Implement flashed error messages

src/main.rs

             }
         });
 
-    app.at("/login").get(|req: tide::Request<()>| async move {
-        let tera = Tera::new("templates/**/*.html")?;
-        let mut context = Context::new();
-        let logged_in: bool = req.session().get("logged_in").unwrap_or(false);
-        context.insert("logged_in", &logged_in);
-        let html = tera.render("login.html", &context)?;
-        Ok(Body::from_string(html))
-    }).post(|mut req: tide::Request<()>| async move {
-        let logged_in: bool = req.session().get("logged_in").unwrap_or(false);
-        if logged_in {
-            return Ok(tide::Redirect::new("/"));
-        }
+    app.at("/login")
+        .get(|mut req: tide::Request<()>| async move {
+            let tera = Tera::new("templates/**/*.html")?;
+            let mut context = Context::new();
+            let logged_in: bool = req.session().get("logged_in").unwrap_or(false);
+            context.insert("logged_in", &logged_in);
+            match req.session_mut().get::<String>("flash_error") {
+                Some(error) => {
+                    req.session_mut().remove("flash_error");
+                    &context.insert("error", &error);
+                }
+                None => {}
+            }
+            let html = tera.render("login.html", &context)?;
+            Ok(Body::from_string(html))
+        })
+        .post(|mut req: tide::Request<()>| async move {
+            let logged_in: bool = req.session().get("logged_in").unwrap_or(false);
+            if logged_in {
+                return Ok(tide::Redirect::new("/"));
+            }
 
-        let username = env::var("ADMIN_USERNAME")?;
-        let password = env::var("ADMIN_PASSWORD")?;
-        let user: User = req.body_form().await?;
-        if user.username == username && user.password == password {
-            req.session_mut().insert("logged_in", true)?;
-            Ok(tide::Redirect::new("/"))
-        } else {
-            Ok(tide::Redirect::new("/login"))
-        }
-    });
+            let username = env::var("ADMIN_USERNAME")?;
+            let password = env::var("ADMIN_PASSWORD")?;
+            let user: User = req.body_form().await?;
+            if user.username == username && user.password == password {
+                req.session_mut().remove("logged_in");
+                req.session_mut().insert("logged_in", true)?;
+                Ok(tide::Redirect::new("/"))
+            } else {
+                req.session_mut().remove("logged_in");
+                req.session_mut().insert("flash_error", "Invalid credentials")?;
+                Ok(tide::Redirect::new("/login"))
+            }
+        });
 
     app.at("/logout").get(|mut req: tide::Request<()>| async move {
         req.session_mut().remove("logged_in");

templates/login.html

   </div>
   {% if error %}
     <div class="error">
-      <h1 class="error__heading">Error:</h1>
       <p class="error__message">{{ error }}</p>
     </div>
   {% endif %}