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Handwritten generic type bounds
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<h1 id="handwritten-generic-type-bounds"><a name="handwritten-generic-type-bounds" class="plugin-anchor" href="#handwritten-generic-type-bounds"><i class="fa fa-link" aria-hidden="true"></i></a>Handwritten generic type bounds</h1>
<p>When deriving <code>Serialize</code> and <code>Deserialize</code> implementations for structs with
generic type parameters, most of the time Serde is able to infer the correct
trait bounds without help from the programmer. It uses several heuristics to
guess the right bound, but most importantly it puts a bound of <code>T: Serialize</code> on
every type parameter <code>T</code> that is part of a serialized field and a bound of <code>T:
Deserialize</code> on every type parameter <code>T</code> that is part of a deserialized field.
As with most heuristics, this is not always right and Serde provides an escape
hatch to replace the automatically generated bound by one written by the
programmer.</p>
<pre><a class="playground-link" target="_blank" href="https://play.rust-lang.org/?edition=2018&gist=d2a50878ab69a5786f5a3a11a9de71ea"></a><code class="lang-rust"><span class="hljs-keyword">use</span> serde::{de, Deserialize, Deserializer};
<span class="hljs-keyword">use</span> std::fmt::Display;
<span class="hljs-keyword">use</span> std::str::FromStr;
<span class="hljs-meta">#[derive(Deserialize, Debug)]</span>
<span class="hljs-class"><span class="hljs-keyword">struct</span> <span class="hljs-title">Outer</span></span><<span class="hljs-symbol">'a</span>, S, T: <span class="hljs-symbol">'a</span> + ?<span class="hljs-built_in">Sized</span>> {
<span class="hljs-comment">// When deriving the Deserialize impl, Serde would want to generate a bound</span>
<span class="hljs-comment">// `S: Deserialize` on the type of this field. But we are going to use the</span>
<span class="hljs-comment">// type's `FromStr` impl instead of its `Deserialize` impl by going through</span>
<span class="hljs-comment">// `deserialize_from_str`, so we override the automatically generated bound</span>
<span class="hljs-comment">// by the one required for `deserialize_from_str`.</span>
<span class="hljs-meta">#[serde(deserialize_with = <span class="hljs-meta-string">"deserialize_from_str"</span>)]</span>
<span class="hljs-meta">#[serde(bound(deserialize = <span class="hljs-meta-string">"S: FromStr, S::Err: Display"</span>))]</span>
s: S,
<span class="hljs-comment">// Here Serde would want to generate a bound `T: Deserialize`. That is a</span>
<span class="hljs-comment">// stricter condition than is necessary. In fact, the `main` function below</span>
<span class="hljs-comment">// uses T=str which does not implement Deserialize. We override the</span>
<span class="hljs-comment">// automatically generated bound by a looser one.</span>
<span class="hljs-meta">#[serde(bound(deserialize = <span class="hljs-meta-string">"Ptr<'a, T>: Deserialize<'de>"</span>))]</span>
ptr: Ptr<<span class="hljs-symbol">'a</span>, T>,
}
<span class="hljs-comment">/// Deserialize a type `S` by deserializing a string, then using the `FromStr`</span>
<span class="hljs-comment">/// impl of `S` to create the result. The generic type `S` is not required to</span>
<span class="hljs-comment">/// implement `Deserialize`.</span>
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">deserialize_from_str</span></span><<span class="hljs-symbol">'de</span>, S, D>(deserializer: D) -> <span class="hljs-built_in">Result</span><S, D::Error>
<span class="hljs-keyword">where</span>
S: FromStr,
S::<span class="hljs-literal">Err</span>: Display,
D: Deserializer<<span class="hljs-symbol">'de</span>>,
{
<span class="hljs-keyword">let</span> s: <span class="hljs-built_in">String</span> = Deserialize::deserialize(deserializer)?;
S::from_str(&s).map_err(de::Error::custom)
}
<span class="hljs-comment">/// A pointer to `T` which may or may not own the data. When deserializing we</span>
<span class="hljs-comment">/// always want to produce owned data.</span>
<span class="hljs-meta">#[derive(Debug)]</span>
<span class="hljs-class"><span class="hljs-keyword">enum</span> <span class="hljs-title">Ptr</span></span><<span class="hljs-symbol">'a</span>, T: <span class="hljs-symbol">'a</span> + ?<span class="hljs-built_in">Sized</span>> {
Ref(&<span class="hljs-symbol">'a</span> T),
Owned(<span class="hljs-built_in">Box</span><T>),
}
<span class="hljs-keyword">impl</span><<span class="hljs-symbol">'de</span>, <span class="hljs-symbol">'a</span>, T: <span class="hljs-symbol">'a</span> + ?<span class="hljs-built_in">Sized</span>> Deserialize<<span class="hljs-symbol">'de</span>> <span class="hljs-keyword">for</span> Ptr<<span class="hljs-symbol">'a</span>, T>
<span class="hljs-keyword">where</span>
<span class="hljs-built_in">Box</span><T>: Deserialize<<span class="hljs-symbol">'de</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>>,
{
Deserialize::deserialize(deserializer).map(Ptr::Owned)
}
}
<span class="hljs-function"><span class="hljs-keyword">fn</span> <span class="hljs-title">main</span></span>() {
<span class="hljs-keyword">let</span> j = r#<span class="hljs-string">"
{
"</span>s<span class="hljs-string">": "</span><span class="hljs-number">1234567890</span><span class="hljs-string">",
"</span>pt<span class="hljs-string">r": "</span>owned<span class="hljs-string">"
}
"</span>#;
<span class="hljs-keyword">let</span> result: Outer<<span class="hljs-keyword">u64</span>, <span class="hljs-keyword">str</span>> = serde_json::from_str(j).unwrap();
<span class="hljs-comment">// result = Outer { s: 1234567890, ptr: Owned("owned") }</span>
<span class="hljs-built_in">println!</span>(<span class="hljs-string">"result = {:?}"</span>, result);
}
</code></pre>
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