Simplify foreign_modules.

compiler/rustc_metadata/src/foreign_modules.rs

@@ -1,19 +1,31 @@
+use rustc_data_structures::fx::FxHashMap;
 use rustc_hir as hir;
 use rustc_hir::def::DefKind;
+use rustc_hir::def_id::DefId;
+use rustc_middle::query::LocalCrate;
 use rustc_middle::ty::TyCtxt;
 use rustc_session::cstore::ForeignModule;
 
-pub(crate) fn collect(tcx: TyCtxt<'_>) -> Vec<ForeignModule> {
-    let mut modules = Vec::new();
+pub(crate) fn collect(
+    tcx: TyCtxt<'_>,
+    LocalCrate: LocalCrate,
+) -> FxHashMap<DefId, ForeignModule> {
+    let mut modules = FxHashMap::default();
+
+    // We need to collect all the `ForeignMod`, even if they are empty.
     for id in tcx.hir().items() {
         if !matches!(tcx.def_kind(id.owner_id), DefKind::ForeignMod) {
             continue;
         }
+
+        let def_id = id.owner_id.to_def_id();
         let item = tcx.hir().item(id);
+
         if let hir::ItemKind::ForeignMod { items, .. } = item.kind {
             let foreign_items = items.iter().map(|it| it.id.owner_id.to_def_id()).collect();
-            modules.push(ForeignModule { foreign_items, def_id: id.owner_id.to_def_id() });
+            modules.insert(def_id, ForeignModule { foreign_items, def_id });
         }
     }
+
     modules
 }

compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs

@@ -404,9 +404,7 @@ pub(in crate::rmeta) fn provide(providers: &mut Providers) {
                 })
         },
         native_libraries: |tcx, LocalCrate| native_libs::collect(tcx),
-        foreign_modules: |tcx, LocalCrate| {
-            foreign_modules::collect(tcx).into_iter().map(|m| (m.def_id, m)).collect()
-        },
+        foreign_modules: foreign_modules::collect,
 
         // Returns a map from a sufficiently visible external item (i.e., an
         // external item that is visible from at least one local module) to a