feat(ui): Add the none filter.

It's the opposite of the `all` filter. This one rejects all entries.

crates/matrix-sdk-ui/src/room_list_service/filters/mod.rs

@@ -1,9 +1,11 @@
 mod all;
 mod fuzzy_match_room_name;
+mod none;
 mod normalized_match_room_name;
 
 pub use all::new_filter as new_filter_all;
 pub use fuzzy_match_room_name::new_filter as new_filter_fuzzy_match_room_name;
+pub use none::new_filter as new_filter_none;
 pub use normalized_match_room_name::new_filter as new_filter_normalized_match_room_name;
 use unicode_normalization::{char::is_combining_mark, UnicodeNormalization};
 

crates/matrix-sdk-ui/src/room_list_service/filters/none.rs

@@ -0,0 +1,25 @@
+use matrix_sdk::RoomListEntry;
+
+/// Create a new filter that will reject all entries.
+pub fn new_filter() -> impl Fn(&RoomListEntry) -> bool {
+    |_room_list_entry| -> bool { false }
+}
+
+#[cfg(test)]
+mod tests {
+    use std::ops::Not;
+
+    use matrix_sdk::RoomListEntry;
+    use ruma::room_id;
+
+    use super::new_filter;
+
+    #[test]
+    fn test_all_kind_of_room_list_entry() {
+        let none = new_filter();
+
+        assert!(none(&RoomListEntry::Empty).not());
+        assert!(none(&RoomListEntry::Filled(room_id!("!r0:bar.org").to_owned())).not());
+        assert!(none(&RoomListEntry::Invalidated(room_id!("!r0:bar.org").to_owned())).not());
+    }
+}