Function rs_roundtrip_serialize_error

crates/ffi/src/compile.rs:1511–1516

(
  obj: LeanIxSerializeError<LeanBorrowed<'_>>,
)

      ),
      2 => {
        let reason = self.get_obj(0).as_string().to_string();
        CompileError::InvalidMutualBlock { reason }
      },
      3 => {
        let desc = self.get_obj(0).as_string().to_string();
        CompileError::UnsupportedExpr { desc }
      },
      4 => {
        let curr = self.get_obj(0).as_string().to_string();