serialize: Avoid stray nul characters when auto-serializing char.

Fixes #19719.

src/libserialize/json.rs

@@ -367,8 +367,8 @@ fn escape_str(writer: &mut io::Writer, v: &str) -> Result<(), io::IoError> {
 
 fn escape_char(writer: &mut io::Writer, v: char) -> Result<(), io::IoError> {
     let mut buf = [0, .. 4];
-    v.encode_utf8(&mut buf);
-    escape_bytes(writer, &mut buf)
+    let len = v.encode_utf8(&mut buf).unwrap();
+    escape_bytes(writer, buf[mut ..len])
 }
 
 fn spaces(wr: &mut io::Writer, mut n: uint) -> Result<(), io::IoError> {
@@ -2694,37 +2694,39 @@ mod tests {
         );
     }
 
+    macro_rules! check_encoder_for_simple(
+        ($value:expr, $expected:expr) => ({
+            let s = with_str_writer(|writer| {
+                let mut encoder = Encoder::new(writer);
+                $value.encode(&mut encoder).unwrap();
+            });
+            assert_eq!(s, $expected);
+
+            let s = with_str_writer(|writer| {
+                let mut encoder = PrettyEncoder::new(writer);
+                $value.encode(&mut encoder).unwrap();
+            });
+            assert_eq!(s, $expected);
+        })
+    )
+
     #[test]
     fn test_write_some() {
-        let value = Some("jodhpurs".into_string());
-        let s = with_str_writer(|writer| {
-            let mut encoder = Encoder::new(writer);
-            value.encode(&mut encoder).unwrap();
-        });
-        assert_eq!(s, "\"jodhpurs\"");
-
-        let value = Some("jodhpurs".into_string());
-        let s = with_str_writer(|writer| {
-            let mut encoder = PrettyEncoder::new(writer);
-            value.encode(&mut encoder).unwrap();
-        });
-        assert_eq!(s, "\"jodhpurs\"");
+        check_encoder_for_simple!(Some("jodhpurs".to_string()), "\"jodhpurs\"");
     }
 
     #[test]
     fn test_write_none() {
-        let value: Option<string::String> = None;
-        let s = with_str_writer(|writer| {
-            let mut encoder = Encoder::new(writer);
-            value.encode(&mut encoder).unwrap();
-        });
-        assert_eq!(s, "null");
+        check_encoder_for_simple!(None::<string::String>, "null");
+    }
 
-        let s = with_str_writer(|writer| {
-            let mut encoder = Encoder::new(writer);
-            value.encode(&mut encoder).unwrap();
-        });
-        assert_eq!(s, "null");
+    #[test]
+    fn test_write_char() {
+        check_encoder_for_simple!('a', "\"a\"");
+        check_encoder_for_simple!('\t', "\"\\t\"");
+        check_encoder_for_simple!('\u00a0', "\"\u00a0\"");
+        check_encoder_for_simple!('\uabcd', "\"\uabcd\"");
+        check_encoder_for_simple!('\U0010ffff', "\"\U0010ffff\"");
     }
 
     #[test]