use @builtin.StringBuilder in json package

json/internal_types.mbt

@@ -61,40 +61,3 @@ priv enum Token {
   Comma
   Colon
 } derive(Eq, Show)
-
-priv struct StringBuilder {
-  mut buffer : String
-}
-
-fn StringBuilder::make() -> StringBuilder {
-  { buffer: "" }
-}
-
-fn add_string(self : StringBuilder, s : String) -> Unit {
-  self.buffer = self.buffer + s
-}
-
-fn add_substring(
-  self : StringBuilder,
-  s : String,
-  start : Int,
-  end : Int
-) -> Unit {
-  self.buffer = self.buffer + s.substring(~start, ~end)
-}
-
-fn add_char(self : StringBuilder, c : Char) -> Unit {
-  self.buffer = self.buffer + c.to_string()
-}
-
-fn to_string(self : StringBuilder) -> String {
-  self.buffer
-}
-
-test "add_string method coverage" {
-  let sb = StringBuilder::make()
-  add_string(sb, "Hello")
-  assert_eq!(sb.buffer, "Hello")
-  add_string(sb, " World")
-  assert_eq!(sb.buffer, "Hello World")
-}

json/json.mbt

@@ -96,7 +96,7 @@ pub fn stringify(
         if members.is_empty() {
           return "{}"
         }
-        let buf = Buffer::new(size_hint=0)
+        let buf = StringBuilder::new(size_hint=0)
         buf.write_char('{')
         buf.write_string(indent_str(level + 1, indent))
         let mut first = true
@@ -125,7 +125,7 @@ pub fn stringify(
         if arr.is_empty() {
           return "[]"
         }
-        let buf = Buffer::new(size_hint=0)
+        let buf = StringBuilder::new(size_hint=0)
         buf.write_char('[')
         buf.write_string(indent_str(level + 1, indent))
         for i, v in arr {
@@ -139,7 +139,7 @@ pub fn stringify(
         buf..write_char(']').to_string()
       }
       String(s) => {
-        let buf = Buffer::new(size_hint=0)
+        let buf = StringBuilder::new(size_hint=0)
         buf
         ..write_char('\"')
         ..write_string(escape(s, ~escape_slash))
@@ -165,7 +165,7 @@ fn escape(str : String, ~escape_slash : Bool) -> String {
     }
   }
 
-  let buf = Buffer::new(size_hint=str.length())
+  let buf = StringBuilder::new(size_hint=str.length())
   for c in str {
     match c {
       '"' | '\\' => buf.write_string("\\\{c}")

json/lex_string.mbt

@@ -13,11 +13,11 @@
 // limitations under the License.
 
 fn lex_string(ctx : ParseContext) -> String!ParseError {
-  let buf = StringBuilder::make()
+  let buf = StringBuilder::new()
   let mut start = ctx.offset
   fn flush(end : Int) {
     if start > 0 && end > start {
-      buf.add_substring(ctx.input, start, end)
+      buf.write_sub_string(ctx.input, start, end - start)
     }
   }
 
@@ -31,17 +31,17 @@ fn lex_string(ctx : ParseContext) -> String!ParseError {
       Some('\\') => {
         flush(ctx.offset - 1)
         match read_char(ctx) {
-          Some('b') => buf.add_char('\b')
-          Some('f') => buf.add_char('\x0C')
-          Some('n') => buf.add_char('\n')
-          Some('r') => buf.add_char('\r')
-          Some('t') => buf.add_char('\t')
-          Some('"') => buf.add_char('"')
-          Some('\\') => buf.add_char('\\')
-          Some('/') => buf.add_char('/')
+          Some('b') => buf.write_char('\b')
+          Some('f') => buf.write_char('\x0C')
+          Some('n') => buf.write_char('\n')
+          Some('r') => buf.write_char('\r')
+          Some('t') => buf.write_char('\t')
+          Some('"') => buf.write_char('"')
+          Some('\\') => buf.write_char('\\')
+          Some('/') => buf.write_char('/')
           Some('u') => {
             let c = lex_hex_digits!(ctx, 4)
-            buf.add_char(Char::from_int(c))
+            buf.write_char(Char::from_int(c))
           }
           Some(_) => invalid_char!(ctx, shift=-1)
           None => raise InvalidEof