Allow to create definitions inside the query system.

compiler/rustc_data_structures/src/lib.rs

@@ -10,6 +10,7 @@
 #![feature(array_windows)]
 #![feature(associated_type_bounds)]
 #![feature(auto_traits)]
+#![feature(cell_leak)]
 #![feature(control_flow_enum)]
 #![feature(extend_one)]
 #![feature(let_else)]