cstore: Use IndexSet as backing store for postorder dependencies

<rustc_metadata::creader::CStore>::push_dependencies_in_postorder showed up in new benchmarks from rust-lang/rustc-perf#2143, hence I gave it a shot to remove an obvious O(n) there.

Author: osiewicz

compiler/rustc_metadata/src/creader.rs

@@ -22,6 +22,7 @@ use rustc_hir::def_id::{CrateNum, LOCAL_CRATE, LocalDefId, StableCrateId};
 use rustc_hir::definitions::Definitions;
 use rustc_index::IndexVec;
 use rustc_middle::bug;
+use rustc_middle::ty::data_structures::IndexSet;
 use rustc_middle::ty::{TyCtxt, TyCtxtFeed};
 use rustc_session::config::{
     self, CrateType, ExtendedTargetModifierInfo, ExternLocation, OptionsTargetModifiers,
@@ -281,7 +282,7 @@ impl CStore {
             .filter_map(|(cnum, data)| data.as_deref_mut().map(|data| (cnum, data)))
     }
 
-    fn push_dependencies_in_postorder(&self, deps: &mut Vec<CrateNum>, cnum: CrateNum) {
+    fn push_dependencies_in_postorder(&self, deps: &mut IndexSet<CrateNum>, cnum: CrateNum) {
         if !deps.contains(&cnum) {
             let data = self.get_crate_data(cnum);
             for dep in data.dependencies() {
@@ -290,12 +291,12 @@ impl CStore {
                 }
             }
 
-            deps.push(cnum);
+            deps.insert(cnum);
         }
     }
 
-    pub(crate) fn crate_dependencies_in_postorder(&self, cnum: CrateNum) -> Vec<CrateNum> {
-        let mut deps = Vec::new();
+    pub(crate) fn crate_dependencies_in_postorder(&self, cnum: CrateNum) -> IndexSet<CrateNum> {
+        let mut deps = IndexSet::default();
         if cnum == LOCAL_CRATE {
             for (cnum, _) in self.iter_crate_data() {
                 self.push_dependencies_in_postorder(&mut deps, cnum);
@@ -306,7 +307,7 @@ impl CStore {
         deps
     }
 
-    fn crate_dependencies_in_reverse_postorder(&self, cnum: CrateNum) -> Vec<CrateNum> {
+    fn crate_dependencies_in_reverse_postorder(&self, cnum: CrateNum) -> IndexSet<CrateNum> {
         let mut deps = self.crate_dependencies_in_postorder(cnum);
         deps.reverse();
         deps

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

@@ -549,8 +549,9 @@ pub(in crate::rmeta) fn provide(providers: &mut Providers) {
         has_global_allocator: |tcx, LocalCrate| CStore::from_tcx(tcx).has_global_allocator(),
         has_alloc_error_handler: |tcx, LocalCrate| CStore::from_tcx(tcx).has_alloc_error_handler(),
         postorder_cnums: |tcx, ()| {
-            tcx.arena
-                .alloc_slice(&CStore::from_tcx(tcx).crate_dependencies_in_postorder(LOCAL_CRATE))
+            tcx.arena.alloc_from_iter(
+                CStore::from_tcx(tcx).crate_dependencies_in_postorder(LOCAL_CRATE).into_iter(),
+            )
         },
         crates: |tcx, ()| {
             // The list of loaded crates is now frozen in query cache,