How to read a connection for a serial?

I'm making a code so i can turn on and off a led using a bluetooth app and a bluetooth device. The point is if i choose a letter, the led turns on, and if i use another, it turns off. I already advanced it but i'm stuck with how can i do to make the card to recieve the signal. I found a portion of the code i need but it's for an arduino and i need it in Rust. Can anyone help me translate it?

This is my code

fn main() -> ! {
    let dp = stm32::Peripherals::take().unwrap();
    let cp = cortex_m::peripheral::Peripherals::take().unwrap();

    let gpioa = dp.GPIOA.split();

    let rcc = dp.RCC.constrain();

    let clocks = rcc.cfgr.use_hse(8.mhz()).freeze();

    let mut delay = hal::delay::Delay::new(cp.SYST, clocks);

    //define pins
    let tx_pin = gpioa.pa9.into_alternate_af7();
    let rx_pin = gpioa.pa10.into_alternate_af7();
    let mut led_pin = gpioa.pa7.into_push_pull_output();

    // configure serial
    let serial = Serial::usart1(
        dp.USART1,
        (tx_pin, rx_pin),
        Config::default().baudrate(9600.bps()),
        clocks,
    ).unwrap();

    let (mut _tx, mut rx) = serial.split();

    let mut state = 0;

    loop {


        if state == 'a' {
            led_pin.set_high().unwrap();
        }

        if state == 'b' {
            led_pin.set_low().unwrap();
        }
    }
}

This is what i used to build the code

int led=13;
int state=0;

void setup(){
    Serial.begin(9600);
    pinMode(led,OUTPUT);
}

void loop(){
    if (Serial.available()>0){
        state = Serial.read();
    } 
    if (state == 'a'){
        digitalWrite(led,'HIGH');
    }
    if (state == 'b'){
        digitalWrite(led,'LOW');
    }
}

Can anyone tell me how to implement it correctly or what am i missing?

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