Change item collection to be on demand

compiler/rustc_smir/src/rustc_internal/mod.rs

@@ -3,10 +3,13 @@
 //! For that, we define APIs that will temporarily be public to 3P that exposes rustc internal APIs
 //! until stable MIR is complete.
 
-use crate::stable_mir::CrateItem;
+use crate::stable_mir;
+pub use rustc_span::def_id::{CrateNum, DefId};
 
-pub type DefId = rustc_span::def_id::DefId;
-
-pub fn item_def_id(item: &CrateItem) -> DefId {
+pub fn item_def_id(item: &stable_mir::CrateItem) -> DefId {
     item.0
 }
+
+pub fn crate_num(item: &stable_mir::Crate) -> CrateNum {
+    item.id.into()
+}