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<a href="." >Deserializer lifetimes</a>
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<h1 id="understanding-deserializer-lifetimes"><a name="understanding-deserializer-lifetimes" class="plugin-anchor" href="#understanding-deserializer-lifetimes"><i class="fa fa-link" aria-hidden="true"></i></a>Understanding deserializer lifetimes</h1>
<p>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/trait.Deserializer.html" target="_blank"><code>Deserializer</code></a> traits both have a lifetime called
<code>'de</code>, as do some of the other deserialization-related traits.</p>
<pre><code class="lang-rust"><span class="hljs-class"><span class="hljs-keyword">trait</span> <span class="hljs-title">Deserialize</span></span><<span class="hljs-symbol">'de</span>>: <span class="hljs-built_in">Sized</span> {
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">deserialize</span></span><D>(deserializer: D) -> <span class="hljs-built_in">Result</span><<span class="hljs-keyword">Self</span>, D::Error>
<span class="hljs-keyword">where</span>
D: Deserializer<<span class="hljs-symbol">'de</span>>;
}
</code></pre>
<p>This lifetime is what enables Serde to safely perform efficient zero-copy
deserialization across a variety of data formats, something that would be
impossible or recklessly unsafe in languages other than Rust.</p>
<pre><code class="lang-rust"><span class="hljs-meta">#[derive(Deserialize)]</span>
<span class="hljs-class"><span class="hljs-keyword">struct</span> <span class="hljs-title">User</span></span><<span class="hljs-symbol">'a</span>> {
id: <span class="hljs-keyword">u32</span>,
name: &<span class="hljs-symbol">'a</span> <span class="hljs-keyword">str</span>,
screen_name: &<span class="hljs-symbol">'a</span> <span class="hljs-keyword">str</span>,
location: &<span class="hljs-symbol">'a</span> <span class="hljs-keyword">str</span>,
}
</code></pre>
<p>Zero-copy deserialization means deserializing into a data structure, like the
<code>User</code> struct above, that borrows string or byte array data from the string or
byte array holding the input. This avoids allocating memory to store a string
for each individual field and then copying string data out of the input over to
the newly allocated field. Rust guarantees that the input data outlives the
period during which the output data structure is in scope, meaning it is
impossible to have dangling pointer errors as a result of losing the input data
while the output data structure still refers to it.</p>
<h2 id="trait-bounds"><a name="trait-bounds" class="plugin-anchor" href="#trait-bounds"><i class="fa fa-link" aria-hidden="true"></i></a>Trait bounds</h2>
<p>There are two main ways to write <code>Deserialize</code> trait bounds, whether on an impl
block or a function or anywhere else.</p>
<ul>
<li><p><strong><code><'de, T> where T: Deserialize<'de></code></strong></p>
<p> This means "T can be deserialized from <strong>some</strong> lifetime." The caller gets
to decide what lifetime that is. Typically this is used when the caller also
provides the data that is being deserialized from, for example in a function
like <a href="https://docs.rs/serde_json/1/serde_json/fn.from_str.html" target="_blank"><code>serde_json::from_str</code></a>. In that case the input data must also have
lifetime <code>'de</code>, for example it could be <code>&'de str</code>.</p>
</li>
<li><p><strong><code><T> where T: DeserializeOwned</code></strong></p>
<p> This means "T can be deserialized from <strong>any</strong> lifetime." The callee gets to
decide what lifetime. Usually this is because the data that is being
deserialized from is going to be thrown away before the function returns, so
T must not be allowed to borrow from it. For example a function that accepts
base64-encoded data as input, decodes it from base64, deserializes a value
of type T, then throws away the result of base64 decoding. Another common
use of this bound is functions that deserialize from an IO stream, such as
<a href="https://docs.rs/serde_json/1/serde_json/fn.from_reader.html" target="_blank"><code>serde_json::from_reader</code></a>.</p>
<p> To say it more technically, the <a href="https://docs.rs/serde/1/serde/de/trait.DeserializeOwned.html" target="_blank"><code>DeserializeOwned</code></a> trait is equivalent to
the <a href="https://doc.rust-lang.org/nomicon/hrtb.html" target="_blank">higher-rank trait bound</a> <code>for<'de> Deserialize<'de></code>. The only
difference is <code>DeserializeOwned</code> is more intuitive to read. It means T owns
all the data that gets deserialized.</p>
</li>
</ul>
<p>Note that <code><T> where T: Deserialize<'static></code> is never what you want. Also
<code>Deserialize<'de> + 'static</code> is never what you want. Generally writing <code>'static</code>
anywhere near <code>Deserialize</code> is a sign of being on the wrong track. Use one of
the above bounds instead.</p>
<h2 id="transient-borrowed-and-owned-data"><a name="transient-borrowed-and-owned-data" class="plugin-anchor" href="#transient-borrowed-and-owned-data"><i class="fa fa-link" aria-hidden="true"></i></a>Transient, borrowed, and owned data</h2>
<p>The Serde data model has three flavors of strings and byte arrays during
deserialization. They correspond to different methods on the <a href="https://docs.rs/serde/1/serde/de/trait.Visitor.html" target="_blank"><code>Visitor</code></a> trait.</p>
<ul>
<li><strong>Transient</strong> — <a href="https://docs.rs/serde/1/serde/de/trait.Visitor.html#method.visit_str" target="_blank"><code>visit_str</code></a> accepts a <code>&str</code>.</li>
<li><strong>Borrowed</strong> — <a href="https://docs.rs/serde/1/serde/de/trait.Visitor.html#method.visit_borrowed_str" target="_blank"><code>visit_borrowed_str</code></a> accepts a <code>&'de str</code>.</li>
<li><strong>Owned</strong> — <a href="https://docs.rs/serde/1/serde/de/trait.Visitor.html#method.visit_string" target="_blank"><code>visit_string</code></a> accepts a <code>String</code>.</li>
</ul>
<p>Transient data is not guaranteed to last beyond the method call it is passed to.
Often this is sufficient, for example when deserializing something like an IP
address from a Serde string using the <a href="https://doc.rust-lang.org/std/str/trait.FromStr.html" target="_blank"><code>FromStr</code></a> trait. When it is not
sufficient, the data can be copied by calling <a href="https://doc.rust-lang.org/std/borrow/trait.ToOwned.html" target="_blank"><code>to_owned()</code></a>. Deserializers
commonly use transient data when input from an IO stream is being buffered in
memory before being passed to the <code>Visitor</code>, or when escape sequences are being
processed so the resulting string is not present verbatim in the input.</p>
<p>Borrowed data is guaranteed to live at least as long as the <code>'de</code> lifetime
parameter of the <code>Deserializer</code>. Not all deserializers support handing out
borrowed data. For example when deserializing from an IO stream no data can be
borrowed.</p>
<p>Owned data is guaranteed to live as long as the <a href="https://docs.rs/serde/1/serde/de/trait.Visitor.html" target="_blank"><code>Visitor</code></a> wants it to. Some
visitors benefit from receiving owned data. For example the <code>Deserialize</code> impl
for Rust's <code>String</code> type benefits from being given ownership of the Serde string
data that has been deserialized.</p>
<h2 id="the-deserializede-lifetime"><a name="the-deserializede-lifetime" class="plugin-anchor" href="#the-deserializede-lifetime"><i class="fa fa-link" aria-hidden="true"></i></a>The Deserialize<'de> lifetime</h2>
<p>This lifetime records the constraints on how long data borrowed by this type
must be valid.</p>
<p>Every lifetime of data borrowed by this type must be a bound on the <code>'de</code>
lifetime of its <code>Deserialize</code> impl. If this type borrows data with lifetime
<code>'a</code>, then <code>'de</code> must be constrained to outlive <code>'a</code>.</p>
<pre><code class="lang-rust"><span class="hljs-class"><span class="hljs-keyword">struct</span> <span class="hljs-title">S</span></span><<span class="hljs-symbol">'a</span>, <span class="hljs-symbol">'b</span>, T> {
a: &<span class="hljs-symbol">'a</span> <span class="hljs-keyword">str</span>,
b: &<span class="hljs-symbol">'b</span> <span class="hljs-keyword">str</span>,
bb: &<span class="hljs-symbol">'b</span> <span class="hljs-keyword">str</span>,
t: T,
}
<span class="hljs-keyword">impl</span><<span class="hljs-symbol">'de</span>: <span class="hljs-symbol">'a</span> + <span class="hljs-symbol">'b</span>, <span class="hljs-symbol">'a</span>, <span class="hljs-symbol">'b</span>, T> Deserialize<<span class="hljs-symbol">'de</span>> <span class="hljs-keyword">for</span> S<<span class="hljs-symbol">'a</span>, <span class="hljs-symbol">'b</span>, T>
<span class="hljs-keyword">where</span>
T: Deserialize<<span class="hljs-symbol">'de</span>>,
{
<span class="hljs-comment">/* ... */</span>
}
</code></pre>
<p>If this type does not borrow any data from the <code>Deserializer</code>, there are simply
no bounds on the <code>'de</code> lifetime. Such types automatically implement the
<a href="https://docs.rs/serde/1/serde/de/trait.DeserializeOwned.html" target="_blank"><code>DeserializeOwned</code></a> trait.</p>
<pre><code class="lang-rust"><span class="hljs-class"><span class="hljs-keyword">struct</span> <span class="hljs-title">S</span></span> {
owned: <span class="hljs-built_in">String</span>,
}
<span class="hljs-keyword">impl</span><<span class="hljs-symbol">'de</span>> Deserialize<<span class="hljs-symbol">'de</span>> <span class="hljs-keyword">for</span> S {
<span class="hljs-comment">/* ... */</span>
}
</code></pre>
<p>The <code>'de</code> lifetime <strong>should not</strong> appear in the type to which the <code>Deserialize</code>
impl applies.</p>
<pre><code class="lang-diff"><span class="hljs-deletion">- // Do not do this. Sooner or later you will be sad.</span>
<span class="hljs-deletion">- impl<'de> Deserialize<'de> for Q<'de> {</span>
<span class="hljs-addition">+ // Do this instead.</span>
<span class="hljs-addition">+ impl<'de: 'a, 'a> Deserialize<'de> for Q<'a> {</span>
</code></pre>
<h2 id="the-deserializerde-lifetime"><a name="the-deserializerde-lifetime" class="plugin-anchor" href="#the-deserializerde-lifetime"><i class="fa fa-link" aria-hidden="true"></i></a>The Deserializer<'de> lifetime</h2>
<p>This is the lifetime of data that can be borrowed from the <code>Deserializer</code>.</p>
<pre><code class="lang-rust"><span class="hljs-class"><span class="hljs-keyword">struct</span> <span class="hljs-title">MyDeserializer</span></span><<span class="hljs-symbol">'de</span>> {
input_data: &<span class="hljs-symbol">'de</span> [<span class="hljs-keyword">u8</span>],
pos: usize,
}
<span class="hljs-keyword">impl</span><<span class="hljs-symbol">'de</span>> Deserializer<<span class="hljs-symbol">'de</span>> <span class="hljs-keyword">for</span> MyDeserializer<<span class="hljs-symbol">'de</span>> {
<span class="hljs-comment">/* ... */</span>
}
</code></pre>
<p>If the <code>Deserializer</code> never invokes <a href="https://docs.rs/serde/1/serde/de/trait.Visitor.html#method.visit_borrowed_str" target="_blank"><code>visit_borrowed_str</code></a> or
<a href="https://docs.rs/serde/1/serde/de/trait.Visitor.html#method.visit_borrowed_bytes" target="_blank"><code>visit_borrowed_bytes</code></a>, the <code>'de</code> lifetime will be an unconstrained lifetime
parameter.</p>
<pre><code class="lang-rust"><span class="hljs-class"><span class="hljs-keyword">struct</span> <span class="hljs-title">MyDeserializer</span></span><R> {
read: R,
}
<span class="hljs-keyword">impl</span><<span class="hljs-symbol">'de</span>, R> Deserializer<<span class="hljs-symbol">'de</span>> <span class="hljs-keyword">for</span> MyDeserializer<R>
<span class="hljs-keyword">where</span>
R: io::Read,
{
<span class="hljs-comment">/* ... */</span>
}
</code></pre>
<h2 id="borrowing-data-in-a-derived-impl"><a name="borrowing-data-in-a-derived-impl" class="plugin-anchor" href="#borrowing-data-in-a-derived-impl"><i class="fa fa-link" aria-hidden="true"></i></a>Borrowing data in a derived impl</h2>
<p>Fields of type <code>&str</code> and <code>&[u8]</code> are implicitly borrowed from the input data by
Serde. Any other type of field can opt in to borrowing by using the
<code>#[serde(borrow)]</code> attribute.</p>
<pre><code class="lang-rust"><span class="hljs-keyword">use</span> serde::Deserialize;
<span class="hljs-keyword">use</span> std::borrow::Cow;
<span class="hljs-meta">#[derive(Deserialize)]</span>
<span class="hljs-class"><span class="hljs-keyword">struct</span> <span class="hljs-title">Inner</span></span><<span class="hljs-symbol">'a</span>, <span class="hljs-symbol">'b</span>> {
<span class="hljs-comment">// &str and &[u8] are implicitly borrowed.</span>
username: &<span class="hljs-symbol">'a</span> <span class="hljs-keyword">str</span>,
<span class="hljs-comment">// Other types must be borrowed explicitly.</span>
<span class="hljs-meta">#[serde(borrow)]</span>
comment: Cow<<span class="hljs-symbol">'b</span>, <span class="hljs-keyword">str</span>>,
}
<span class="hljs-meta">#[derive(Deserialize)]</span>
<span class="hljs-class"><span class="hljs-keyword">struct</span> <span class="hljs-title">Outer</span></span><<span class="hljs-symbol">'a</span>, <span class="hljs-symbol">'b</span>, <span class="hljs-symbol">'c</span>> {
owned: <span class="hljs-built_in">String</span>,
<span class="hljs-meta">#[serde(borrow)]</span>
inner: Inner<<span class="hljs-symbol">'a</span>, <span class="hljs-symbol">'b</span>>,
<span class="hljs-comment">// This field is never borrowed.</span>
not_borrowed: Cow<<span class="hljs-symbol">'c</span>, <span class="hljs-keyword">str</span>>,
}
</code></pre>
<p>This attribute works by placing bounds on the <code>'de</code> lifetime of the generated
<code>Deserialize</code> impl. For example the impl for the struct <code>Outer</code> defined above
looks like this:</p>
<pre><code class="lang-rust"><span class="hljs-comment">// The lifetimes 'a and 'b are borrowed while 'c is not.</span>
<span class="hljs-keyword">impl</span><<span class="hljs-symbol">'de</span>: <span class="hljs-symbol">'a</span> + <span class="hljs-symbol">'b</span>, <span class="hljs-symbol">'a</span>, <span class="hljs-symbol">'b</span>, <span class="hljs-symbol">'c</span>> Deserialize<<span class="hljs-symbol">'de</span>> <span class="hljs-keyword">for</span> Outer<<span class="hljs-symbol">'a</span>, <span class="hljs-symbol">'b</span>, <span class="hljs-symbol">'c</span>> {
<span class="hljs-comment">/* ... */</span>
}
</code></pre>
<p>The attribute may specify explicitly which lifetimes should be borrowed.</p>
<pre><code class="lang-rust"><span class="hljs-keyword">use</span> std::marker::PhantomData;
<span class="hljs-comment">// This struct borrows the first two lifetimes but not the third.</span>
<span class="hljs-meta">#[derive(Deserialize)]</span>
<span class="hljs-class"><span class="hljs-keyword">struct</span> <span class="hljs-title">Three</span></span><<span class="hljs-symbol">'a</span>, <span class="hljs-symbol">'b</span>, <span class="hljs-symbol">'c</span>> {
a: &<span class="hljs-symbol">'a</span> <span class="hljs-keyword">str</span>,
b: &<span class="hljs-symbol">'b</span> <span class="hljs-keyword">str</span>,
c: PhantomData<&<span class="hljs-symbol">'c</span> <span class="hljs-keyword">str</span>>,
}
<span class="hljs-meta">#[derive(Deserialize)]</span>
<span class="hljs-class"><span class="hljs-keyword">struct</span> <span class="hljs-title">Example</span></span><<span class="hljs-symbol">'a</span>, <span class="hljs-symbol">'b</span>, <span class="hljs-symbol">'c</span>> {
<span class="hljs-comment">// Borrow 'a and 'b only, not 'c.</span>
<span class="hljs-meta">#[serde(borrow = <span class="hljs-meta-string">"'a + 'b"</span>)]</span>
three: Three<<span class="hljs-symbol">'a</span>, <span class="hljs-symbol">'b</span>, <span class="hljs-symbol">'c</span>>,
}
</code></pre>
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