From 580253e6a299b62b2ef6492584b923195945dcad Mon Sep 17 00:00:00 2001
From: LorrensP-2158466 <lorrens.pantelis@student.uhasselt.be>
Date: Tue, 28 Jul 2026 20:22:14 +0200
Subject: [PATCH] split module resolutions into local and external types, with
 external having a `OnceLock` around it for parallel import resolution

---
 .../rustc_resolve/src/build_reduced_graph.rs  |  5 +-
 compiler/rustc_resolve/src/lib.rs             | 49 +++++++++++--------
 2 files changed, 31 insertions(+), 23 deletions(-)

diff --git a/compiler/rustc_resolve/src/build_reduced_graph.rs b/compiler/rustc_resolve/src/build_reduced_graph.rs
index 970d7486d640c..5dd810f7df91b 100644
--- a/compiler/rustc_resolve/src/build_reduced_graph.rs
+++ b/compiler/rustc_resolve/src/build_reduced_graph.rs
@@ -41,7 +41,8 @@ use crate::ref_mut::CmCell;
 use crate::{
     BindingKey, Decl, DeclData, DeclKind, DelayedVisResolutionError, ExternModule,
     ExternPreludeEntry, Finalize, IdentKey, LocalModule, Module, ModuleKind, ModuleOrUniformRoot,
-    ParentScope, PathResult, Res, Resolver, Segment, Used, VisResolutionError, diagnostics,
+    ParentScope, PathResult, Res, ResolutionTable, Resolver, Segment, Used, VisResolutionError,
+    diagnostics,
 };
 
 impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
@@ -336,7 +337,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
     pub(crate) fn build_reduced_graph_external(
         &self,
         module: ExternModule<'ra>,
-    ) -> FxIndexMap<BindingKey, NameResolutionRef<'ra>> {
+    ) -> ResolutionTable<'ra> {
         let mut resolutions = FxIndexMap::default();
         let def_id = module.def_id();
         let children = self.tcx.module_children(def_id);
diff --git a/compiler/rustc_resolve/src/lib.rs b/compiler/rustc_resolve/src/lib.rs
index 111eae6a4134f..5e31f01cbd48b 100644
--- a/compiler/rustc_resolve/src/lib.rs
+++ b/compiler/rustc_resolve/src/lib.rs
@@ -24,7 +24,7 @@
 use std::cell::Ref;
 use std::collections::BTreeSet;
 use std::ops::ControlFlow;
-use std::sync::{Arc, Once};
+use std::sync::{Arc, OnceLock};
 use std::{fmt, mem};
 
 use diagnostics::{ParamKindInEnumDiscriminant, ParamKindInNonTrivialAnonConst};
@@ -633,7 +633,22 @@ impl BindingKey {
     }
 }
 
-type Resolutions<'ra> = CmRefCell<FxIndexMap<BindingKey, NameResolutionRef<'ra>>>;
+type ResolutionTable<'ra> = FxIndexMap<BindingKey, NameResolutionRef<'ra>>;
+
+enum Resolutions<'ra> {
+    Local(CmRefCell<ResolutionTable<'ra>>),
+    Extern(OnceLock<CmRefCell<ResolutionTable<'ra>>>),
+}
+
+impl<'ra> Resolutions<'ra> {
+    fn new(local: bool) -> Self {
+        if local {
+            Resolutions::Local(Default::default())
+        } else {
+            Resolutions::Extern(Default::default())
+        }
+    }
+}
 
 /// One node in the tree of modules.
 ///
@@ -655,8 +670,6 @@ struct ModuleData<'ra> {
     /// Mapping between names and their (possibly in-progress) resolutions in this module.
     /// Resolutions in modules from other crates are not populated until accessed.
     lazy_resolutions: Resolutions<'ra>,
-    /// True if this is a module from other crate that needs to be populated on access.
-    populate_on_access: Once,
     /// Used to disambiguate underscore items (`const _: T = ...`) in the module.
     underscore_disambiguator: CmCell<u32>,
 
@@ -710,6 +723,7 @@ impl<'ra> ModuleData<'ra> {
         vis: Visibility<ModId>,
         arenas: &'ra ResolverArenas<'ra>,
     ) -> Self {
+        let lazy_resolutions = Resolutions::new(kind.is_local());
         let self_decl = match kind {
             ModuleKind::Def(def_kind, def_id, ..) => {
                 let expn_id = expansion.as_local().unwrap_or(LocalExpnId::ROOT);
@@ -720,8 +734,7 @@ impl<'ra> ModuleData<'ra> {
         ModuleData {
             parent,
             kind,
-            lazy_resolutions: Default::default(),
-            populate_on_access: Once::new(),
+            lazy_resolutions,
             underscore_disambiguator: CmCell::new(0),
             unexpanded_invocations: Default::default(),
             no_implicit_prelude,
@@ -2159,15 +2172,16 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
         self.tcx.hir_arena.alloc_slice(&import_ids)
     }
 
-    fn resolutions(&self, module: Module<'ra>) -> &'ra Resolutions<'ra> {
-        if !module.is_local() {
-            // as long as 1 thread is building this external table, all other threads will wait
-            module.populate_on_access.call_once(|| {
-                *module.lazy_resolutions.borrow_mut_unchecked() =
-                    self.build_reduced_graph_external(module.expect_extern());
-            });
+    fn resolutions(&self, module: Module<'ra>) -> &'ra CmRefCell<ResolutionTable<'ra>> {
+        match &module.0.0.lazy_resolutions {
+            Resolutions::Local(local_res) => local_res,
+            Resolutions::Extern(extern_res) => {
+                // as long as 1 thread is building this external table, all other threads will wait
+                extern_res.get_or_init(|| {
+                    CmRefCell::new(self.build_reduced_graph_external(module.expect_extern()))
+                })
+            }
         }
-        &module.0.0.lazy_resolutions
     }
 
     fn resolution(
@@ -2920,13 +2934,6 @@ mod ref_mut {
             CmRefCell(RefCell::new(value))
         }
 
-        #[track_caller]
-        // FIXME: this should be eliminated in the process of migration
-        // to parallel name resolution.
-        pub(crate) fn borrow_mut_unchecked(&self) -> RefMut<'_, T> {
-            self.0.borrow_mut()
-        }
-
         #[track_caller]
         pub(crate) fn borrow_mut<'ra, 'tcx>(&self, r: &Resolver<'ra, 'tcx>) -> RefMut<'_, T> {
             if r.assert_speculative {