 id='associatedtype.Value' class='method'><span id='Value.t' class='invisible'><code>type <a href='#associatedtype.Value' class="type">Value</a></code></span></h3><div class='docblock'><p>The type produced by using this seed.</p>
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</div></div>
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<h2 id='required-methods'>Required Methods</h2>
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<div class='methods'>
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<h3 id='tymethod.deserialize' class='method'><span id='deserialize.v' class='invisible'><code>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></span></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>
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<h2 id='implementors'>Implementors</h2>
|
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<ul class='item-list' id='implementors-list'>
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<li><code>impl<'de, T> DeserializeSeed<'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></li>
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</ul><script type="text/javascript" async
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