Added support for feature = preserve_order

Cargo.toml

@@ -10,9 +10,13 @@ readme = "README.md"
 keywords = ["serde"]
 license = "MIT"
 
+[features]
+preserve_order = ["indexmap"]
+
 [dependencies]
 serde = "^1.0.0"
 ordered-float = "^1.0.1"
+indexmap = { version = "1.0.2", optional = true, features = ["serde-1"] }
 
 [dev-dependencies]
 serde_derive = "^1.0.0"

src/de.rs

@@ -1,9 +1,8 @@
 use serde::de;
-use std::collections::BTreeMap;
 use std::error::Error;
 use std::fmt;
 
-use Value;
+use {Value, MapImpl};
 
 #[derive(Debug)]
 pub enum Unexpected {
@@ -274,7 +273,7 @@ impl<'de> de::Visitor<'de> for ValueVisitor {
     }
 
     fn visit_map<V: de::MapAccess<'de>>(self, mut visitor: V) -> Result<Value, V::Error> {
-        let mut values = BTreeMap::new();
+        let mut values = MapImpl::new();
         while let Some((key, value)) = try!(visitor.next_entry()) {
             values.insert(key, value);
         }

src/lib.rs

@@ -3,12 +3,19 @@
 #[macro_use]
 extern crate serde;
 extern crate ordered_float;
+#[cfg(feature = "preserve_order")]
+extern crate indexmap;
 
 #[cfg(test)]
 #[macro_use]
 extern crate serde_derive;
 
-use std::collections::BTreeMap;
+#[cfg(feature = "preserve_order")]
+pub(crate) use indexmap::IndexMap as MapImpl;
+
+#[cfg(not(feature = "preserve_order"))]
+pub(crate) use std::collections::BTreeMap as MapImpl;
+
 use std::cmp::Ordering;
 use std::hash::{Hash, Hasher};
 use serde::Deserialize;
@@ -44,7 +51,7 @@ pub enum Value {
     Option(Option<Box<Value>>),
     Newtype(Box<Value>),
     Seq(Vec<Value>),
-    Map(BTreeMap<Value, Value>),
+    Map(MapImpl<Value, Value>),
     Bytes(Vec<u8>),
 }
 
@@ -72,7 +79,17 @@ impl Hash for Value {
             Value::Option(ref v) => v.hash(hasher),
             Value::Newtype(ref v) => v.hash(hasher),
             Value::Seq(ref v) => v.hash(hasher),
-            Value::Map(ref v) => v.hash(hasher),
+            Value::Map(ref v) => {
+                #[cfg(feature = "preserve_order")] {
+                    for (key, value) in v.iter() {
+                        key.hash(hasher);
+                        value.hash(hasher);
+                    }
+                }
+                #[cfg(not(feature = "preserve_order"))] {
+                    v.hash(hasher)
+                }
+            }
             Value::Bytes(ref v) => v.hash(hasher),
         }
     }
@@ -125,7 +142,14 @@ impl Ord for Value {
             (&Value::Option(ref v0), &Value::Option(ref v1)) => v0.cmp(v1),
             (&Value::Newtype(ref v0), &Value::Newtype(ref v1)) => v0.cmp(v1),
             (&Value::Seq(ref v0), &Value::Seq(ref v1)) => v0.cmp(v1),
-            (&Value::Map(ref v0), &Value::Map(ref v1)) => v0.cmp(v1),
+            (&Value::Map(ref v0), &Value::Map(ref v1)) => {
+                #[cfg(feature = "preserve_order")] {
+                    v0.iter().cmp(v1.iter())
+                }
+                #[cfg(not(feature = "preserve_order"))] {
+                    v0.cmp(v1)
+                }
+            },
             (&Value::Bytes(ref v0), &Value::Bytes(ref v1)) => v0.cmp(v1),
             (ref v0, ref v1) => v0.discriminant().cmp(&v1.discriminant()),
         }

src/ser.rs

@@ -1,9 +1,8 @@
 use serde::ser;
-use std::collections::BTreeMap;
 use std::error::Error;
 use std::fmt;
 
-use Value;
+use {Value, MapImpl};
 
 #[derive(Debug)]
 pub enum SerializerError {
@@ -224,15 +223,15 @@ impl ser::Serializer for Serializer {
         self,
         _len: Option<usize>
     ) -> Result<Self::SerializeMap, Self::Error> {
-        Ok(SerializeMap { map: BTreeMap::new(), key: None })
+        Ok(SerializeMap { map: MapImpl::new(), key: None })
     }
 
     fn serialize_struct(
         self,
         _name: &'static str,
         _len: usize
     ) -> Result<Self::SerializeStruct, Self::Error> {
-        Ok(SerializeStruct(BTreeMap::new()))
+        Ok(SerializeStruct(MapImpl::new()))
     }
 
     fn serialize_struct_variant(
@@ -242,7 +241,7 @@ impl ser::Serializer for Serializer {
         _variant: &'static str,
         _len: usize
     ) -> Result<Self::SerializeStructVariant, Self::Error> {
-        Ok(SerializeStructVariant(BTreeMap::new()))
+        Ok(SerializeStructVariant(MapImpl::new()))
     }
 }
 
@@ -339,7 +338,7 @@ impl ser::SerializeTupleVariant for SerializeTupleVariant {
 }
 
 struct SerializeMap {
-    map: BTreeMap<Value, Value>,
+    map: MapImpl<Value, Value>,
     key: Option<Value>,
 }
 
@@ -373,7 +372,7 @@ impl ser::SerializeMap for SerializeMap {
     }
 }
 
-struct SerializeStruct(BTreeMap<Value, Value>);
+struct SerializeStruct(MapImpl<Value, Value>);
 
 impl ser::SerializeStruct for SerializeStruct {
     type Ok = Value;
@@ -398,7 +397,7 @@ impl ser::SerializeStruct for SerializeStruct {
     }
 }
 
-struct SerializeStructVariant(BTreeMap<Value, Value>);
+struct SerializeStructVariant(MapImpl<Value, Value>);
 
 impl ser::SerializeStructVariant for SerializeStructVariant {
     type Ok = Value;