Allow to create definitions inside the query system.

compiler/rustc_middle/src/query/mod.rs

@@ -20,6 +20,12 @@ rustc_queries! {
         desc { "trigger a delay span bug" }
     }
 
+    /// Create a new definition within the incr. comp. engine.
+    query register_def(_: ty::RawLocalDefId) -> LocalDefId {
+        eval_always
+        desc { "register a DefId with the incr. comp. engine" }
+    }
+
     query resolutions(_: ()) -> &'tcx ty::ResolverOutputs {
         eval_always
         no_hash