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<a href="." >Error handling</a>
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<h1 id="error-handling"><a name="error-handling" class="plugin-anchor" href="#error-handling"><i class="fa fa-link" aria-hidden="true"></i></a>Error handling</h1>
<p>During serialization, the <a href="https://docs.rs/serde/1/serde/trait.Serialize.html" target="_blank"><code>Serialize</code></a> trait maps a Rust data structure into
Serde's <a href="data-model.html">data model</a> and the <a href="https://docs.rs/serde/1/serde/ser/trait.Serializer.html" target="_blank"><code>Serializer</code></a> trait maps the data model into the
output format. During deserialization, the <a href="https://docs.rs/serde/1/serde/trait.Deserializer.html" target="_blank"><code>Deserializer</code></a> maps the input data
into Serde's data model and the <a href="https://docs.rs/serde/1/serde/trait.Deserialize.html" target="_blank"><code>Deserialize</code></a> and <a href="https://docs.rs/serde/1/serde/de/trait.Visitor.html" target="_blank"><code>Visitor</code></a> traits map the
data model into the resulting data structure. Any of these steps can fail.</p>
<ul>
<li><code>Serialize</code> can fail, for example when a <code>Mutex<T></code> is being serialized and
the mutex happens to be poisoned.</li>
<li><code>Serializer</code> can fail, for example the Serde data model allows maps with
non-string keys but JSON does not.</li>
<li><code>Deserializer</code> can fail, especially if the input data is syntactically
invalid.</li>
<li><code>Deserialize</code> can fail, usually because the input is the wrong type for the
value it is being deserialized into.</li>
</ul>
<p>In Serde, errors from the <code>Serializer</code> and <code>Deserializer</code> work just like they
would in any other Rust library. The crate defines an error type, public
functions return a Result with that error type, and there are variants for the
various possible failure modes.</p>
<p>Handling of errors from the <code>Serialize</code> and <code>Deserialize</code>, the data structure
being processed by the library, is built around the <a href="https://docs.rs/serde/1/serde/ser/trait.Error.html" target="_blank"><code>ser::Error</code></a> and
<a href="https://docs.rs/serde/1/serde/de/trait.Error.html" target="_blank"><code>de::Error</code></a> traits. These traits allow the data format to expose constructors
for its error type for the data structure to use in various situations.</p>
<div><p class="code-filename">src/error.rs</p></div>
<pre><code class="lang-rust"><span class="hljs-keyword">use</span> std;
<span class="hljs-keyword">use</span> std::fmt::{<span class="hljs-keyword">self</span>, Display};
<span class="hljs-keyword">use</span> serde::{de, ser};
<span class="hljs-keyword">pub</span> <span class="hljs-class"><span class="hljs-keyword">type</span> <span class="hljs-title">Result</span></span><T> = std::result::<span class="hljs-built_in">Result</span><T, Error>;
<span class="hljs-comment">// This is a bare-bones implementation. A real library would provide additional</span>
<span class="hljs-comment">// information in its error type, for example the line and column at which the</span>
<span class="hljs-comment">// error occurred, the byte offset into the input, or the current key being</span>
<span class="hljs-comment">// processed.</span>
<span class="hljs-meta">#[derive(Debug)]</span>
<span class="hljs-keyword">pub</span> <span class="hljs-class"><span class="hljs-keyword">enum</span> <span class="hljs-title">Error</span></span> {
<span class="hljs-comment">// One or more variants that can be created by data structures through the</span>
<span class="hljs-comment">// `ser::Error` and `de::Error` traits. For example the Serialize impl for</span>
<span class="hljs-comment">// Mutex<T> might return an error because the mutex is poisoned, or the</span>
<span class="hljs-comment">// Deserialize impl for a struct may return an error because a required</span>
<span class="hljs-comment">// field is missing.</span>
Message(<span class="hljs-built_in">String</span>),
<span class="hljs-comment">// Zero or more variants that can be created directly by the Serializer and</span>
<span class="hljs-comment">// Deserializer without going through `ser::Error` and `de::Error`. These</span>
<span class="hljs-comment">// are specific to the format, in this case JSON.</span>
Eof,
Syntax,
ExpectedBoolean,
ExpectedInteger,
ExpectedString,
ExpectedNull,
ExpectedArray,
ExpectedArrayComma,
ExpectedArrayEnd,
ExpectedMap,
ExpectedMapColon,
ExpectedMapComma,
ExpectedMapEnd,
ExpectedEnum,
TrailingCharacters,
}
<span class="hljs-keyword">impl</span> ser::Error <span class="hljs-keyword">for</span> Error {
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">custom</span></span><T: Display>(msg: T) -> <span class="hljs-keyword">Self</span> {
Error::Message(msg.to_string())
}
}
<span class="hljs-keyword">impl</span> de::Error <span class="hljs-keyword">for</span> Error {
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">custom</span></span><T: Display>(msg: T) -> <span class="hljs-keyword">Self</span> {
Error::Message(msg.to_string())
}
}
<span class="hljs-keyword">impl</span> Display <span class="hljs-keyword">for</span> Error {
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">fmt</span></span>(&<span class="hljs-keyword">self</span>, formatter: &<span class="hljs-keyword">mut</span> fmt::Formatter) -> fmt::<span class="hljs-built_in">Result</span> {
<span class="hljs-keyword">match</span> <span class="hljs-keyword">self</span> {
Error::Message(msg) => formatter.write_str(msg),
Error::Eof => formatter.write_str(<span class="hljs-string">"unexpected end of input"</span>),
<span class="hljs-comment">/* and so forth */</span>
}
}
}
<span class="hljs-keyword">impl</span> std::error::Error <span class="hljs-keyword">for</span> Error {}
</code></pre>
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