Add dc_msg_set_subject() C FFI (#4057)

src/message.rs

@@ -797,6 +797,12 @@ impl Message {
         self.text = text;
     }
 
+    /// Sets the email's subject. If it's empty, a default subject
+    /// will be used (e.g. `Message from Alice` or `Re: <last subject>`).
+    pub fn set_subject(&mut self, subject: String) {
+        self.subject = subject;
+    }
+
     /// Sets the file associated with a message.
     ///
     /// This function does not use the file or check if it exists,

src/mimefactory.rs

@@ -1612,6 +1612,22 @@ mod tests {
         assert_eq!(maybe_encode_words("äöü"), "=?utf-8?b?w6TDtsO8?=");
     }
 
+    #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+    async fn test_manually_set_subject() -> Result<()> {
+        let t = TestContext::new_alice().await;
+        let chat = t.create_chat_with_contact("bob", "bob@example.org").await;
+
+        let mut msg = Message::new(Viewtype::Text);
+        msg.set_subject("Subjeeeeect".to_string());
+
+        let sent_msg = t.send_msg(chat.id, &mut msg).await;
+        let payload = sent_msg.payload();
+
+        assert_eq!(payload.match_indices("Subject: Subjeeeeect").count(), 1);
+
+        Ok(())
+    }
+
     #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
     async fn test_subject_from_mua() {
         // 1.: Receive a mail from an MUA