Change serde_json::from_value to use pass-by-reference

src/value/mod.rs

@@ -256,6 +256,12 @@ impl Display for Value {
     }
 }
 
+impl AsRef<Value> for Value {
+    fn as_ref(&self) -> &Value {
+        self
+    }
+}
+
 fn parse_index(s: &str) -> Option<usize> {
     if s.starts_with('+') || (s.starts_with('0') && s.len() != 1) {
         return None;
@@ -987,8 +993,13 @@ where
 ///         "location": "Menlo Park, CA"
 ///     });
 ///
-///     let u: User = serde_json::from_value(j).unwrap();
+///     // from_value accepts both reference ...
+///     let u: User = serde_json::from_value(&j).unwrap();
 ///     println!("{:#?}", u);
+///
+///     // ... and moved value
+///     let u2: User = serde_json::from_value(j).unwrap();
+///     println!("{:#?}", u2);
 /// }
 /// ```
 ///
@@ -1001,9 +1012,9 @@ where
 /// is wrong with the data, for example required struct fields are missing from
 /// the JSON map or some number is too big to fit in the expected primitive
 /// type.
-pub fn from_value<T>(value: Value) -> Result<T, Error>
+pub fn from_value<T, V: AsRef<Value>>(value: V) -> Result<T, Error>
 where
     T: DeserializeOwned,
 {
-    T::deserialize(value)
+    T::deserialize(value.as_ref())
 }

tests/regression/issue520.rs

@@ -12,7 +12,7 @@ enum E {
 fn test() {
     let e = E::Float(159.1);
     let v = serde_json::to_value(e).unwrap();
-    let e = serde_json::from_value::<E>(v).unwrap();
+    let e = serde_json::from_value::<E, serde_json::Value>(v).unwrap();
 
     match e {
         E::Float(f) => assert_eq!(f, 159.1),

tests/regression/issue795.rs

@@ -58,5 +58,5 @@ fn test() {
     assert!(serde_json::from_str::<Enum>(s).is_err());
 
     let j = json!({"Variant":{"x":0,"y":0}});
-    assert!(serde_json::from_value::<Enum>(j).is_err());
+    assert!(serde_json::from_value::<Enum, serde_json::Value>(j).is_err());
 }

tests/test.rs

@@ -1899,13 +1899,13 @@ fn test_integer_key() {
         (r#"{"123 ":null}"#, "expected `\"` at line 1 column 6"),
     ]);
 
-    let err = from_value::<BTreeMap<i32, ()>>(json!({" 123":null})).unwrap_err();
+    let err = from_value::<BTreeMap<i32, ()>, Value>(json!({" 123":null})).unwrap_err();
     assert_eq!(
         err.to_string(),
         "invalid value: expected key to be a number in quotes",
     );
 
-    let err = from_value::<BTreeMap<i32, ()>>(json!({"123 ":null})).unwrap_err();
+    let err = from_value::<BTreeMap<i32, ()>, Value>(json!({"123 ":null})).unwrap_err();
     assert_eq!(
         err.to_string(),
         "invalid value: expected key to be a number in quotes",