To send an OTP (one-time password) with Rust, you will need to use a service that can send SMS or phone calls to deliver the OTP to your users. Here are three options:

1. Use a third-party service: There are many services that allow you to send SMS or phone calls with an API, such as Twilio, Nexmo, and Plivo. These services typically provide Rust libraries or SDKs that you can use to send OTPs.
2. Use the curl command: Some SMS and phone call services provide a simple HTTP API that you can use to send messages. You can use Rust's std::process::Command to invoke the curl command to send an HTTP request to one of these APIs.
3. Use a web API: Some email services (e.g., Gmail, Outlook, etc.) allow you to send SMS messages through their APIs. This approach requires an internet connection and an API key, but it allows you to send SMS messages from Rust without needing to install any libraries or utilities.

Here's an example of how you could use the twilio-rs library and the Twilio API to send an SMS with an OTP from Rust:

send an OTP with rust

use twilio::{Client, OutboundMessage};

fn main() {
    // Set up the Twilio client
    let client = Client::new("ACCOUNT_SID", "AUTH_TOKEN").unwrap();

    // Generate the OTP
    let otp = generate_otp();

    // Send the SMS
    let message = OutboundMessage::send_sms("+1234567890", "+1234567890", &format!("Your OTP is: {}", otp), None, None).unwrap();
    client.send_message(message);
}

fn generate_otp() -> String {
    // Generate a random 6-digit OTP
    let mut rng = rand::thread_rng();
    let otp: u32 = rng.gen_range(100000, 1000000);
    otp.to_string()
}

Note that this example uses the twilio-rs library and the rand crate. You will need to add these dependencies to your project's Cargo.toml file.

One-time passwords (OTPs) are a common way to provide an additional layer of security for sensitive information, such as online banking transactions or access to sensitive systems. Here is a general outline of how you could implement OTPs in Rust:

1. Generate a random OTP and store it in a secure location, such as a database.
2. Send the OTP to the user via a secure channel, such as email or SMS.
3. When the user attempts to access a protected resource, prompt them for the OTP.
4. Check the user-provided OTP against the stored OTP to verify their identity.

Here is some example code that demonstrates how to generate and send an OTP in Rust using the rand and chrono crates:

extern crate rand;
extern crate chrono;

use rand::Rng;
use chrono::Utc;

fn generate_otp() -> String {
    // Generate a random 6-digit OTP
    let otp: u32 = rand::thread_rng().gen_range(100000, 1000000);
    otp.to_string()
}

fn send_otp(otp: &str) {
    // Send the OTP via email or SMS
    println!("Sending OTP: {}", otp);
}

fn main() {
    let otp = generate_otp();
    send_otp(&otp);
}

Keep in mind that this is just a basic example, and a production-ready OTP implementation would likely need to consider additional factors such as expiration times, rate limiting, and secure storage of the OTP.