fix(json): use two-char short forms \b and \f in JSON.stringify (#5047)

crates/perry-runtime/src/json/mod.rs

@@ -758,4 +758,19 @@ mod tests {
         let output = unsafe { js_json_stringify(outer_boxed, TYPE_UNKNOWN) };
         assert_eq!(unsafe { str_from_header(output).unwrap() }, r#"{"o":{}}"#);
     }
+
+    #[test]
+    fn stringify_uses_two_char_short_forms_for_backspace_and_formfeed() {
+        // #5047: QuoteJSONString mandates the two-character escapes for
+        // U+0008 (\b) and U+000C (\f); Perry emitted the 4-hex-digit form for both. Other
+        // control characters keep the 4-hex-digit form.
+        let input = b"a\x08b\x0cc\x0bd";
+        let s = js_string_from_bytes(input.as_ptr(), input.len() as u32);
+        let boxed = f64::from_bits(STRING_TAG | (s as u64 & POINTER_MASK));
+        let output = unsafe { js_json_stringify(boxed, TYPE_UNKNOWN) };
+        assert_eq!(
+            unsafe { str_from_header(output).unwrap() },
+            "\"a\\bb\\fc\\u000bd\""
+        );
+    }
 }

crates/perry-runtime/src/json/stringify.rs

@@ -333,6 +333,8 @@ pub(crate) unsafe fn write_escaped_string(buf: &mut String, s: &str) {
             b'\n' => Some("\\n"),
             b'\r' => Some("\\r"),
             b'\t' => Some("\\t"),
+            0x08 => Some("\\b"),
+            0x0c => Some("\\f"),
             0..=0x1f => {
                 if start < i {
                     buf_vec.extend_from_slice(&bytes[start..i]);