more clippy fixes:

clippy::match_like_matches_macro
clippy::to_string_in_format_args
clippy::single_char_add_str
clippy::filter_map_identity
clippy::clone_on_copy
clippy::useless_format
clippy::unused_unit

crates/ide/src/file_structure.rs

@@ -138,7 +138,7 @@ fn structure_node(node: &SyntaxNode) -> Option<StructureNode> {
                     collapse_ws(param_list.syntax(), &mut detail);
                 }
                 if let Some(ret_type) = it.ret_type() {
-                    detail.push_str(" ");
+                    detail.push(' ');
                     collapse_ws(ret_type.syntax(), &mut detail);
                 }