Use Rc<str> instead of string. (#52)

This reduces std::mem::size_of::<Value>() to 16 bytes.
String values are also efficiently copied like Objects, arrays, sets etc.

When multiple function rules are evaluated, ensure that constant argument values
match before running the rule. If actual parameter does not match the constant parameter,
then the rule is skipped.

Signed-off-by: Anand Krishnamoorthi <anakrish@microsoft.com>

src/value.rs

@@ -90,7 +90,7 @@ pub enum Value {
     Null,
     Bool(bool),
     Number(Number),
-    String(String),
+    String(Rc<str>),
     Array(Rc<Vec<Value>>),
     Object(Rc<BTreeMap<Value, Value>>),
 
@@ -110,7 +110,7 @@ impl Serialize for Value {
         match self {
             Value::Null => serializer.serialize_none(),
             Value::Bool(b) => serializer.serialize_bool(*b),
-            Value::String(s) => serializer.serialize_str(s.as_str()),
+            Value::String(s) => serializer.serialize_str(s.as_ref()),
             Value::Number(n) => n.serialize(serializer),
             Value::Array(a) => a.serialize(serializer),
             Value::Object(fields) => {
@@ -233,14 +233,14 @@ impl Value {
         }
     }
 
-    pub fn as_string(&self) -> Result<&String> {
+    pub fn as_string(&self) -> Result<&Rc<str>> {
         match self {
             Value::String(s) => Ok(s),
             _ => Err(anyhow!("not a string")),
         }
     }
 
-    pub fn as_string_mut(&mut self) -> Result<&mut String> {
+    pub fn as_string_mut(&mut self) -> Result<&mut Rc<str>> {
         match self {
             Value::String(s) => Ok(s),
             _ => Err(anyhow!("not a string")),
@@ -310,7 +310,7 @@ impl Value {
             return Ok(self);
         }
 
-        let key = Value::String(paths[0].to_owned());
+        let key = Value::String(paths[0].into());
         if self == &Value::Undefined {
             *self = Value::new_object();
         }
@@ -379,7 +379,7 @@ impl ops::Index<&str> for Value {
     type Output = Value;
 
     fn index(&self, key: &str) -> &Self::Output {
-        &self[&Value::String(key.to_owned())]
+        &self[&Value::String(key.into())]
     }
 }
 
@@ -387,7 +387,7 @@ impl ops::Index<&String> for Value {
     type Output = Value;
 
     fn index(&self, key: &String) -> &Self::Output {
-        &self[&Value::String(key.clone())]
+        &self[&Value::String(key.clone().into())]
     }
 }