Add and derive deserialize_from

serde/src/de/mod.rs

@@ -504,6 +504,34 @@ pub trait Deserialize<'de>: Sized {
     fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
     where
         D: Deserializer<'de>;
+
+    /// Deserializes a value into `self` from the given Deserializer.
+    ///
+    /// The purpose of this method is to allow the deserializer to reuse
+    /// resources and avoid copies. As such, if this method returns an error,
+    /// `self` will be in an indeterminate state where some parts of the struct
+    /// have been overwritten. Although whatever state that is will be
+    /// memory-safe.
+    ///
+    /// This is generally useful when repeateadly deserializing values that
+    /// are processed one at a time, where the value of `self` doesn't matter
+    /// when the next deserialization occurs.
+    ///
+    /// If you manually implement this, your recursive deserializations should
+    /// use `deserialize_from`.
+    ///
+    /// This method is stable and an official public API, but hidden from the
+    /// documentation because it is almost never what newbies are looking for.
+    /// Showing it in rustdoc would cause it to be featured more prominently
+    /// than it deserves.
+    #[doc(hidden)]
+    fn deserialize_from<D>(&mut self, deserializer: D) -> Result<(), D::Error>
+        where D: Deserializer<'de>
+    {
+        // Default implementation just delegates to `deserialize` impl.
+        *self = Deserialize::deserialize(deserializer)?;
+        Ok(())
+    }
 }
 
 /// A data structure that can be deserialized without borrowing any data from

serde/src/private/de.rs

@@ -8,7 +8,7 @@
 
 use lib::*;
 
-use de::{Deserialize, Deserializer, IntoDeserializer, Error, Visitor};
+use de::{Deserialize, Deserializer, DeserializeSeed, IntoDeserializer, Error, Visitor};
 
 #[cfg(any(feature = "std", feature = "alloc"))]
 use de::Unexpected;
@@ -2009,3 +2009,20 @@ where
         map struct enum identifier ignored_any
     }
 }
+
+/// A DeserializeSeed helper for implementing deserialize_from Visitors.
+///
+/// Wraps a mutable reference and calls deserialize_from on it.
+pub struct DeserializeFromSeed<'a, T: 'a>(pub &'a mut T);
+
+impl<'a, 'de, T> DeserializeSeed<'de> for DeserializeFromSeed<'a, T>
+    where T: Deserialize<'de>,
+{
+    type Value = ();
+    fn deserialize<D>(self, deserializer: D) -> Result<Self::Value, D::Error>
+    where
+        D: Deserializer<'de>,
+    {
+       self.0.deserialize_from(deserializer)
+    }
+}

serde_derive/Cargo.toml

@@ -14,6 +14,10 @@ include = ["Cargo.toml", "src/**/*.rs", "README.md", "LICENSE-APACHE", "LICENSE-
 [badges]
 travis-ci = { repository = "serde-rs/serde" }
 
+[features]
+default = []
+deserialize_from = []
+
 [lib]
 name = "serde_derive"
 proc-macro = true