Use an intermediate function to build the handlers

Such a function may later be generated using a procedural macro.

examples/dummy.rs

@@ -1,14 +1,21 @@
 use log::debug;
 use env_logger::{Builder, Env};
-use opensmtpd::{Entry, EventHandler, MatchEvent, SmtpIn};
+use opensmtpd::{handlers, Entry, EventHandler, MatchEvent, SmtpIn};
 
 fn on_event(entry: &Entry) -> bool {
     debug!("Event received: {:?}", entry);
     true
 }
 
+// This function should be replaced by a procedural macro on
+// the `on_event` function.
+fn on_event_builder() -> EventHandler {
+    EventHandler::new(MatchEvent::All, on_event)
+}
+
 fn main() {
     Builder::from_env(Env::default().default_filter_or("debug")).init();
-    let h = vec![EventHandler::new(MatchEvent::All, on_event)];
+    SmtpIn::new()
-    SmtpIn::new().event_handlers(h).run();
+        .event_handlers(handlers!(on_event_builder))
+        .run();
 }

src/lib.rs

@@ -12,6 +12,19 @@ pub use crate::entry::{Entry, Event};
 pub use crate::errors::Error;
 pub use crate::event_handlers::{EventHandler, MatchEvent};
 
+#[macro_export]
+macro_rules! handlers {
+    ( $( $x:expr ),* ) => {
+        {
+            let mut temp_vec = Vec::new();
+            $(
+                temp_vec.push(($x)());
+            )*
+            temp_vec
+        }
+    };
+}
+
 struct SessionHandler {
     entry_rx: mpsc::Receiver<Entry>,
     event_handlers: Vec<EventHandler>,
@@ -19,7 +32,10 @@ struct SessionHandler {
 
 impl SessionHandler {
     fn new(entry_rx: mpsc::Receiver<Entry>, handlers_rx: mpsc::Receiver<EventHandler>) -> Self {
-        debug!("New thread for session {}", thread::current().name().unwrap());
+        debug!(
+            "New thread for session {}",
+            thread::current().name().unwrap()
+        );
         let mut event_handlers = Vec::new();
         for h in handlers_rx.iter() {
             debug!("Event handler registered");