Revert "Add "opaque interface declarations""

This reverts commit 396e0a3567.

bindings/matrix-sdk-ffi/src/api.udl

@@ -8,10 +8,6 @@ callback interface NotificationDelegate {
     void did_receive_notification(NotificationItem notification);
 };
 
-interface AuthenticationService {};
-interface Span {};
-interface NotificationService {};
-
 dictionary NotificationItem {
     TimelineEvent event;
     string room_id;

bindings/matrix-sdk-ffi/src/authentication_service.rs

@@ -11,6 +11,7 @@ use zeroize::Zeroize;
 use super::{client::Client, client_builder::ClientBuilder, RUNTIME};
 use crate::error::ClientError;
 
+#[derive(uniffi::Object)]
 pub struct AuthenticationService {
     base_path: String,
     passphrase: Option<String>,

bindings/matrix-sdk-ffi/src/notification_service.rs

@@ -51,6 +51,7 @@ impl NotificationItem {
 }
 
 #[allow(dead_code)]
+#[derive(uniffi::Object)]
 pub struct NotificationService {
     base_path: String,
     user_id: String,

bindings/matrix-sdk-ffi/src/tracing.rs

@@ -95,6 +95,7 @@ fn span_or_event_enabled(callsite: &'static DefaultCallsite) -> bool {
     }
 }
 
+#[derive(uniffi::Object)]
 pub struct Span(tracing::Span);
 
 #[uniffi::export]