imap: treat \NoSelect mailboxes as empty

\NoSelect are mailboxes that can't be selected, thus treat them as if
they are empty.

melib/src/backends/imap.rs

@@ -157,12 +157,17 @@ impl MailBackend for ImapType {
             let can_create_flags = self.can_create_flags.clone();
             let folder_path = folder.path().to_string();
             let folder_hash = folder.hash();
-            let (permissions, folder_exists) = {
+            let (permissions, folder_exists, no_select) = {
                 let f = &self.folders.read().unwrap()[&folder_hash];
-                (f.permissions.clone(), f.exists.clone())
+                (f.permissions.clone(), f.exists.clone(), f.no_select)
             };
             let connection = self.connection.clone();
             let closure = move |_work_context| {
+                if no_select {
+                    tx.send(AsyncStatus::Payload(Ok(Vec::new()))).unwrap();
+                    tx.send(AsyncStatus::Finished).unwrap();
+                    return;
+                }
                 let connection = connection.clone();
                 let tx = tx.clone();
                 let mut response = String::with_capacity(8 * 1024);

melib/src/backends/imap/folder.rs

@@ -28,6 +28,7 @@ pub struct ImapFolder {
     pub(super) name: String,
     pub(super) parent: Option<FolderHash>,
     pub(super) children: Vec<FolderHash>,
+    pub no_select: bool,
 
     pub permissions: Arc<Mutex<FolderPermissions>>,
     pub exists: Arc<Mutex<usize>>,
@@ -61,6 +62,7 @@ impl BackendFolder for ImapFolder {
             name: self.name.clone(),
             parent: self.parent,
             children: self.children.clone(),
+            no_select: self.no_select,
             permissions: self.permissions.clone(),
             exists: self.exists.clone(),
         })

melib/src/backends/imap/protocol_parser.rs

@@ -54,6 +54,12 @@ named!(
             >> ({
                 let separator: u8 = separator[0];
                 let mut f = ImapFolder::default();
+                f.no_select = false;
+                for p in properties.split(|&b| b == b' ') {
+                    if p.eq_ignore_ascii_case(b"\\NoSelect") {
+                        f.no_select = true;
+                    }
+                }
                 f.hash = get_path_hash!(path);
                 f.path = String::from_utf8_lossy(path).into();
                 f.name = if let Some(pos) = path.iter().rposition(|&c| c == separator) {