email: Reduce monomorphization overhead for the Emails::build_message() fn (#10304)

Author: Turbo87

src/email.rs

@@ -84,7 +84,12 @@ impl Emails {
         }
     }
 
-    fn build_message<E: Email>(&self, recipient: &str, email: E) -> Result<Message, EmailError> {
+    fn build_message(
+        &self,
+        recipient: &str,
+        subject: String,
+        body: String,
+    ) -> Result<Message, EmailError> {
         // The message ID is normally generated by the SMTP server, but if we let it generate the
         // ID there will be no way for the crates.io application to know the ID of the message it
         // just sent, as it's not included in the SMTP response.
@@ -100,9 +105,6 @@ impl Emails {
 
         let from = Mailbox::new(Some(self.domain.clone()), self.from.clone());
 
-        let subject = email.subject();
-        let body = email.body();
-
         let message = Message::builder()
             .message_id(Some(message_id.clone()))
             .to(recipient.parse()?)
@@ -115,7 +117,7 @@ impl Emails {
     }
 
     pub async fn send<E: Email>(&self, recipient: &str, email: E) -> Result<(), EmailError> {
-        let email = self.build_message(recipient, email)?;
+        let email = self.build_message(recipient, email.subject(), email.body())?;
 
         self.backend
             .send(email)