Add deserialize_from to Deserialize

serde/src/de/mod.rs

@@ -504,6 +504,35 @@ 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`.
+    ///
+    /// TODO: example
+    ///
+    /// ```
+    /// // Something with a loop that returns on error.
+    ///
+    /// ```
+    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