Feature: Support string prefix parsing

src/de.rs

@@ -60,6 +60,11 @@ where
             disable_recursion_limit: false,
         }
     }
+
+    /// Consume the deserializer and return its inner reader.
+    pub fn into_reader(self) -> R {
+        self.read
+    }
 }
 
 #[cfg(feature = "std")]

src/read.rs

@@ -412,6 +412,11 @@ impl<'a> SliceRead<'a> {
         }
     }
 
+    /// Get the byte index of the data that will be read next
+    pub fn index(&self) -> usize {
+        self.index
+    }
+
     fn position_of_index(&self, i: usize) -> Position {
         let mut position = Position { line: 1, column: 0 };
         for ch in &self.slice[..i] {
@@ -623,6 +628,11 @@ impl<'a> StrRead<'a> {
             data: s,
         }
     }
+
+    /// Get the byte index of the data that will be read next
+    pub fn index(&self) -> usize {
+        self.delegate.index()
+    }
 }
 
 impl<'a> private::Sealed for StrRead<'a> {}

tests/test.rs

@@ -2407,3 +2407,22 @@ fn hash_positive_and_negative_zero() {
         assert_eq!(hash(k1), hash(k2));
     }
 }
+
+#[test]
+fn parse_string_prefix() {
+    #[derive(PartialEq, Deserialize, Debug)]
+    struct S {
+        a: u32
+    }
+
+    let data = "{\"a\": 42} tail";
+
+    let mut de = serde_json::Deserializer::from_str(data);
+    let val = S::deserialize(&mut de).unwrap();
+
+    assert_eq!(val, S { a: 42 });
+
+    let str_read = de.into_reader();
+    let tail = &data[str_read.index()..];
+    assert_eq!(tail, " tail");
+}