struct DcdWriter<'a>(&'a mut [u8]);
impl<'a> DcdWriter<'a> {
    fn write_len<T: num_traits::ToPrimitive>(
        &'a mut self,
        value: T,
        len: usize,
    ) -> &'a mut DcdWriter<'a> {
        dcd_encode(value.to_u32().unwrap(), &mut self.0[0..len]);
        self.0 = &mut self.0[len..];
        self
    }

    fn write_var<T: num_traits::ToPrimitive>(&'a mut self, value: &T) -> &'a mut DcdWriter<'a> {
        let len = std::mem::size_of_val(&value);
        self.write_len((*value).to_u32().unwrap(), len)
    }

    fn write_simple<'b: 'a>(&'b mut self, value: u32, len: usize) {
        dcd_encode(value, &mut self.0[0..len]);
        self.0 = &mut self.0[len..];
    }
}

impl BeQueryData {
    pub fn to_packet(&self, packet: &mut PckStruct) {
        packet.p_param.resize(48, 0x00);

        let mut writer = DcdWriter(&mut packet.p_param);
        writer
            .write_len(self.status_flags, 6)
            .write_var(&self.vcharge_max_pos)
            .write_var(&self.vcharge_max_neg)
            .write_len(self.icharge_max_pos, 3)
            .write_len(self.icharge_max_neg, 3)
            .write_var(&self.vbat_pos)
            .write_var(&self.vbat_neg)
            .write_len(self.ibat_pos, 5)
            .write_len(self.ibat_neg, 5)
            .write_var(&self.aut_time)
            .write_var(&self.soc)
            .write_var(&self.soh)
            .write_var(&self.tbd);

        // this does not compile
        writer.write_simple(self.status_flags, 6);
        writer.write_simple(self.vcharge_max_pos as u32, 6);
    }
}

Why the code above gives an error for the last two lines (‘cannot borrow writer as mutable more than once at a time’)?
I understand that it is related to the lifetime ‘b but why writer is still considered borrowed after the call has to write_simple is returned?
And why the calls to the other methods does not represent any problem?
Finally is there a way to eliminate the build error on write_simple?