Use function wrapper to annotate lifetimes on closures.

See discussion rust-lang/rust#58052

Author: djg

fluent-fallback/tests/l10nregistry.rs

@@ -28,10 +28,14 @@ fn localization_format_sync() {
 
     reg.register_sources(vec![main_fs]).unwrap();
 
-    let generate_messages = |res_ids: &[PathBuf]| {
+    fn annotate<'a, T: 'a, U: 'a, F>(f: F) -> F where F: Fn(T) -> U {
+        f
+    }
+
+    let generate_messages = annotate(|res_ids: &[PathBuf]| {
         let locales = get_app_locales();
-        reg.generate_bundles_sync(locales, res_ids)
-    };
+         reg.generate_bundles_sync(locales, res_ids)
+    });
 
     let loc = Localization::new(resource_ids.clone(), Some(generate_messages));