diff --git a/compiler/rustc_middle/src/mir/mono.rs b/compiler/rustc_middle/src/mir/mono.rs
index 892808386dee..13c325a14e40 100644
--- a/compiler/rustc_middle/src/mir/mono.rs
+++ b/compiler/rustc_middle/src/mir/mono.rs
@@ -7,6 +7,7 @@
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher};
 use rustc_hir::def_id::{CrateNum, DefId, LOCAL_CRATE};
 use rustc_hir::ItemId;
+use rustc_index::vec::Idx;
 use rustc_query_system::ich::{NodeIdHashingMode, StableHashingContext};
 use rustc_session::config::OptLevel;
 use rustc_span::source_map::Span;
@@ -380,7 +381,7 @@ fn item_sort_key<'tcx>(tcx: TyCtxt<'tcx>, item: MonoItem<'tcx>) -> ItemSortKey<'
                             // instances into account. The others don't matter for
                             // the codegen tests and can even make item order
                             // unstable.
-                            InstanceDef::Item(def) => Some(def.did.index.as_usize()),
+                            InstanceDef::Item(def) => def.did.as_local().map(Idx::index),
                             InstanceDef::VtableShim(..)
                             | InstanceDef::ReifyShim(..)
                             | InstanceDef::Intrinsic(..)
@@ -391,10 +392,8 @@ fn item_sort_key<'tcx>(tcx: TyCtxt<'tcx>, item: MonoItem<'tcx>) -> ItemSortKey<'
                             | InstanceDef::CloneShim(..) => None,
                         }
                     }
-                    MonoItem::Static(def_id) => Some(def_id.index.as_usize()),
-                    MonoItem::GlobalAsm(item_id) => {
-                        Some(item_id.def_id.to_def_id().index.as_usize())
-                    }
+                    MonoItem::Static(def_id) => def_id.as_local().map(Idx::index),
+                    MonoItem::GlobalAsm(item_id) => Some(item_id.def_id.index()),
                 },
                 item.symbol_name(tcx),
             )