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<a href="." >Implementing a Deserializer</a>
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<h1 id="implementing-a-deserializer"><a name="implementing-a-deserializer" class="plugin-anchor" href="#implementing-a-deserializer"><i class="fa fa-link" aria-hidden="true"></i></a>Implementing a Deserializer</h1>
<p>This page gives a basic but functional implementation of a JSON deserializer
using Serde.</p>
<p>As with the serializer, the <a href="https://docs.rs/serde/1/serde/de/trait.Deserializer.html" target="_blank"><code>Deserializer</code></a> trait has a lot of methods but none
of them are complicated in this implementation. The deserializer is responsible
for mapping the input data into <a href="data-model.html">Serde's data model</a> by invoking exactly one of
the methods on the <a href="https://docs.rs/serde/1/serde/de/trait.Visitor.html" target="_blank"><code>Visitor</code></a> that it receives.</p>
<p>The <code>Deserializer</code> methods are called by a <code>Deserialize</code> impl as a hint to
indicate what Serde data model type the <code>Deserialize</code> type expects to see in the
input. For self-describing formats like JSON, it is fine for the <code>Deserializer</code>
to ignore this hint and just call whichever <code>Visitor</code> method corresponds to the
content of the input data. Other formats, especially compact binary formats like
Postcard, rely on the hint to determine how the input data is to be interpreted.</p>
<p>Self-describing formats can save a lot of code by using the
<a href="https://docs.rs/serde/1/serde/macro.forward_to_deserialize_any.html" target="_blank"><code>forward_to_deserialize_any!</code></a> macro to ignore hints and forward some or all of
the methods of the <code>Deserializer</code> trait to the <code>deserialize_any</code> method.</p>
<p>The code below implements every method explicitly for documentation purposes but
there is no advantage to that.</p>
<p><a href="lifetimes.html">Deserializer lifetimes</a> have their own dedicated page.</p>
<div><p class="code-filename">src/de.rs</p></div>
<pre><code class="lang-rust"><span class="hljs-keyword">use</span> std::ops::{AddAssign, MulAssign, Neg};
<span class="hljs-keyword">use</span> serde::Deserialize;
<span class="hljs-keyword">use</span> serde::de::{
<span class="hljs-keyword">self</span>, DeserializeSeed, EnumAccess, IntoDeserializer, MapAccess, SeqAccess,
VariantAccess, Visitor,
};
<span class="hljs-keyword">use</span> error::{Error, <span class="hljs-built_in">Result</span>};
<span class="hljs-keyword">pub</span> <span class="hljs-class"><span class="hljs-keyword">struct</span> <span class="hljs-title">Deserializer</span></span><<span class="hljs-symbol">'de</span>> {
<span class="hljs-comment">// This string starts with the input data and characters are truncated off</span>
<span class="hljs-comment">// the beginning as data is parsed.</span>
input: &<span class="hljs-symbol">'de</span> <span class="hljs-keyword">str</span>,
}
<span class="hljs-keyword">impl</span><<span class="hljs-symbol">'de</span>> Deserializer<<span class="hljs-symbol">'de</span>> {
<span class="hljs-comment">// By convention, `Deserializer` constructors are named like `from_xyz`.</span>
<span class="hljs-comment">// That way basic use cases are satisfied by something like</span>
<span class="hljs-comment">// `serde_json::from_str(...)` while advanced use cases that require a</span>
<span class="hljs-comment">// deserializer can make one with `serde_json::Deserializer::from_str(...)`.</span>
<span class="hljs-keyword">pub</span> <span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">from_str</span></span>(input: &<span class="hljs-symbol">'de</span> <span class="hljs-keyword">str</span>) -> <span class="hljs-keyword">Self</span> {
Deserializer { input }
}
}
<span class="hljs-comment">// By convention, the public API of a Serde deserializer is one or more</span>
<span class="hljs-comment">// `from_xyz` methods such as `from_str`, `from_bytes`, or `from_reader`</span>
<span class="hljs-comment">// depending on what Rust types the deserializer is able to consume as input.</span>
<span class="hljs-comment">//</span>
<span class="hljs-comment">// This basic deserializer supports only `from_str`.</span>
<span class="hljs-keyword">pub</span> <span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">from_str</span></span><<span class="hljs-symbol">'a</span>, T>(s: &<span class="hljs-symbol">'a</span> <span class="hljs-keyword">str</span>) -> <span class="hljs-built_in">Result</span><T>
<span class="hljs-keyword">where</span>
T: Deserialize<<span class="hljs-symbol">'a</span>>,
{
<span class="hljs-keyword">let</span> <span class="hljs-keyword">mut</span> deserializer = Deserializer::from_str(s);
<span class="hljs-keyword">let</span> t = T::deserialize(&<span class="hljs-keyword">mut</span> deserializer)?;
<span class="hljs-keyword">if</span> deserializer.input.is_empty() {
<span class="hljs-literal">Ok</span>(t)
} <span class="hljs-keyword">else</span> {
<span class="hljs-literal">Err</span>(Error::TrailingCharacters)
}
}
<span class="hljs-comment">// SERDE IS NOT A PARSING LIBRARY. This impl block defines a few basic parsing</span>
<span class="hljs-comment">// functions from scratch. More complicated formats may wish to use a dedicated</span>
<span class="hljs-comment">// parsing library to help implement their Serde deserializer.</span>
<span class="hljs-keyword">impl</span><<span class="hljs-symbol">'de</span>> Deserializer<<span class="hljs-symbol">'de</span>> {
<span class="hljs-comment">// Look at the first character in the input without consuming it.</span>
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">peek_char</span></span>(&<span class="hljs-keyword">mut</span> <span class="hljs-keyword">self</span>) -> <span class="hljs-built_in">Result</span><<span class="hljs-keyword">char</span>> {
<span class="hljs-keyword">self</span>.input.chars().next().ok_or(Error::Eof)
}
<span class="hljs-comment">// Consume the first character in the input.</span>
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">next_char</span></span>(&<span class="hljs-keyword">mut</span> <span class="hljs-keyword">self</span>) -> <span class="hljs-built_in">Result</span><<span class="hljs-keyword">char</span>> {
<span class="hljs-keyword">let</span> ch = <span class="hljs-keyword">self</span>.peek_char()?;
<span class="hljs-keyword">self</span>.input = &<span class="hljs-keyword">self</span>.input[ch.len_utf8()..];
<span class="hljs-literal">Ok</span>(ch)
}
<span class="hljs-comment">// Parse the JSON identifier `true` or `false`.</span>
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">parse_bool</span></span>(&<span class="hljs-keyword">mut</span> <span class="hljs-keyword">self</span>) -> <span class="hljs-built_in">Result</span><<span class="hljs-keyword">bool</span>> {
<span class="hljs-keyword">if</span> <span class="hljs-keyword">self</span>.input.starts_with(<span class="hljs-string">"true"</span>) {
<span class="hljs-keyword">self</span>.input = &<span class="hljs-keyword">self</span>.input[<span class="hljs-string">"true"</span>.len()..];
<span class="hljs-literal">Ok</span>(<span class="hljs-literal">true</span>)
} <span class="hljs-keyword">else</span> <span class="hljs-keyword">if</span> <span class="hljs-keyword">self</span>.input.starts_with(<span class="hljs-string">"false"</span>) {
<span class="hljs-keyword">self</span>.input = &<span class="hljs-keyword">self</span>.input[<span class="hljs-string">"false"</span>.len()..];
<span class="hljs-literal">Ok</span>(<span class="hljs-literal">false</span>)
} <span class="hljs-keyword">else</span> {
<span class="hljs-literal">Err</span>(Error::ExpectedBoolean)
}
}
<span class="hljs-comment">// Parse a group of decimal digits as an unsigned integer of type T.</span>
<span class="hljs-comment">//</span>
<span class="hljs-comment">// This implementation is a bit too lenient, for example `001` is not</span>
<span class="hljs-comment">// allowed in JSON. Also the various arithmetic operations can overflow and</span>
<span class="hljs-comment">// panic or return bogus data. But it is good enough for example code!</span>
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">parse_unsigned</span></span><T>(&<span class="hljs-keyword">mut</span> <span class="hljs-keyword">self</span>) -> <span class="hljs-built_in">Result</span><T>
<span class="hljs-keyword">where</span>
T: AddAssign<T> + MulAssign<T> + <span class="hljs-built_in">From</span><<span class="hljs-keyword">u8</span>>,
{
<span class="hljs-keyword">let</span> <span class="hljs-keyword">mut</span> <span class="hljs-keyword">int</span> = <span class="hljs-keyword">match</span> <span class="hljs-keyword">self</span>.next_char()? {
ch @ <span class="hljs-string">'0'</span>..=<span class="hljs-string">'9'</span> => T::from(ch <span class="hljs-keyword">as</span> <span class="hljs-keyword">u8</span> - <span class="hljs-string">b'0'</span>),
_ => {
<span class="hljs-keyword">return</span> <span class="hljs-literal">Err</span>(Error::ExpectedInteger);
}
};
<span class="hljs-keyword">loop</span> {
<span class="hljs-keyword">match</span> <span class="hljs-keyword">self</span>.input.chars().next() {
<span class="hljs-literal">Some</span>(ch @ <span class="hljs-string">'0'</span>..=<span class="hljs-string">'9'</span>) => {
<span class="hljs-keyword">self</span>.input = &<span class="hljs-keyword">self</span>.input[<span class="hljs-number">1</span>..];
<span class="hljs-keyword">int</span> *= T::from(<span class="hljs-number">10</span>);
<span class="hljs-keyword">int</span> += T::from(ch <span class="hljs-keyword">as</span> <span class="hljs-keyword">u8</span> - <span class="hljs-string">b'0'</span>);
}
_ => {
<span class="hljs-keyword">return</span> <span class="hljs-literal">Ok</span>(<span class="hljs-keyword">int</span>);
}
}
}
}
<span class="hljs-comment">// Parse a possible minus sign followed by a group of decimal digits as a</span>
<span class="hljs-comment">// signed integer of type T.</span>
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">parse_signed</span></span><T>(&<span class="hljs-keyword">mut</span> <span class="hljs-keyword">self</span>) -> <span class="hljs-built_in">Result</span><T>
<span class="hljs-keyword">where</span>
T: Neg<Output = T> + AddAssign<T> + MulAssign<T> + <span class="hljs-built_in">From</span><<span class="hljs-keyword">i8</span>>,
{
<span class="hljs-comment">// Optional minus sign, delegate to `parse_unsigned`, negate if negative.</span>
<span class="hljs-built_in">unimplemented!</span>()
}
<span class="hljs-comment">// Parse a string until the next '"' character.</span>
<span class="hljs-comment">//</span>
<span class="hljs-comment">// Makes no attempt to handle escape sequences. What did you expect? This is</span>
<span class="hljs-comment">// example code!</span>
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">parse_string</span></span>(&<span class="hljs-keyword">mut</span> <span class="hljs-keyword">self</span>) -> <span class="hljs-built_in">Result</span><&<span class="hljs-symbol">'de</span> <span class="hljs-keyword">str</span>> {
<span class="hljs-keyword">if</span> <span class="hljs-keyword">self</span>.next_char()? != <span class="hljs-string">'"'</span> {
<span class="hljs-keyword">return</span> <span class="hljs-literal">Err</span>(Error::ExpectedString);
}
<span class="hljs-keyword">match</span> <span class="hljs-keyword">self</span>.input.find(<span class="hljs-string">'"'</span>) {
<span class="hljs-literal">Some</span>(len) => {
<span class="hljs-keyword">let</span> s = &<span class="hljs-keyword">self</span>.input[..len];
<span class="hljs-keyword">self</span>.input = &<span class="hljs-keyword">self</span>.input[len + <span class="hljs-number">1</span>..];
<span class="hljs-literal">Ok</span>(s)
}
<span class="hljs-literal">None</span> => <span class="hljs-literal">Err</span>(Error::Eof),
}
}
}
<span class="hljs-keyword">impl</span><<span class="hljs-symbol">'de</span>, <span class="hljs-symbol">'a</span>> de::Deserializer<<span class="hljs-symbol">'de</span>> <span class="hljs-keyword">for</span> &<span class="hljs-symbol">'a</span> <span class="hljs-keyword">mut</span> Deserializer<<span class="hljs-symbol">'de</span>> {
<span class="hljs-class"><span class="hljs-keyword">type</span> <span class="hljs-title">Error</span></span> = Error;
<span class="hljs-comment">// Look at the input data to decide what Serde data model type to</span>
<span class="hljs-comment">// deserialize as. Not all data formats are able to support this operation.</span>
<span class="hljs-comment">// Formats that support `deserialize_any` are known as self-describing.</span>
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">deserialize_any</span></span><V>(<span class="hljs-keyword">self</span>, visitor: V) -> <span class="hljs-built_in">Result</span><V::Value>
<span class="hljs-keyword">where</span>
V: Visitor<<span class="hljs-symbol">'de</span>>,
{
<span class="hljs-keyword">match</span> <span class="hljs-keyword">self</span>.peek_char()? {
<span class="hljs-string">'n'</span> => <span class="hljs-keyword">self</span>.deserialize_unit(visitor),
<span class="hljs-string">'t'</span> | <span class="hljs-string">'f'</span> => <span class="hljs-keyword">self</span>.deserialize_bool(visitor),
<span class="hljs-string">'"'</span> => <span class="hljs-keyword">self</span>.deserialize_str(visitor),
<span class="hljs-string">'0'</span>..=<span class="hljs-string">'9'</span> => <span class="hljs-keyword">self</span>.deserialize_u64(visitor),
<span class="hljs-string">'-'</span> => <span class="hljs-keyword">self</span>.deserialize_i64(visitor),
<span class="hljs-string">'['</span> => <span class="hljs-keyword">self</span>.deserialize_seq(visitor),
<span class="hljs-string">'{'</span> => <span class="hljs-keyword">self</span>.deserialize_map(visitor),
_ => <span class="hljs-literal">Err</span>(Error::Syntax),
}
}
<span class="hljs-comment">// Uses the `parse_bool` parsing function defined above to read the JSON</span>
<span class="hljs-comment">// identifier `true` or `false` from the input.</span>
<span class="hljs-comment">//</span>
<span class="hljs-comment">// Parsing refers to looking at the input and deciding that it contains the</span>
<span class="hljs-comment">// JSON value `true` or `false`.</span>
<span class="hljs-comment">//</span>
<span class="hljs-comment">// Deserialization refers to mapping that JSON value into Serde's data</span>
<span class="hljs-comment">// model by invoking one of the `Visitor` methods. In the case of JSON and</span>
<span class="hljs-comment">// bool that mapping is straightforward so the distinction may seem silly,</span>
<span class="hljs-comment">// but in other cases Deserializers sometimes perform non-obvious mappings.</span>
<span class="hljs-comment">// For example the TOML format has a Datetime type and Serde's data model</span>
<span class="hljs-comment">// does not. In the `toml` crate, a Datetime in the input is deserialized by</span>
<span class="hljs-comment">// mapping it to a Serde data model "struct" type with a special name and a</span>
<span class="hljs-comment">// single field containing the Datetime represented as a string.</span>
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">deserialize_bool</span></span><V>(<span class="hljs-keyword">self</span>, visitor: V) -> <span class="hljs-built_in">Result</span><V::Value>
<span class="hljs-keyword">where</span>
V: Visitor<<span class="hljs-symbol">'de</span>>,
{
visitor.visit_bool(<span class="hljs-keyword">self</span>.parse_bool()?)
}
<span class="hljs-comment">// The `parse_signed` function is generic over the integer type `T` so here</span>
<span class="hljs-comment">// it is invoked with `T=i8`. The next 8 methods are similar.</span>
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">deserialize_i8</span></span><V>(<span class="hljs-keyword">self</span>, visitor: V) -> <span class="hljs-built_in">Result</span><V::Value>
<span class="hljs-keyword">where</span>
V: Visitor<<span class="hljs-symbol">'de</span>>,
{
visitor.visit_i8(<span class="hljs-keyword">self</span>.parse_signed()?)
}
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">deserialize_i16</span></span><V>(<span class="hljs-keyword">self</span>, visitor: V) -> <span class="hljs-built_in">Result</span><V::Value>
<span class="hljs-keyword">where</span>
V: Visitor<<span class="hljs-symbol">'de</span>>,
{
visitor.visit_i16(<span class="hljs-keyword">self</span>.parse_signed()?)
}
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">deserialize_i32</span></span><V>(<span class="hljs-keyword">self</span>, visitor: V) -> <span class="hljs-built_in">Result</span><V::Value>
<span class="hljs-keyword">where</span>
V: Visitor<<span class="hljs-symbol">'de</span>>,
{
visitor.visit_i32(<span class="hljs-keyword">self</span>.parse_signed()?)
}
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">deserialize_i64</span></span><V>(<span class="hljs-keyword">self</span>, visitor: V) -> <span class="hljs-built_in">Result</span><V::Value>
<span class="hljs-keyword">where</span>
V: Visitor<<span class="hljs-symbol">'de</span>>,
{
visitor.visit_i64(<span class="hljs-keyword">self</span>.parse_signed()?)
}
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">deserialize_u8</span></span><V>(<span class="hljs-keyword">self</span>, visitor: V) -> <span class="hljs-built_in">Result</span><V::Value>
<span class="hljs-keyword">where</span>
V: Visitor<<span class="hljs-symbol">'de</span>>,
{
visitor.visit_u8(<span class="hljs-keyword">self</span>.parse_unsigned()?)
}
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">deserialize_u16</span></span><V>(<span class="hljs-keyword">self</span>, visitor: V) -> <span class="hljs-built_in">Result</span><V::Value>
<span class="hljs-keyword">where</span>
V: Visitor<<span class="hljs-symbol">'de</span>>,
{
visitor.visit_u16(<span class="hljs-keyword">self</span>.parse_unsigned()?)
}
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">deserialize_u32</span></span><V>(<span class="hljs-keyword">self</span>, visitor: V) -> <span class="hljs-built_in">Result</span><V::Value>
<span class="hljs-keyword">where</span>
V: Visitor<<span class="hljs-symbol">'de</span>>,
{
visitor.visit_u32(<span class="hljs-keyword">self</span>.parse_unsigned()?)
}
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">deserialize_u64</span></span><V>(<span class="hljs-keyword">self</span>, visitor: V) -> <span class="hljs-built_in">Result</span><V::Value>
<span class="hljs-keyword">where</span>
V: Visitor<<span class="hljs-symbol">'de</span>>,
{
visitor.visit_u64(<span class="hljs-keyword">self</span>.parse_unsigned()?)
}
<span class="hljs-comment">// Float parsing is stupidly hard.</span>
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">deserialize_f32</span></span><V>(<span class="hljs-keyword">self</span>, _visitor: V) -> <span class="hljs-built_in">Result</span><V::Value>
<span class="hljs-keyword">where</span>
V: Visitor<<span class="hljs-symbol">'de</span>>,
{
<span class="hljs-built_in">unimplemented!</span>()
}
<span class="hljs-comment">// Float parsing is stupidly hard.</span>
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">deserialize_f64</span></span><V>(<span class="hljs-keyword">self</span>, _visitor: V) -> <span class="hljs-built_in">Result</span><V::Value>
<span class="hljs-keyword">where</span>
V: Visitor<<span class="hljs-symbol">'de</span>>,
{
<span class="hljs-built_in">unimplemented!</span>()
}
<span class="hljs-comment">// The `Serializer` implementation on the previous page serialized chars as</span>
<span class="hljs-comment">// single-character strings so handle that representation here.</span>
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">deserialize_char</span></span><V>(<span class="hljs-keyword">self</span>, _visitor: V) -> <span class="hljs-built_in">Result</span><V::Value>
<span class="hljs-keyword">where</span>
V: Visitor<<span class="hljs-symbol">'de</span>>,
{
<span class="hljs-comment">// Parse a string, check that it is one character, call `visit_char`.</span>
<span class="hljs-built_in">unimplemented!</span>()
}
<span class="hljs-comment">// Refer to the "Understanding deserializer lifetimes" page for information</span>
<span class="hljs-comment">// about the three deserialization flavors of strings in Serde.</span>
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">deserialize_str</span></span><V>(<span class="hljs-keyword">self</span>, visitor: V) -> <span class="hljs-built_in">Result</span><V::Value>
<span class="hljs-keyword">where</span>
V: Visitor<<span class="hljs-symbol">'de</span>>,
{
visitor.visit_borrowed_str(<span class="hljs-keyword">self</span>.parse_string()?)
}
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">deserialize_string</span></span><V>(<span class="hljs-keyword">self</span>, visitor: V) -> <span class="hljs-built_in">Result</span><V::Value>
<span class="hljs-keyword">where</span>
V: Visitor<<span class="hljs-symbol">'de</span>>,
{
<span class="hljs-keyword">self</span>.deserialize_str(visitor)
}
<span class="hljs-comment">// The `Serializer` implementation on the previous page serialized byte</span>
<span class="hljs-comment">// arrays as JSON arrays of bytes. Handle that representation here.</span>
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">deserialize_bytes</span></span><V>(<span class="hljs-keyword">self</span>, _visitor: V) -> <span class="hljs-built_in">Result</span><V::Value>
<span class="hljs-keyword">where</span>
V: Visitor<<span class="hljs-symbol">'de</span>>,
{
<span class="hljs-built_in">unimplemented!</span>()
}
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">deserialize_byte_buf</span></span><V>(<span class="hljs-keyword">self</span>, _visitor: V) -> <span class="hljs-built_in">Result</span><V::Value>
<span class="hljs-keyword">where</span>
V: Visitor<<span class="hljs-symbol">'de</span>>,
{
<span class="hljs-built_in">unimplemented!</span>()
}
<span class="hljs-comment">// An absent optional is represented as the JSON `null` and a present</span>
<span class="hljs-comment">// optional is represented as just the contained value.</span>
<span class="hljs-comment">//</span>
<span class="hljs-comment">// As commented in `Serializer` implementation, this is a lossy</span>
<span class="hljs-comment">// representation. For example the values `Some(())` and `None` both</span>
<span class="hljs-comment">// serialize as just `null`. Unfortunately this is typically what people</span>
<span class="hljs-comment">// expect when working with JSON. Other formats are encouraged to behave</span>
<span class="hljs-comment">// more intelligently if possible.</span>
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">deserialize_option</span></span><V>(<span class="hljs-keyword">self</span>, visitor: V) -> <span class="hljs-built_in">Result</span><V::Value>
<span class="hljs-keyword">where</span>
V: Visitor<<span class="hljs-symbol">'de</span>>,
{
<span class="hljs-keyword">if</span> <span class="hljs-keyword">self</span>.input.starts_with(<span class="hljs-string">"null"</span>) {
<span class="hljs-keyword">self</span>.input = &<span class="hljs-keyword">self</span>.input[<span class="hljs-string">"null"</span>.len()..];
visitor.visit_none()
} <span class="hljs-keyword">else</span> {
visitor.visit_some(<span class="hljs-keyword">self</span>)
}
}
<span class="hljs-comment">// In Serde, unit means an anonymous value containing no data.</span>
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">deserialize_unit</span></span><V>(<span class="hljs-keyword">self</span>, visitor: V) -> <span class="hljs-built_in">Result</span><V::Value>
<span class="hljs-keyword">where</span>
V: Visitor<<span class="hljs-symbol">'de</span>>,
{
<span class="hljs-keyword">if</span> <span class="hljs-keyword">self</span>.input.starts_with(<span class="hljs-string">"null"</span>) {
<span class="hljs-keyword">self</span>.input = &<span class="hljs-keyword">self</span>.input[<span class="hljs-string">"null"</span>.len()..];
visitor.visit_unit()
} <span class="hljs-keyword">else</span> {
<span class="hljs-literal">Err</span>(Error::ExpectedNull)
}
}
<span class="hljs-comment">// Unit struct means a named value containing no data.</span>
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">deserialize_unit_struct</span></span><V>(
<span class="hljs-keyword">self</span>,
_name: &<span class="hljs-symbol">'static</span> <span class="hljs-keyword">str</span>,
visitor: V,
) -> <span class="hljs-built_in">Result</span><V::Value>
<span class="hljs-keyword">where</span>
V: Visitor<<span class="hljs-symbol">'de</span>>,