Email over SMTP

use smtp.qweruse send_email from smtp      // Quillo 0.6: only these

lettre = { version = "0.11", default-features = false, features = ["builder", "smtp-transport", "rustls-tls", "hostname"] }

// A welcome email, sent in the background so the sign-up answers at once.////   SMTP_URL=smtps://user:password@smtp.example.com SMTP_FROM="Shop <shop@example.com>" quillo run smtp.qwe//   then open http://localhost:8080/welcome?to=ada@example.comuse "smtp.qwer"route GET /welcome ? { to: str } {    send_email(to, "Welcome to the shop", "Thanks for signing up!", "<h1>Welcome!</h1><p>Thanks for signing up.</p>")?    { sent: true }}

//! Email sent over SMTP, through any provider (Postmark, SES, Mailgun, SendGrid, Fastmail, Gmail): text, HTML, or both.//! version: 1.0.0//!//! [dependencies]//! lettre = { version = "0.11", default-features = false, features = ["builder", "smtp-transport", "rustls-tls", "hostname"] }use lettre::message::{header::ContentType, Mailbox, MultiPart};use lettre::{Message, SmtpTransport, Transport};use std::sync::OnceLock;fn mailer() -> Result<&'static SmtpTransport, String> {    static M: OnceLock<SmtpTransport> = OnceLock::new();    if let Some(m) = M.get() {        return Ok(m);    }    let url = std::env::var("SMTP_URL").map_err(|_| "SMTP_URL is not set: smtps://user:password@smtp.example.com:465".to_string())?;    let m = SmtpTransport::from_url(&url).map_err(|e| e.to_string())?.timeout(Some(std::time::Duration::from_secs(20))).build();    Ok(M.get_or_init(|| m))}fn mailbox(s: &str) -> Result<Mailbox, String> {    s.trim().parse::<Mailbox>().map_err(|e| format!("`{}`: {}", s, e))}/// Sends a message from `SMTP_FROM` to `to` (one address, or several with commas): `text`, and `html` too unless it is empty. Waits for the server's answer.pub fn send_email(to: &str, subject: &str, text: &str, html: &str) -> Result<(), String> {    let from = std::env::var("SMTP_FROM").map_err(|_| "SMTP_FROM is not set: Shop <shop@example.com>".to_string())?;    let mut b = Message::builder().from(mailbox(&from)?).subject(subject);    for addr in to.split(',').filter(|a| !a.trim().is_empty()) {        b = b.to(mailbox(addr)?);    }    let m = if html.is_empty() {        b.header(ContentType::TEXT_PLAIN).body(text.to_string())    } else {        b.multipart(MultiPart::alternative_plain_html(text.to_string(), html.to_string()))    }    .map_err(|e| e.to_string())?;    mailer()?.send(&m).map(|_| ()).map_err(|e| e.to_string())}