to GBK and to UTF8 is not right work

[whereisyou]

i have a GBK string, GBK.decode(rst_raw, DecoderTrap::Strict).is_err() and UTF_8.decode(rst_raw, DecoderTrap::Strict).is_err() can not judge right result.i don't why, so i writed judge "utf8 str" code: fn is_utf8(data: &[u8]) -> bool { let mut i = 0; while i < data.len() { let num = preNUm(data[i]); if data[i] & 0x80 == 0x00 { i += 1; continue; } else if num > 2 { i += 1; let mut j = 0; while j < num -1 { if data[i] & 0xc0 != 0x80 { return false; } j += 1; i += 1; } } else { return false; }

}
return true;

}

fn preNUm(data: u8) -> i32 { let rst = format!("{:b}", data); let mut i = 0; for j in rst.chars() { if j != '1' { break; } i += 1; } return i; }