feat: Remove subject prefix from ad-hoc group names (#5385)

Delta Chat -style groups have names w/o prefixes like "Re: " even if the user is added to an already
existing group, so let's remove prefixes from ad-hoc group names too. Usually it's not very
important that the group is a classic email thread existed before, this info just eats up screen
space. Also this way a group name is likely to preserve if the first message was missed.

src/receive_imf.rs

@@ -40,7 +40,7 @@ use crate::simplify;
 use crate::sql;
 use crate::stock_str;
 use crate::sync::Sync::*;
-use crate::tools::{self, buf_compress};
+use crate::tools::{self, buf_compress, remove_subject_prefix};
 use crate::{chatlist_events, location};
 use crate::{contact, imap};
 use iroh_net::NodeAddr;
@@ -1936,8 +1936,9 @@ async fn create_group(
         let grpname = mime_parser
             .get_header(HeaderDef::ChatGroupName)
             .context("Chat-Group-Name vanished")?
-            // W/a for "Space added before long group names after MIME serialization/deserialization
-            // #3650" issue. DC itself never creates group names with leading/trailing whitespace.
+            // Workaround for the "Space added before long group names after MIME
+            // serialization/deserialization #3650" issue. DC itself never creates group names with
+            // leading/trailing whitespace.
             .trim();
         let new_chat_id = ChatId::create_multiuser_record(
             context,
@@ -2127,12 +2128,10 @@ async fn apply_group_changes(
         }
     } else if let Some(old_name) = mime_parser
         .get_header(HeaderDef::ChatGroupNameChanged)
-        // See create_or_lookup_group() for explanation
         .map(|s| s.trim())
     {
         if let Some(grpname) = mime_parser
             .get_header(HeaderDef::ChatGroupName)
-            // See create_or_lookup_group() for explanation
             .map(|grpname| grpname.trim())
             .filter(|grpname| grpname.len() < 200)
         {
@@ -2547,9 +2546,9 @@ async fn create_adhoc_group(
         return Ok(None);
     }
 
-    // use subject as initial chat name
     let grpname = mime_parser
         .get_subject()
+        .map(|s| remove_subject_prefix(&s))
         .unwrap_or_else(|| "Unnamed group".to_string());
 
     let new_chat_id: ChatId = ChatId::create_multiuser_record(

src/receive_imf/tests.rs

@@ -4228,6 +4228,33 @@ async fn test_keep_member_list_if_possibly_nomember() -> Result<()> {
     Ok(())
 }
 
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn test_adhoc_grp_name_no_prefix() -> Result<()> {
+    let mut tcm = TestContextManager::new();
+    let alice = &tcm.alice().await;
+    let chat_id = receive_imf(
+        alice,
+        b"Subject: Re: Once upon a time this was with the only Re: here\n\
+          From: <[email protected]>\n\
+          To: <[email protected]>, <[email protected]>\n\
+          Date: Mon, 12 Dec 3000 14:32:39 +0000\n\
+          Message-ID: <[email protected]>\n\
+          In-Reply-To: <[email protected]>\n\
+          \n\
+          Adding Alice the Delta Chat lover",
+        false,
+    )
+    .await?
+    .unwrap()
+    .chat_id;
+    let chat = Chat::load_from_db(alice, chat_id).await.unwrap();
+    assert_eq!(
+        chat.get_name(),
+        "Once upon a time this was with the only Re: here"
+    );
+    Ok(())
+}
+
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn test_download_later() -> Result<()> {
     let mut tcm = TestContextManager::new();