From e87ad19ddbb790f4ca998eee5c9499ba063915cf Mon Sep 17 00:00:00 2001
From: Conrad Ludgate <conrad.ludgate@databricks.com>
Date: Thu, 10 Jul 2025 09:36:24 +0100
Subject: [PATCH] support seeded deserialize for stream

---
 src/de.rs       | 65 ++++++++++++++++++++++++++++++++++++++++---------
 tests/stream.rs | 53 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 107 insertions(+), 11 deletions(-)

diff --git a/src/de.rs b/src/de.rs
index ffd0d48c2..c4bca26a2 100644
--- a/src/de.rs
+++ b/src/de.rs
@@ -2268,7 +2268,6 @@ pub struct StreamDeserializer<'de, R, T> {
 impl<'de, R, T> StreamDeserializer<'de, R, T>
 where
     R: read::Read<'de>,
-    T: de::Deserialize<'de>,
 {
     /// Create a JSON stream deserializer from one of the possible serde_json
     /// input sources.
@@ -2338,16 +2337,48 @@ where
             }
         }
     }
-}
-
-impl<'de, R, T> Iterator for StreamDeserializer<'de, R, T>
-where
-    R: Read<'de>,
-    T: de::Deserialize<'de>,
-{
-    type Item = Result<T>;
 
-    fn next(&mut self) -> Option<Result<T>> {
+    /// Deserialize the next JSON value using the provided seed.
+    ///
+    /// ```
+    /// use serde_json::{Deserializer, Value};
+    ///
+    /// /// A simple `DeserializeSeed` impl that deserializes into a vec,
+    /// /// rather than returning the deserialized value.
+    /// struct Collect<'a, T: 'a>(&'a mut Vec<T>);
+    ///
+    /// impl<'de, 'a, T> serde::de::DeserializeSeed<'de> for Collect<'a, T>
+    /// where
+    ///     T: serde::de::Deserialize<'de>,
+    /// {
+    ///     type Value = ();
+    ///
+    ///     fn deserialize<D>(self, deserializer: D) -> Result<(), D::Error>
+    ///     where
+    ///         D: serde::de::Deserializer<'de>,
+    ///     {
+    ///         self.0.push(T::deserialize(deserializer)?);
+    ///         Ok(())
+    ///     }
+    /// }
+    ///
+    /// fn main() {
+    ///     let data = "{\"k\": 3}1\"cool\"\"stuff\" 3{}  [0, 1, 2]";
+    ///
+    ///     let mut stream = Deserializer::from_str(data).into_iter();
+    ///
+    ///     let mut values: Vec<Value> = vec![];
+    ///     while let Some(res) = stream.next_seed(Collect(&mut values)) {
+    ///         res.unwrap();
+    ///     }
+    ///
+    ///     println!("{values:?}")
+    /// }
+    /// ```
+    pub fn next_seed<S>(&mut self, seed: S) -> Option<Result<T>>
+    where
+        S: de::DeserializeSeed<'de, Value = T>,
+    {
         if R::should_early_return_if_failed && self.failed {
             return None;
         }
@@ -2369,7 +2400,7 @@ where
                     _ => false,
                 };
                 self.offset = self.de.read.byte_offset();
-                let result = de::Deserialize::deserialize(&mut self.de);
+                let result = seed.deserialize(&mut self.de);
 
                 Some(match result {
                     Ok(value) => {
@@ -2394,6 +2425,18 @@ where
     }
 }
 
+impl<'de, R, T> Iterator for StreamDeserializer<'de, R, T>
+where
+    R: Read<'de>,
+    T: de::Deserialize<'de>,
+{
+    type Item = Result<T>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        self.next_seed(self.output)
+    }
+}
+
 impl<'de, R, T> FusedIterator for StreamDeserializer<'de, R, T>
 where
     R: Read<'de> + Fused,
diff --git a/tests/stream.rs b/tests/stream.rs
index ca54e9a15..84a156275 100644
--- a/tests/stream.rs
+++ b/tests/stream.rs
@@ -150,6 +150,59 @@ fn test_json_stream_primitive() {
     });
 }
 
+#[test]
+fn test_json_stream_seed() {
+    struct Seed;
+    impl<'de> serde::de::DeserializeSeed<'de> for Seed {
+        type Value = Value;
+        fn deserialize<D>(self, deserializer: D) -> Result<Value, D::Error>
+        where
+            D: serde::Deserializer<'de>,
+        {
+            let value: Value = serde::Deserialize::deserialize(deserializer)?;
+
+            // apply some modifications to test that the seed is working
+            Ok(match value {
+                Value::Bool(b) => Value::Bool(!b),
+                Value::Number(n) => Value::String(n.to_string()),
+                Value::String(s) => Value::String(s.to_uppercase()),
+                v => v,
+            })
+        }
+    }
+
+    let data = "{} true{}1[]\nfalse\"hey\"2 ";
+
+    test_stream!(data, Value, |stream| {
+        assert_eq!(stream.next_seed(Seed).unwrap().unwrap(), json!({}));
+        assert_eq!(stream.byte_offset(), 2);
+
+        assert_eq!(stream.next_seed(Seed).unwrap().unwrap(), false);
+        assert_eq!(stream.byte_offset(), 7);
+
+        assert_eq!(stream.next_seed(Seed).unwrap().unwrap(), json!({}));
+        assert_eq!(stream.byte_offset(), 9);
+
+        assert_eq!(stream.next_seed(Seed).unwrap().unwrap(), "1");
+        assert_eq!(stream.byte_offset(), 10);
+
+        assert_eq!(stream.next_seed(Seed).unwrap().unwrap(), json!([]));
+        assert_eq!(stream.byte_offset(), 12);
+
+        assert_eq!(stream.next_seed(Seed).unwrap().unwrap(), true);
+        assert_eq!(stream.byte_offset(), 18);
+
+        assert_eq!(stream.next_seed(Seed).unwrap().unwrap(), "HEY");
+        assert_eq!(stream.byte_offset(), 23);
+
+        assert_eq!(stream.next_seed(Seed).unwrap().unwrap(), "2");
+        assert_eq!(stream.byte_offset(), 24);
+
+        assert!(stream.next_seed(Seed).is_none());
+        assert_eq!(stream.byte_offset(), 25);
+    });
+}
+
 #[test]
 fn test_json_stream_invalid_literal() {
     let data = "truefalse";