Use dtoa to not allocate a String to serialize a float

Cargo.toml

@@ -12,6 +12,7 @@ keywords = ["serde", "serialization", "urlencoded"]
 test = false
 
 [dependencies]
+dtoa = "0.4.0"
 itoa = "0.3.0"
 serde = "0.9.2"
 url = "1.0.0"

src/lib.rs

@@ -1,6 +1,7 @@
 //! `x-www-form-urlencoded` meets Serde
 
 extern crate itoa;
+extern crate dtoa;
 #[macro_use]
 extern crate serde;
 extern crate url;

src/ser/part.rs

@@ -1,3 +1,4 @@
+use dtoa;
 use itoa;
 use ser::Error;
 use ser::void::VoidSerializer;
@@ -83,11 +84,11 @@ impl<S: Sink> ser::Serializer for PartSerializer<S> {
     }
 
     fn serialize_f32(self, v: f32) -> Result<S::Ok, Error> {
-        self.sink.serialize_string(v.to_string())
+        self.serialize_floating(v)
     }
 
     fn serialize_f64(self, v: f64) -> Result<S::Ok, Error> {
-        self.sink.serialize_string(v.to_string())
+        self.serialize_floating(v)
     }
 
     fn serialize_char(self, v: char) -> Result<S::Ok, Error> {
@@ -218,4 +219,13 @@ impl<S: Sink> PartSerializer<S> {
         let part = unsafe { str::from_utf8_unchecked(&buf[0..len]) };
         ser::Serializer::serialize_str(self, part)
     }
+
+    fn serialize_floating<F>(self, value: F) -> Result<S::Ok, Error>
+        where F: dtoa::Floating
+    {
+        let mut buf = [b'\0'; 24];
+        let len = dtoa::write(&mut buf[..], value).unwrap();
+        let part = unsafe { str::from_utf8_unchecked(&buf[0..len]) };
+        ser::Serializer::serialize_str(self, part)
+    }
 }