Add some details about DefId creation

Author: oli-obk

src/lowering.md

@@ -32,4 +32,16 @@ sanity checks in `src/librustc/hir/map/hir_id_validator.rs`:
 4. If you are creating new nodes that didn't exist in the `AST`, you *must*
   create new ids for them. This is done by calling the `next_id` method,
   which produces both a new `NodeId` as well as automatically lowering it
-  for you so you also get the `HirId`.
+  for you so you also get the `HirId`.
+
+If you are creating new `DefId`s, since each `DefId` needs to have a corresponding
+`NodeId`, it is adviseable to add these `NodeId`s to the `AST` so you don't have
+to generate new ones during lowering. This has the advantage of creating a
+way to find the `DefId` of something via its `NodeId`. If lowering needs this
+`DefId` in multiple places, you can't generate a new `NodeId` in all those places
+because you'd also get a new `DefId` then. With a `NodeId` from the `AST` this is
+not an issue.
+
+Having the `NodeId` also allows the `DefCollector` to generate the `DefId`s instead
+of lowering having to do it on the fly. Centralizing the `DefId` generation in one
+place makes it easier to refactor and reason about.