fix: escape json for string input (moonbitlang#858)

builtin/json.mbt

@@ -47,8 +47,42 @@ pub fn Double::to_json(self : Double) -> Json {
   Number(self)
 }
 
+fn escape_json_string(str : String) -> String {
+  fn to_hex_digit(i : Int) -> Char {
+    if i < 10 {
+      Char::from_int('0'.to_int() + i)
+    } else {
+      Char::from_int('a'.to_int() + (i - 10))
+    }
+  }
+
+  let buf = Buffer::new(size_hint=str.length())
+  for i = 0; i < str.length(); i = i + 1 {
+    let c = str[i]
+    match c {
+      '\n' => buf.write_string("\\n")
+      '\r' => buf.write_string("\\r")
+      '\b' => buf.write_string("\\b")
+      '\t' => buf.write_string("\\t")
+      _ => {
+        let code = c.to_int()
+        if code == 0x0C {
+          buf.write_string("\\f")
+        } else if code < 0x20 {
+          buf.write_string("\\u00")
+          buf.write_char(to_hex_digit(code / 16))
+          buf.write_char(to_hex_digit(code % 16))
+        } else {
+          buf.write_char(c)
+        }
+      }
+    }
+  }
+  buf.to_string()
+}
+
 pub fn String::to_json(self : String) -> Json {
-  String(self)
+  String(escape_json_string(self))
 }
 
 pub fn Char::to_json(self : Char) -> Json {

json/json_test.mbt

@@ -295,11 +295,12 @@ test "stringify escape" {
     "\n": "\n\r\t\x0C",
     "\\": "/",
     "\x1F": "\x01",
+    "actual": "\x00",
   }
   inspect!(
     json.stringify(),
     content=
-      #|{"\b":"\"hello\"","\n":"\n\r\t\f","\\":"\/","\u001f":"\u0001"}
+      #|{"\b":"\"hello\"","\n":"\n\r\t\f","\\":"\/","\u001f":"\u0001","actual":"\u0000"}
     ,
   )
   match @json.parse?(json.stringify()) {

json/to_json_test.mbt

@@ -42,6 +42,30 @@ test "String::to_json" {
       #|String("a,\"b\",c")
     ,
   )
+  inspect!(
+    "\"".to_json(),
+    content=
+      #|String("\"")
+    ,
+  )
+  inspect!(
+    "\x00".to_json(),
+    content=
+      #|String("\\u0000")
+    ,
+  )
+  inspect!(
+    "\n".to_json(),
+    content=
+      #|String("\\n")
+    ,
+  )
+  inspect!(
+    "\\".to_json(),
+    content=
+      #|String("\\")
+    ,
+  )
 }
 
 test "Char::to_json" {