From 30306fdecab6a8d63f3ddc59f119f83b94e65dbb Mon Sep 17 00:00:00 2001
From: LorrensP-2158466 <lorrens.pantelis@student.uhasselt.be>
Date: Sat, 12 Jul 2025 15:01:39 +0200
Subject: [PATCH 1/8] split up define into define_extern and define_local

---
 .../rustc_resolve/src/build_reduced_graph.rs  | 68 ++++++++++++-------
 compiler/rustc_resolve/src/imports.rs         | 25 +++++--
 compiler/rustc_resolve/src/lib.rs             |  4 +-
 3 files changed, 67 insertions(+), 30 deletions(-)

diff --git a/compiler/rustc_resolve/src/build_reduced_graph.rs b/compiler/rustc_resolve/src/build_reduced_graph.rs
index 675ea9d1e98ee..2774ed0190f39 100644
--- a/compiler/rustc_resolve/src/build_reduced_graph.rs
+++ b/compiler/rustc_resolve/src/build_reduced_graph.rs
@@ -42,19 +42,19 @@ type Res = def::Res<NodeId>;
 impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
     /// Defines `name` in namespace `ns` of module `parent` to be `def` if it is not yet defined;
     /// otherwise, reports an error.
-    pub(crate) fn define_binding(
+    pub(crate) fn define_binding_local(
         &mut self,
         parent: Module<'ra>,
         ident: Ident,
         ns: Namespace,
         binding: NameBinding<'ra>,
     ) {
-        if let Err(old_binding) = self.try_define(parent, ident, ns, binding, false) {
+        if let Err(old_binding) = self.try_define_local(parent, key, binding, false) {
             self.report_conflict(parent, ident, ns, old_binding, binding);
         }
     }
 
-    fn define(
+    fn define_local(
         &mut self,
         parent: Module<'ra>,
         ident: Ident,
@@ -65,7 +65,29 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
         expn_id: LocalExpnId,
     ) {
         let binding = self.arenas.new_res_binding(res, vis.to_def_id(), span, expn_id);
-        self.define_binding(parent, ident, ns, binding)
+        self.define_binding_local(parent, ident, ns, binding)
+    }
+
+    // Panics when a binding already exists.
+    fn define_extern(
+        &self,
+        parent: Module<'ra>,
+        ident: Ident,
+        ns: Namespace,
+        res: Res,
+        vis: Visibility<impl Into<DefId>>,
+        span: Span,
+        expn_id: LocalExpnId,
+    ) {
+        let binding = self.arenas.new_res_binding(res, vis.to_def_id(), span, expn_id);
+        let key = self.new_disambiguated_key(ident, ns);
+        self.check_reserved_macro_name(key.ident, binding.res());
+        let resolution = &mut *self.resolution(parent, key).borrow_mut();
+        if resolution.binding.is_some() {
+            panic!("An external binding was already defined");
+        }
+        // FIXME: maybe some handling of glob-importers
+        resolution.binding = Some(binding);
     }
 
     /// Walks up the tree of definitions starting at `def_id`,
@@ -188,7 +210,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
         visitor.parent_scope.macro_rules
     }
 
-    pub(crate) fn build_reduced_graph_external(&mut self, module: Module<'ra>) {
+    pub(crate) fn build_reduced_graph_external(&self, module: Module<'ra>) {
         for child in self.tcx.module_children(module.def_id()) {
             let parent_scope = ParentScope::module(module, self);
             self.build_reduced_graph_for_external_crate_res(child, parent_scope)
@@ -197,7 +219,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
 
     /// Builds the reduced graph for a single item in an external crate.
     fn build_reduced_graph_for_external_crate_res(
-        &mut self,
+        &self,
         child: &ModChild,
         parent_scope: ParentScope<'ra>,
     ) {
@@ -228,7 +250,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                 _,
             )
             | Res::PrimTy(..)
-            | Res::ToolMod => self.define(parent, ident, TypeNS, res, vis, span, expansion),
+            | Res::ToolMod => self.define_extern(parent, ident, TypeNS, res, vis, span, expansion),
             Res::Def(
                 DefKind::Fn
                 | DefKind::AssocFn
@@ -237,9 +259,9 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                 | DefKind::AssocConst
                 | DefKind::Ctor(..),
                 _,
-            ) => self.define(parent, ident, ValueNS, res, vis, span, expansion),
+            ) => self.define_extern(parent, ident, ValueNS, res, vis, span, expansion),
             Res::Def(DefKind::Macro(..), _) | Res::NonMacroAttr(..) => {
-                self.define(parent, ident, MacroNS, res, vis, span, expansion)
+                self.define_extern(parent, ident, MacroNS, res, vis, span, expansion)
             }
             Res::Def(
                 DefKind::TyParam
@@ -705,7 +727,7 @@ impl<'a, 'ra, 'tcx> BuildReducedGraphVisitor<'a, 'ra, 'tcx> {
         let expansion = parent_scope.expansion;
 
         // Define a name in the type namespace if it is not anonymous.
-        self.r.define(parent, ident, TypeNS, adt_res, adt_vis, adt_span, expansion);
+        self.r.define_local(parent, ident, TypeNS, adt_res, adt_vis, adt_span, expansion);
         self.r.feed_visibility(feed, adt_vis);
         let def_id = feed.key();
 
@@ -757,7 +779,7 @@ impl<'a, 'ra, 'tcx> BuildReducedGraphVisitor<'a, 'ra, 'tcx> {
             }
 
             ItemKind::Mod(_, ident, ref mod_kind) => {
-                self.r.define(parent, ident, TypeNS, res, vis, sp, expansion);
+                self.r.define_local(parent, ident, TypeNS, res, vis, sp, expansion);
 
                 if let ast::ModKind::Loaded(_, _, _, Err(_)) = mod_kind {
                     self.r.mods_with_parse_errors.insert(def_id);
@@ -776,10 +798,10 @@ impl<'a, 'ra, 'tcx> BuildReducedGraphVisitor<'a, 'ra, 'tcx> {
             ItemKind::Const(box ConstItem { ident, .. })
             | ItemKind::Delegation(box Delegation { ident, .. })
             | ItemKind::Static(box StaticItem { ident, .. }) => {
-                self.r.define(parent, ident, ValueNS, res, vis, sp, expansion);
+                self.r.define_local(parent, ident, ValueNS, res, vis, sp, expansion);
             }
             ItemKind::Fn(box Fn { ident, .. }) => {
-                self.r.define(parent, ident, ValueNS, res, vis, sp, expansion);
+                self.r.define_local(parent, ident, ValueNS, res, vis, sp, expansion);
 
                 // Functions introducing procedural macros reserve a slot
                 // in the macro namespace as well (see #52225).
@@ -788,11 +810,11 @@ impl<'a, 'ra, 'tcx> BuildReducedGraphVisitor<'a, 'ra, 'tcx> {
 
             // These items live in the type namespace.
             ItemKind::TyAlias(box TyAlias { ident, .. }) | ItemKind::TraitAlias(ident, ..) => {
-                self.r.define(parent, ident, TypeNS, res, vis, sp, expansion);
+                self.r.define_local(parent, ident, TypeNS, res, vis, sp, expansion);
             }
 
             ItemKind::Enum(ident, _, _) | ItemKind::Trait(box ast::Trait { ident, .. }) => {
-                self.r.define(parent, ident, TypeNS, res, vis, sp, expansion);
+                self.r.define_local(parent, ident, TypeNS, res, vis, sp, expansion);
 
                 self.parent_scope.module = self.r.new_local_module(
                     Some(parent),
@@ -844,7 +866,7 @@ impl<'a, 'ra, 'tcx> BuildReducedGraphVisitor<'a, 'ra, 'tcx> {
                     let feed = self.r.feed(ctor_node_id);
                     let ctor_def_id = feed.key();
                     let ctor_res = self.res(ctor_def_id);
-                    self.r.define(parent, ident, ValueNS, ctor_res, ctor_vis, sp, expansion);
+                    self.r.define_local(parent, ident, ValueNS, ctor_res, ctor_vis, sp, expansion);
                     self.r.feed_visibility(feed, ctor_vis);
                     // We need the field visibility spans also for the constructor for E0603.
                     self.insert_field_visibilities_local(ctor_def_id.to_def_id(), vdata.fields());
@@ -965,7 +987,7 @@ impl<'a, 'ra, 'tcx> BuildReducedGraphVisitor<'a, 'ra, 'tcx> {
                 );
             }
         }
-        self.r.define_binding(parent, ident, TypeNS, imported_binding);
+        self.r.define_binding_local(parent, ident, TypeNS, imported_binding);
     }
 
     /// Constructs the reduced graph for one foreign item.
@@ -982,7 +1004,7 @@ impl<'a, 'ra, 'tcx> BuildReducedGraphVisitor<'a, 'ra, 'tcx> {
         let parent = self.parent_scope.module;
         let expansion = self.parent_scope.expansion;
         let vis = self.resolve_visibility(&item.vis);
-        self.r.define(parent, ident, ns, self.res(def_id), vis, item.span, expansion);
+        self.r.define_local(parent, ident, ns, self.res(def_id), vis, item.span, expansion);
         self.r.feed_visibility(feed, vis);
     }
 
@@ -1241,7 +1263,7 @@ impl<'a, 'ra, 'tcx> BuildReducedGraphVisitor<'a, 'ra, 'tcx> {
                 });
                 self.r.import_use_map.insert(import, Used::Other);
                 let import_binding = self.r.import(binding, import);
-                self.r.define_binding(self.r.graph_root, ident, MacroNS, import_binding);
+                self.r.define_binding_local(self.r.graph_root, ident, MacroNS, import_binding);
             } else {
                 self.r.check_reserved_macro_name(ident, res);
                 self.insert_unused_macro(ident, def_id, item.id);
@@ -1269,7 +1291,7 @@ impl<'a, 'ra, 'tcx> BuildReducedGraphVisitor<'a, 'ra, 'tcx> {
             if !vis.is_public() {
                 self.insert_unused_macro(ident, def_id, item.id);
             }
-            self.r.define(module, ident, MacroNS, res, vis, span, expansion);
+            self.r.define_local(module, ident, MacroNS, res, vis, span, expansion);
             self.r.feed_visibility(feed, vis);
             self.parent_scope.macro_rules
         }
@@ -1405,7 +1427,7 @@ impl<'a, 'ra, 'tcx> Visitor<'a> for BuildReducedGraphVisitor<'a, 'ra, 'tcx> {
         if ctxt == AssocCtxt::Trait {
             let parent = self.parent_scope.module;
             let expansion = self.parent_scope.expansion;
-            self.r.define(parent, ident, ns, self.res(def_id), vis, item.span, expansion);
+            self.r.define_local(parent, ident, ns, self.res(def_id), vis, item.span, expansion);
         } else if !matches!(&item.kind, AssocItemKind::Delegation(deleg) if deleg.from_glob)
             && ident.name != kw::Underscore
         {
@@ -1493,7 +1515,7 @@ impl<'a, 'ra, 'tcx> Visitor<'a> for BuildReducedGraphVisitor<'a, 'ra, 'tcx> {
         let feed = self.r.feed(variant.id);
         let def_id = feed.key();
         let vis = self.resolve_visibility(&variant.vis);
-        self.r.define(parent, ident, TypeNS, self.res(def_id), vis, variant.span, expn_id);
+        self.r.define_local(parent, ident, TypeNS, self.res(def_id), vis, variant.span, expn_id);
         self.r.feed_visibility(feed, vis);
 
         // If the variant is marked as non_exhaustive then lower the visibility to within the crate.
@@ -1509,7 +1531,7 @@ impl<'a, 'ra, 'tcx> Visitor<'a> for BuildReducedGraphVisitor<'a, 'ra, 'tcx> {
             let feed = self.r.feed(ctor_node_id);
             let ctor_def_id = feed.key();
             let ctor_res = self.res(ctor_def_id);
-            self.r.define(parent, ident, ValueNS, ctor_res, ctor_vis, variant.span, expn_id);
+            self.r.define_local(parent, ident, ValueNS, ctor_res, ctor_vis, variant.span, expn_id);
             self.r.feed_visibility(feed, ctor_vis);
         }
 
diff --git a/compiler/rustc_resolve/src/imports.rs b/compiler/rustc_resolve/src/imports.rs
index b4c15ed1ca7e9..962e128c4c285 100644
--- a/compiler/rustc_resolve/src/imports.rs
+++ b/compiler/rustc_resolve/src/imports.rs
@@ -333,9 +333,24 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
         })
     }
 
+    // pub(crate) fn try_define(
+    //     &mut self,
+    //     module: Module<'ra>,
+    //     key: BindingKey,
+    //     binding: NameBinding<'ra>,
+    //     warn_ambiguity: bool,
+    // ) -> Result<(), NameBinding<'ra>> {
+    //     if module.is_local_module() {
+    //         self.try_define_local(module, key, binding, warn_ambiguity)
+    //     } else {
+    //         self.define_extern(module, key, binding);
+    //         Ok(())
+    //     }
+    // }
+
     /// Define the name or return the existing binding if there is a collision.
     /// `update` indicates if the definition is a redefinition of an existing binding.
-    pub(crate) fn try_define(
+    pub(crate) fn try_define_local(
         &mut self,
         module: Module<'ra>,
         ident: Ident,
@@ -496,7 +511,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
             };
             if self.is_accessible_from(binding.vis, scope) {
                 let imported_binding = self.import(binding, *import);
-                let _ = self.try_define(
+                let _ = self.try_define_local(
                     import.parent_scope.module,
                     ident,
                     key.ns,
@@ -522,7 +537,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
             let dummy_binding = self.import(dummy_binding, import);
             self.per_ns(|this, ns| {
                 let module = import.parent_scope.module;
-                let _ = this.try_define(module, target, ns, dummy_binding, false);
+                let _ = this.try_define_local(module, target, ns, dummy_binding, false);
                 // Don't remove underscores from `single_imports`, they were never added.
                 if target.name != kw::Underscore {
                     let key = BindingKey::new(target, ns);
@@ -902,7 +917,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                         }
                         // We need the `target`, `source` can be extracted.
                         let imported_binding = this.import(binding, import);
-                        this.define_binding(parent, target, ns, imported_binding);
+                        this.define_binding_local(parent, target, ns, imported_binding);
                         PendingBinding::Ready(Some(imported_binding))
                     }
                     Err(Determinacy::Determined) => {
@@ -1519,7 +1534,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                     .borrow()
                     .binding()
                     .is_some_and(|binding| binding.warn_ambiguity_recursive());
-                let _ = self.try_define(
+                let _ = self.try_define_local(
                     import.parent_scope.module,
                     key.ident,
                     key.ns,
diff --git a/compiler/rustc_resolve/src/lib.rs b/compiler/rustc_resolve/src/lib.rs
index f38fee8dea5d9..4a85300d1265d 100644
--- a/compiler/rustc_resolve/src/lib.rs
+++ b/compiler/rustc_resolve/src/lib.rs
@@ -1895,7 +1895,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
         import_ids
     }
 
-    fn resolutions(&mut self, module: Module<'ra>) -> &'ra Resolutions<'ra> {
+    fn resolutions(&self, module: Module<'ra>) -> &'ra Resolutions<'ra> {
         if module.populate_on_access.get() {
             module.populate_on_access.set(false);
             self.build_reduced_graph_external(module);
@@ -1904,7 +1904,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
     }
 
     fn resolution(
-        &mut self,
+        &self,
         module: Module<'ra>,
         key: BindingKey,
     ) -> &'ra RefCell<NameResolution<'ra>> {

From 37904a1512ad19794b14707f89a95064279afa82 Mon Sep 17 00:00:00 2001
From: LorrensP-2158466 <lorrens.pantelis@student.uhasselt.be>
Date: Sun, 13 Jul 2025 12:17:09 +0200
Subject: [PATCH 2/8] cleanup old stuff

---
 compiler/rustc_resolve/src/imports.rs | 15 ---------------
 1 file changed, 15 deletions(-)

diff --git a/compiler/rustc_resolve/src/imports.rs b/compiler/rustc_resolve/src/imports.rs
index 962e128c4c285..aec730189bf8d 100644
--- a/compiler/rustc_resolve/src/imports.rs
+++ b/compiler/rustc_resolve/src/imports.rs
@@ -333,21 +333,6 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
         })
     }
 
-    // pub(crate) fn try_define(
-    //     &mut self,
-    //     module: Module<'ra>,
-    //     key: BindingKey,
-    //     binding: NameBinding<'ra>,
-    //     warn_ambiguity: bool,
-    // ) -> Result<(), NameBinding<'ra>> {
-    //     if module.is_local_module() {
-    //         self.try_define_local(module, key, binding, warn_ambiguity)
-    //     } else {
-    //         self.define_extern(module, key, binding);
-    //         Ok(())
-    //     }
-    // }
-
     /// Define the name or return the existing binding if there is a collision.
     /// `update` indicates if the definition is a redefinition of an existing binding.
     pub(crate) fn try_define_local(

From 7ce099ab698445d2dc56b05e126b64cd55c8035f Mon Sep 17 00:00:00 2001
From: LorrensP-2158466 <lorrens.pantelis@student.uhasselt.be>
Date: Tue, 15 Jul 2025 10:54:03 +0200
Subject: [PATCH 3/8] address review

---
 compiler/rustc_resolve/src/build_reduced_graph.rs | 12 +++++++-----
 compiler/rustc_resolve/src/lib.rs                 |  4 ++++
 2 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/compiler/rustc_resolve/src/build_reduced_graph.rs b/compiler/rustc_resolve/src/build_reduced_graph.rs
index 2774ed0190f39..88af22b032d8a 100644
--- a/compiler/rustc_resolve/src/build_reduced_graph.rs
+++ b/compiler/rustc_resolve/src/build_reduced_graph.rs
@@ -60,10 +60,12 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
         ident: Ident,
         ns: Namespace,
         res: Res,
-        vis: Visibility<impl Into<DefId>>,
+        vis: Visibility,
         span: Span,
         expn_id: LocalExpnId,
     ) {
+        assert!(parent.is_local());
+        assert!(res.opt_def_id().is_none_or(|def_id| def_id.is_local()));
         let binding = self.arenas.new_res_binding(res, vis.to_def_id(), span, expn_id);
         self.define_binding_local(parent, ident, ns, binding)
     }
@@ -75,18 +77,18 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
         ident: Ident,
         ns: Namespace,
         res: Res,
-        vis: Visibility<impl Into<DefId>>,
+        vis: Visibility<DefId>,
         span: Span,
         expn_id: LocalExpnId,
     ) {
-        let binding = self.arenas.new_res_binding(res, vis.to_def_id(), span, expn_id);
+        assert!(!parent.is_local());
+        assert!(!res.opt_def_id().is_some_and(|def_id| def_id.is_local()), "res: {res:?} is local");
+        let binding = self.arenas.new_res_binding(res, vis, span, expn_id);
         let key = self.new_disambiguated_key(ident, ns);
-        self.check_reserved_macro_name(key.ident, binding.res());
         let resolution = &mut *self.resolution(parent, key).borrow_mut();
         if resolution.binding.is_some() {
             panic!("An external binding was already defined");
         }
-        // FIXME: maybe some handling of glob-importers
         resolution.binding = Some(binding);
     }
 
diff --git a/compiler/rustc_resolve/src/lib.rs b/compiler/rustc_resolve/src/lib.rs
index 4a85300d1265d..1d5749bcbfbc3 100644
--- a/compiler/rustc_resolve/src/lib.rs
+++ b/compiler/rustc_resolve/src/lib.rs
@@ -702,6 +702,10 @@ impl<'ra> Module<'ra> {
         }
     }
 
+    fn is_local(self) -> bool {
+        self.opt_def_id().is_none_or(|def_id| def_id.is_local())
+    }
+
     // `self` resolves to the first module ancestor that `is_normal`.
     fn is_normal(self) -> bool {
         matches!(self.kind, ModuleKind::Def(DefKind::Mod, _, _))

From 01c162365cb936932d397b35057085ef80457460 Mon Sep 17 00:00:00 2001
From: LorrensP-2158466 <lorrens.pantelis@student.uhasselt.be>
Date: Tue, 15 Jul 2025 15:08:14 +0200
Subject: [PATCH 4/8] check that vis is extern aswell

---
 compiler/rustc_resolve/src/build_reduced_graph.rs | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/compiler/rustc_resolve/src/build_reduced_graph.rs b/compiler/rustc_resolve/src/build_reduced_graph.rs
index 88af22b032d8a..58d1166fdc298 100644
--- a/compiler/rustc_resolve/src/build_reduced_graph.rs
+++ b/compiler/rustc_resolve/src/build_reduced_graph.rs
@@ -83,6 +83,10 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
     ) {
         assert!(!parent.is_local());
         assert!(!res.opt_def_id().is_some_and(|def_id| def_id.is_local()), "res: {res:?} is local");
+        let vis = vis.map_id(|def_id| {
+            assert!(!def_id.is_local());
+            def_id
+        });
         let binding = self.arenas.new_res_binding(res, vis, span, expn_id);
         let key = self.new_disambiguated_key(ident, ns);
         let resolution = &mut *self.resolution(parent, key).borrow_mut();

From 9e5b09979a68016a20e6416b03a567ce5aece9b8 Mon Sep 17 00:00:00 2001
From: LorrensP-2158466 <lorrens.pantelis@student.uhasselt.be>
Date: Tue, 15 Jul 2025 16:24:50 +0200
Subject: [PATCH 5/8] split `for_each_child` into immutable and mutable
 versions

---
 compiler/rustc_expand/src/base.rs             |  2 +-
 .../rustc_resolve/src/build_reduced_graph.rs  |  2 +-
 compiler/rustc_resolve/src/diagnostics.rs     |  4 +--
 .../rustc_resolve/src/late/diagnostics.rs     |  6 ++--
 compiler/rustc_resolve/src/lib.rs             | 28 +++++++++++++++----
 compiler/rustc_resolve/src/macros.rs          |  2 +-
 6 files changed, 31 insertions(+), 13 deletions(-)

diff --git a/compiler/rustc_expand/src/base.rs b/compiler/rustc_expand/src/base.rs
index 1928cfd90482b..ba76ec71aed11 100644
--- a/compiler/rustc_expand/src/base.rs
+++ b/compiler/rustc_expand/src/base.rs
@@ -1144,7 +1144,7 @@ pub trait ResolverExpand {
 
     /// Names of specific methods to which glob delegation expands.
     fn glob_delegation_suffixes(
-        &mut self,
+        &self,
         trait_def_id: DefId,
         impl_def_id: LocalDefId,
     ) -> Result<Vec<(Ident, Option<Ident>)>, Indeterminate>;
diff --git a/compiler/rustc_resolve/src/build_reduced_graph.rs b/compiler/rustc_resolve/src/build_reduced_graph.rs
index 58d1166fdc298..8b006e575b36b 100644
--- a/compiler/rustc_resolve/src/build_reduced_graph.rs
+++ b/compiler/rustc_resolve/src/build_reduced_graph.rs
@@ -1104,7 +1104,7 @@ impl<'a, 'ra, 'tcx> BuildReducedGraphVisitor<'a, 'ra, 'tcx> {
         if let Some(span) = import_all {
             let import = macro_use_import(self, span, false);
             self.r.potentially_unused_imports.push(import);
-            module.for_each_child(self, |this, ident, ns, binding| {
+            module.for_each_child_mut(self, |this, ident, ns, binding| {
                 if ns == MacroNS {
                     let import = if this.r.is_accessible_from(binding.vis, this.parent_scope.module)
                     {
diff --git a/compiler/rustc_resolve/src/diagnostics.rs b/compiler/rustc_resolve/src/diagnostics.rs
index f6f45adabe98b..d9ae8f6e06fee 100644
--- a/compiler/rustc_resolve/src/diagnostics.rs
+++ b/compiler/rustc_resolve/src/diagnostics.rs
@@ -523,7 +523,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
     }
 
     pub(crate) fn add_module_candidates(
-        &mut self,
+        &self,
         module: Module<'ra>,
         names: &mut Vec<TypoSuggestion>,
         filter_fn: &impl Fn(Res) -> bool,
@@ -1155,7 +1155,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
     }
 
     fn lookup_import_candidates_from_module<FilterFn>(
-        &mut self,
+        &self,
         lookup_ident: Ident,
         namespace: Namespace,
         parent_scope: &ParentScope<'ra>,
diff --git a/compiler/rustc_resolve/src/late/diagnostics.rs b/compiler/rustc_resolve/src/late/diagnostics.rs
index ee462d9076491..d85125623ccd1 100644
--- a/compiler/rustc_resolve/src/late/diagnostics.rs
+++ b/compiler/rustc_resolve/src/late/diagnostics.rs
@@ -2629,7 +2629,7 @@ impl<'ast, 'ra, 'tcx> LateResolutionVisitor<'_, 'ast, 'ra, 'tcx> {
         false
     }
 
-    fn find_module(&mut self, def_id: DefId) -> Option<(Module<'ra>, ImportSuggestion)> {
+    fn find_module(&self, def_id: DefId) -> Option<(Module<'ra>, ImportSuggestion)> {
         let mut result = None;
         let mut seen_modules = FxHashSet::default();
         let root_did = self.r.graph_root.def_id();
@@ -2686,7 +2686,7 @@ impl<'ast, 'ra, 'tcx> LateResolutionVisitor<'_, 'ast, 'ra, 'tcx> {
         result
     }
 
-    fn collect_enum_ctors(&mut self, def_id: DefId) -> Option<Vec<(Path, DefId, CtorKind)>> {
+    fn collect_enum_ctors(&self, def_id: DefId) -> Option<Vec<(Path, DefId, CtorKind)>> {
         self.find_module(def_id).map(|(enum_module, enum_import_suggestion)| {
             let mut variants = Vec::new();
             enum_module.for_each_child(self.r, |_, ident, _, name_binding| {
@@ -2703,7 +2703,7 @@ impl<'ast, 'ra, 'tcx> LateResolutionVisitor<'_, 'ast, 'ra, 'tcx> {
 
     /// Adds a suggestion for using an enum's variant when an enum is used instead.
     fn suggest_using_enum_variant(
-        &mut self,
+        &self,
         err: &mut Diag<'_>,
         source: PathSource<'_, '_, '_>,
         def_id: DefId,
diff --git a/compiler/rustc_resolve/src/lib.rs b/compiler/rustc_resolve/src/lib.rs
index 1d5749bcbfbc3..397a898a8da66 100644
--- a/compiler/rustc_resolve/src/lib.rs
+++ b/compiler/rustc_resolve/src/lib.rs
@@ -652,7 +652,7 @@ impl<'ra> ModuleData<'ra> {
 }
 
 impl<'ra> Module<'ra> {
-    fn for_each_child<'tcx, R, F>(self, resolver: &mut R, mut f: F)
+    fn for_each_child_mut<'tcx, R, F>(self, resolver: &mut R, mut f: F)
     where
         R: AsMut<Resolver<'ra, 'tcx>>,
         F: FnMut(&mut R, Ident, Namespace, NameBinding<'ra>),
@@ -664,10 +664,22 @@ impl<'ra> Module<'ra> {
         }
     }
 
+    fn for_each_child<'tcx, R, F>(self, resolver: &R, mut f: F)
+    where
+        R: AsRef<Resolver<'ra, 'tcx>>,
+        F: FnMut(&R, Ident, Namespace, NameBinding<'ra>),
+    {
+        for (key, name_resolution) in resolver.as_ref().resolutions(self).borrow().iter() {
+            if let Some(binding) = name_resolution.borrow().binding {
+                f(resolver, key.ident, key.ns, binding);
+            }
+        }
+    }
+
     /// This modifies `self` in place. The traits will be stored in `self.traits`.
-    fn ensure_traits<'tcx, R>(self, resolver: &mut R)
+    fn ensure_traits<'tcx, R>(self, resolver: &R)
     where
-        R: AsMut<Resolver<'ra, 'tcx>>,
+        R: AsRef<Resolver<'ra, 'tcx>>,
     {
         let mut traits = self.traits.borrow_mut();
         if traits.is_none() {
@@ -677,7 +689,7 @@ impl<'ra> Module<'ra> {
                     return;
                 }
                 if let Res::Def(DefKind::Trait | DefKind::TraitAlias, def_id) = binding.res() {
-                    collected_traits.push((name, binding, r.as_mut().get_module(def_id)))
+                    collected_traits.push((name, binding, r.as_ref().get_module(def_id)))
                 }
             });
             *traits = Some(collected_traits.into_boxed_slice());
@@ -1341,6 +1353,12 @@ impl<'ra, 'tcx> AsMut<Resolver<'ra, 'tcx>> for Resolver<'ra, 'tcx> {
     }
 }
 
+impl<'ra, 'tcx> AsRef<Resolver<'ra, 'tcx>> for Resolver<'ra, 'tcx> {
+    fn as_ref(&self) -> &Resolver<'ra, 'tcx> {
+        self
+    }
+}
+
 impl<'tcx> Resolver<'_, 'tcx> {
     fn opt_local_def_id(&self, node: NodeId) -> Option<LocalDefId> {
         self.opt_feed(node).map(|f| f.key())
@@ -1867,7 +1885,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
     // We don't reject trait aliases (`trait_module == None`) because we don't have access to their
     // associated items.
     fn trait_may_have_item(
-        &mut self,
+        &self,
         trait_module: Option<Module<'ra>>,
         assoc_item: Option<(Symbol, Namespace)>,
     ) -> bool {
diff --git a/compiler/rustc_resolve/src/macros.rs b/compiler/rustc_resolve/src/macros.rs
index 0e8904a7deab6..46ff52f6a9922 100644
--- a/compiler/rustc_resolve/src/macros.rs
+++ b/compiler/rustc_resolve/src/macros.rs
@@ -506,7 +506,7 @@ impl<'ra, 'tcx> ResolverExpand for Resolver<'ra, 'tcx> {
     }
 
     fn glob_delegation_suffixes(
-        &mut self,
+        &self,
         trait_def_id: DefId,
         impl_def_id: LocalDefId,
     ) -> Result<Vec<(Ident, Option<Ident>)>, Indeterminate> {

From e941e0c5cf9e25436edd4b709ed02f0a63b0962c Mon Sep 17 00:00:00 2001
From: LorrensP-2158466 <lorrens.pantelis@student.uhasselt.be>
Date: Thu, 17 Jul 2025 20:39:06 +0200
Subject: [PATCH 6/8] some cleanup/rename

---
 compiler/rustc_resolve/src/build_reduced_graph.rs |  2 +-
 compiler/rustc_resolve/src/imports.rs             | 10 +++++-----
 2 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/compiler/rustc_resolve/src/build_reduced_graph.rs b/compiler/rustc_resolve/src/build_reduced_graph.rs
index 8b006e575b36b..79f7fc7b8bd7e 100644
--- a/compiler/rustc_resolve/src/build_reduced_graph.rs
+++ b/compiler/rustc_resolve/src/build_reduced_graph.rs
@@ -82,7 +82,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
         expn_id: LocalExpnId,
     ) {
         assert!(!parent.is_local());
-        assert!(!res.opt_def_id().is_some_and(|def_id| def_id.is_local()), "res: {res:?} is local");
+        assert!(!res.opt_def_id().is_some_and(|def_id| def_id.is_local()));
         let vis = vis.map_id(|def_id| {
             assert!(!def_id.is_local());
             def_id
diff --git a/compiler/rustc_resolve/src/imports.rs b/compiler/rustc_resolve/src/imports.rs
index aec730189bf8d..e05e4b4d7fea8 100644
--- a/compiler/rustc_resolve/src/imports.rs
+++ b/compiler/rustc_resolve/src/imports.rs
@@ -334,7 +334,6 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
     }
 
     /// Define the name or return the existing binding if there is a collision.
-    /// `update` indicates if the definition is a redefinition of an existing binding.
     pub(crate) fn try_define_local(
         &mut self,
         module: Module<'ra>,
@@ -352,7 +351,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
         let key = BindingKey::new_disambiguated(ident, ns, || {
             (module.0.0.lazy_resolutions.borrow().len() + 1).try_into().unwrap()
         });
-        self.update_resolution(module, key, warn_ambiguity, |this, resolution| {
+        self.update_local_resolution(module, key, warn_ambiguity, |this, resolution| {
             if let Some(old_binding) = resolution.best_binding() {
                 if res == Res::Err && old_binding.res() != Res::Err {
                     // Do not override real bindings with `Res::Err`s from error recovery.
@@ -455,7 +454,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
 
     // Use `f` to mutate the resolution of the name in the module.
     // If the resolution becomes a success, define it in the module's glob importers.
-    fn update_resolution<T, F>(
+    fn update_local_resolution<T, F>(
         &mut self,
         module: Module<'ra>,
         key: BindingKey,
@@ -465,6 +464,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
     where
         F: FnOnce(&Resolver<'ra, 'tcx>, &mut NameResolution<'ra>) -> T,
     {
+        assert!(module.is_local());
         // Ensure that `resolution` isn't borrowed when defining in the module's glob importers,
         // during which the resolution might end up getting re-defined via a glob cycle.
         let (binding, t, warn_ambiguity) = {
@@ -526,7 +526,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                 // Don't remove underscores from `single_imports`, they were never added.
                 if target.name != kw::Underscore {
                     let key = BindingKey::new(target, ns);
-                    this.update_resolution(module, key, false, |_, resolution| {
+                    this.update_local_resolution(module, key, false, |_, resolution| {
                         resolution.single_imports.swap_remove(&import);
                     })
                 }
@@ -909,7 +909,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                         // Don't remove underscores from `single_imports`, they were never added.
                         if target.name != kw::Underscore {
                             let key = BindingKey::new(target, ns);
-                            this.update_resolution(parent, key, false, |_, resolution| {
+                            this.update_local_resolution(parent, key, false, |_, resolution| {
                                 resolution.single_imports.swap_remove(&import);
                             });
                         }

From aa0c3692df4e62c176aed1afd6b0b204c68ea1a9 Mon Sep 17 00:00:00 2001
From: LorrensP-2158466 <lorrens.pantelis@student.uhasselt.be>
Date: Fri, 18 Jul 2025 19:10:16 +0200
Subject: [PATCH 7/8] account for `(non)_glob_binding` change

---
 .../rustc_resolve/src/build_reduced_graph.rs  | 42 ++++++++++++++-----
 compiler/rustc_resolve/src/lib.rs             |  2 +-
 2 files changed, 33 insertions(+), 11 deletions(-)

diff --git a/compiler/rustc_resolve/src/build_reduced_graph.rs b/compiler/rustc_resolve/src/build_reduced_graph.rs
index 79f7fc7b8bd7e..faeef51558063 100644
--- a/compiler/rustc_resolve/src/build_reduced_graph.rs
+++ b/compiler/rustc_resolve/src/build_reduced_graph.rs
@@ -49,7 +49,8 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
         ns: Namespace,
         binding: NameBinding<'ra>,
     ) {
-        if let Err(old_binding) = self.try_define_local(parent, key, binding, false) {
+        assert!(parent.is_local());
+        if let Err(old_binding) = self.try_define_local(parent, ident, ns, binding, false) {
             self.report_conflict(parent, ident, ns, old_binding, binding);
         }
     }
@@ -60,17 +61,43 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
         ident: Ident,
         ns: Namespace,
         res: Res,
-        vis: Visibility,
+        vis: Visibility, // Implictly local
         span: Span,
         expn_id: LocalExpnId,
     ) {
         assert!(parent.is_local());
         assert!(res.opt_def_id().is_none_or(|def_id| def_id.is_local()));
         let binding = self.arenas.new_res_binding(res, vis.to_def_id(), span, expn_id);
-        self.define_binding_local(parent, ident, ns, binding)
+        self.define_binding_local(parent, ident, ns, binding);
+    }
+
+    /// Defines `name` in namespace `ns` of module `parent` to be `def` if it is not yet defined;
+    /// otherwise, panic.
+    pub(crate) fn define_binding_extern(
+        &self,
+        parent: Module<'ra>,
+        ident: Ident,
+        ns: Namespace,
+        binding: NameBinding<'ra>,
+    ) {
+        assert!(!parent.is_local());
+        // Even if underscore names cannot be looked up, we still need to add them to modules,
+        // because they can be fetched by glob imports from those modules, and bring traits
+        // into scope both directly and through glob imports.
+        let key = BindingKey::new_disambiguated(ident, ns, || {
+            (parent.0.0.lazy_resolutions.borrow().len() + 1).try_into().unwrap()
+        });
+        let resolution = &mut *self.resolution(parent, key).borrow_mut();
+        let resolution_binding = if binding.is_glob_import() {
+            &mut resolution.glob_binding
+        } else {
+            &mut resolution.non_glob_binding
+        };
+        if resolution_binding.replace(binding).is_some() {
+            panic!("An external binding was already defined");
+        }
     }
 
-    // Panics when a binding already exists.
     fn define_extern(
         &self,
         parent: Module<'ra>,
@@ -88,12 +115,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
             def_id
         });
         let binding = self.arenas.new_res_binding(res, vis, span, expn_id);
-        let key = self.new_disambiguated_key(ident, ns);
-        let resolution = &mut *self.resolution(parent, key).borrow_mut();
-        if resolution.binding.is_some() {
-            panic!("An external binding was already defined");
-        }
-        resolution.binding = Some(binding);
+        self.define_binding_extern(parent, ident, ns, binding);
     }
 
     /// Walks up the tree of definitions starting at `def_id`,
diff --git a/compiler/rustc_resolve/src/lib.rs b/compiler/rustc_resolve/src/lib.rs
index 397a898a8da66..8cd48ded68ea2 100644
--- a/compiler/rustc_resolve/src/lib.rs
+++ b/compiler/rustc_resolve/src/lib.rs
@@ -670,7 +670,7 @@ impl<'ra> Module<'ra> {
         F: FnMut(&R, Ident, Namespace, NameBinding<'ra>),
     {
         for (key, name_resolution) in resolver.as_ref().resolutions(self).borrow().iter() {
-            if let Some(binding) = name_resolution.borrow().binding {
+            if let Some(binding) = name_resolution.borrow().best_binding() {
                 f(resolver, key.ident, key.ns, binding);
             }
         }

From 110a3d96639ce2f9613f320b46f04b94478dce0c Mon Sep 17 00:00:00 2001
From: LorrensP-2158466 <lorrens.pantelis@student.uhasselt.be>
Date: Sat, 19 Jul 2025 03:51:17 +0200
Subject: [PATCH 8/8] spell check

---
 compiler/rustc_resolve/src/build_reduced_graph.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/compiler/rustc_resolve/src/build_reduced_graph.rs b/compiler/rustc_resolve/src/build_reduced_graph.rs
index faeef51558063..823b51986fdce 100644
--- a/compiler/rustc_resolve/src/build_reduced_graph.rs
+++ b/compiler/rustc_resolve/src/build_reduced_graph.rs
@@ -61,7 +61,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
         ident: Ident,
         ns: Namespace,
         res: Res,
-        vis: Visibility, // Implictly local
+        vis: Visibility, // Implicitly local
         span: Span,
         expn_id: LocalExpnId,
     ) {