Resolver: Introduce CmRef which has a speclative borrow variant for CmRefCell

compiler/rustc_resolve/src/check_unused.rs

@@ -558,7 +558,7 @@ impl Resolver<'_, '_> {
         let unused_imports = visitor.unused_imports;
         let mut check_redundant_imports = FxIndexSet::default();
         for module in &self.local_modules {
-            for (_key, resolution) in self.resolutions(module.to_module()).borrow().iter() {
+            for (_key, resolution) in self.resolutions(module.to_module()).iter() {
                 if let Some(decl) = resolution.borrow().best_decl()
                     && let DeclKind::Import { import, .. } = decl.kind
                     && let ImportKind::Single { id, .. } = import.kind

compiler/rustc_resolve/src/diagnostics/impls.rs

@@ -1870,24 +1870,22 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                 // If so, we have to disambiguate the potential import suggestions by making
                 // the paths *global* (i.e., by prefixing them with `::`).
                 let needs_disambiguation =
-                    self.resolutions(parent_scope.module).borrow().iter().any(
-                        |(key, name_resolution)| {
-                            if key.ns == TypeNS
-                                && key.ident == *ident
-                                && let Some(decl) = name_resolution.borrow().best_decl()
-                            {
-                                match decl.res() {
-                                    // No disambiguation needed if the identically named item we
-                                    // found in scope actually refers to the crate in question.
-                                    Res::Def(_, def_id) => def_id != crate_def_id,
-                                    Res::PrimTy(_) => true,
-                                    _ => false,
-                                }
-                            } else {
-                                false
+                    self.resolutions(parent_scope.module).iter().any(|(key, name_resolution)| {
+                        if key.ns == TypeNS
+                            && key.ident == *ident
+                            && let Some(decl) = name_resolution.borrow().best_decl()
+                        {
+                            match decl.res() {
+                                // No disambiguation needed if the identically named item we
+                                // found in scope actually refers to the crate in question.
+                                Res::Def(_, def_id) => def_id != crate_def_id,
+                                Res::PrimTy(_) => true,
+                                _ => false,
                             }
-                        },
-                    );
+                        } else {
+                            false
+                        }
+                    });
                 let mut crate_path = ThinVec::new();
                 if needs_disambiguation {
                     crate_path.push(ast::PathSegment::path_root(rustc_span::DUMMY_SP));

compiler/rustc_resolve/src/effective_visibilities.rs

@@ -125,7 +125,7 @@ impl<'a, 'ra, 'tcx> EffectiveVisibilitiesVisitor<'a, 'ra, 'tcx> {
     /// including their whole reexport chains.
     fn set_bindings_effective_visibilities(&mut self, module_id: LocalDefId) {
         let module = self.r.expect_module(module_id.to_def_id());
-        for (_, name_resolution) in self.r.resolutions(module).borrow().iter() {
+        for (_, name_resolution) in self.r.resolutions(module).iter() {
             let Some(decl) = name_resolution.borrow().best_decl() else {
                 continue;
             };
@@ -309,7 +309,7 @@ impl<'a, 'ra, 'tcx> EffectiveVisibilitiesVisitor<'a, 'ra, 'tcx> {
     ) {
         if self.macro_reachable.insert((module_def_id, defining_mod)) {
             let module = self.r.expect_module(module_def_id.to_def_id());
-            for (_, name_resolution) in self.r.resolutions(module).borrow().iter() {
+            for (_, name_resolution) in self.r.resolutions(module).iter() {
                 let Some(decl) = name_resolution.borrow().best_decl() else {
                     continue;
                 };

compiler/rustc_resolve/src/imports.rs

@@ -1002,7 +1002,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
 
     pub(crate) fn lint_reexports(&mut self, exported_ambiguities: FxHashSet<Decl<'ra>>) {
         for module in &self.local_modules {
-            for (key, resolution) in self.resolutions(module.to_module()).borrow().iter() {
+            for (key, resolution) in self.resolutions(module.to_module()).iter() {
                 let resolution = resolution.borrow();
                 let Some(binding) = resolution.best_decl() else { continue };
 
@@ -1481,7 +1481,6 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                 let names = match module {
                     ModuleOrUniformRoot::Module(module) => {
                         self.resolutions(module)
-                            .borrow()
                             .iter()
                             .filter_map(|(BindingKey { ident: i, .. }, resolution)| {
                                 if i.name == ident.name {
@@ -1799,7 +1798,6 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
         let import_bindings = match imported_module {
             ModuleOrUniformRoot::Module(module) if module != import.parent_scope.module => self
                 .resolutions(module)
-                .borrow()
                 .iter()
                 .filter_map(|(key, resolution)| {
                     let res = resolution.borrow();

compiler/rustc_resolve/src/late/diagnostics.rs

@@ -190,7 +190,7 @@ impl<'ast, 'ra, 'tcx> LateResolutionVisitor<'_, 'ast, 'ra, 'tcx> {
         assoc_name: Symbol,
     ) -> Option<DefId> {
         let module = self.r.get_module(trait_def_id)?;
-        self.r.resolutions(module).borrow().iter().find_map(|(key, resolution)| {
+        self.r.resolutions(module).iter().find_map(|(key, resolution)| {
             if key.ident.name != assoc_name {
                 return None;
             }
@@ -648,7 +648,6 @@ impl<'ast, 'ra, 'tcx> LateResolutionVisitor<'_, 'ast, 'ra, 'tcx> {
                             && self
                                 .r
                                 .resolutions(module)
-                                .borrow()
                                 .iter()
                                 .any(|(key, _r)| key.ident.name == following_seg.ident.name)
                     } else {
@@ -1164,7 +1163,7 @@ impl<'ast, 'ra, 'tcx> LateResolutionVisitor<'_, 'ast, 'ra, 'tcx> {
 
     fn lookup_doc_alias_name(&mut self, path: &[Segment], ns: Namespace) -> Option<(DefId, Ident)> {
         let find_doc_alias_name = |r: &mut Resolver<'ra, '_>, m: Module<'ra>, item_name: Symbol| {
-            for resolution in r.resolutions(m).borrow().values() {
+            for resolution in r.resolutions(m).values() {
                 let Some(did) =
                     resolution.borrow().best_decl().and_then(|binding| binding.res().opt_def_id())
                 else {
@@ -1904,7 +1903,6 @@ impl<'ast, 'ra, 'tcx> LateResolutionVisitor<'_, 'ast, 'ra, 'tcx> {
                 let targets: Vec<_> = self
                     .r
                     .resolutions(module)
-                    .borrow()
                     .iter()
                     .filter_map(|(key, resolution)| {
                         let resolution = resolution.borrow();
@@ -2767,7 +2765,6 @@ impl<'ast, 'ra, 'tcx> LateResolutionVisitor<'_, 'ast, 'ra, 'tcx> {
         let targets = self
             .r
             .resolutions(*module)
-            .borrow()
             .iter()
             .filter_map(|(key, res)| res.borrow().best_decl().map(|binding| (key, binding.res())))
             .filter(|(_, res)| match (kind, res) {
@@ -2970,7 +2967,6 @@ impl<'ast, 'ra, 'tcx> LateResolutionVisitor<'_, 'ast, 'ra, 'tcx> {
                     let module = self.r.expect_module(def_id);
                     self.r
                         .resolutions(module)
-                        .borrow()
                         .iter()
                         .any(|(key, _)| key.ident.name == following_seg.ident.name)
                 }

compiler/rustc_resolve/src/lib.rs

@@ -21,7 +21,7 @@
 #![recursion_limit = "256"]
 // tidy-alphabetical-end
 
-use std::cell::Ref;
+use std::cell::{Ref, RefMut};
 use std::collections::BTreeSet;
 use std::ops::ControlFlow;
 use std::sync::{Arc, OnceLock};
@@ -81,7 +81,7 @@ use crate::diagnostics::impls::{
     ImportSuggestion, LabelSuggestion, OnUnknownData, StructCtor, Suggestion,
 };
 use crate::imports::{ImportResolution, NameResolutionRef};
-use crate::ref_mut::{CmCell, CmRefCell};
+use crate::ref_mut::{CmCell, CmRef, CmRefCell};
 
 mod build_reduced_graph;
 mod check_unused;
@@ -637,7 +637,7 @@ type ResolutionTable<'ra> = FxIndexMap<BindingKey, NameResolutionRef<'ra>>;
 
 enum Resolutions<'ra> {
     Local(CmRefCell<ResolutionTable<'ra>>),
-    Extern(OnceLock<CmRefCell<ResolutionTable<'ra>>>),
+    Extern(OnceLock<ResolutionTable<'ra>>),
 }
 
 impl<'ra> Resolutions<'ra> {
@@ -792,7 +792,7 @@ impl<'ra> Module<'ra> {
         resolver: &R,
         mut f: impl FnMut(&R, IdentKey, Span, Namespace, Decl<'ra>),
     ) {
-        for (key, name_resolution) in resolver.as_ref().resolutions(self).borrow().iter() {
+        for (key, name_resolution) in resolver.as_ref().resolutions(self).iter() {
             let name_resolution = name_resolution.borrow();
             if let Some(decl) = name_resolution.best_decl() {
                 f(resolver, key.ident, name_resolution.orig_ident_span, key.ns, decl);
@@ -805,7 +805,7 @@ impl<'ra> Module<'ra> {
         resolver: &mut R,
         mut f: impl FnMut(&mut R, IdentKey, Span, Namespace, Decl<'ra>),
     ) {
-        for (key, name_resolution) in resolver.as_mut().resolutions(self).borrow().iter() {
+        for (key, name_resolution) in resolver.as_mut().resolutions(self).iter() {
             let name_resolution = name_resolution.borrow();
             if let Some(decl) = name_resolution.best_decl() {
                 f(resolver, key.ident, name_resolution.orig_ident_span, key.ns, decl);
@@ -2152,7 +2152,6 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
         match (trait_module, assoc_item) {
             (Some(trait_module), Some((name, ns))) => self
                 .resolutions(trait_module)
-                .borrow()
                 .iter()
                 .any(|(key, _name_resolution)| key.ns == ns && key.ident.name == name),
             _ => true,
@@ -2177,14 +2176,28 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
         self.tcx.hir_arena.alloc_slice(&import_ids)
     }
 
-    fn resolutions(&self, module: Module<'ra>) -> &'ra CmRefCell<ResolutionTable<'ra>> {
+    fn resolutions(&self, module: Module<'ra>) -> CmRef<'ra, ResolutionTable<'ra>> {
         match &module.0.0.lazy_resolutions {
-            Resolutions::Local(local_res) => local_res,
+            Resolutions::Local(local_res) => CmRef::Tracked(local_res.borrow()),
             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()))
-                })
+                // It is fine to return a `CmRef::Untracked`, we never give out a `&mut`
+                // to an external table.
+                CmRef::Untracked(
+                    // As long as 1 thread is building this external table, all other threads will wait.
+                    extern_res
+                        .get_or_init(|| self.build_reduced_graph_external(module.expect_extern())),
+                )
+            }
+        }
+    }
+
+    fn resolutions_mut(&self, module: Module<'ra>) -> RefMut<'ra, ResolutionTable<'ra>> {
+        match &module.0.0.lazy_resolutions {
+            Resolutions::Local(local_res) => local_res.borrow_mut(self),
+            Resolutions::Extern(_) => {
+                // We do not allow in place mutations of the external resolution table. In fact,
+                // we never attempt it.
+                unreachable!("Attempted to mutably borrow an extenral resolution table")
             }
         }
     }
@@ -2194,7 +2207,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
         module: Module<'ra>,
         key: BindingKey,
     ) -> Option<Ref<'ra, NameResolution<'ra>>> {
-        self.resolutions(module).borrow().get(&key).map(|resolution| resolution.0.borrow())
+        self.resolutions(module).get(&key).map(|resolution| resolution.0.borrow())
     }
 
     #[track_caller]
@@ -2204,7 +2217,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
         key: BindingKey,
         orig_ident_span: Span,
     ) -> NameResolutionRef<'ra> {
-        *self.resolutions(module).borrow_mut(self).entry(key).or_insert_with(|| {
+        *self.resolutions_mut(module).entry(key).or_insert_with(|| {
             self.arenas.alloc_name_resolution(NameResolution::new(orig_ident_span))
         })
     }
@@ -2915,6 +2928,24 @@ mod ref_mut {
         }
     }
 
+    pub(crate) enum CmRef<'b, T> {
+        /// A tracked borrow of a [`CmRefCell`]
+        Tracked(Ref<'b, T>),
+        /// An untracked or normal reference (not dynamically borrow-checked by `RefCell`)
+        Untracked(&'b T),
+    }
+
+    impl<'b, T> Deref for CmRef<'b, T> {
+        type Target = T;
+
+        fn deref(&self) -> &Self::Target {
+            match self {
+                CmRef::Tracked(r) => r,
+                CmRef::Untracked(r) => r,
+            }
+        }
+    }
+
     /// A wrapper around a [`RefCell`] that only allows writes (mutable borrows) based on a condition in the resolver.
     #[derive(Default)]
     pub(crate) struct CmRefCell<T>(RefCell<T>);
@@ -2926,10 +2957,7 @@ mod ref_mut {
 
         #[track_caller]
         pub(crate) fn borrow_mut<'ra, 'tcx>(&self, r: &Resolver<'ra, 'tcx>) -> RefMut<'_, T> {
-            if r.assert_speculative {
-                panic!("not allowed to mutably borrow a `CmRefCell` during speculative resolution");
-            }
-            self.0.borrow_mut()
+            self.try_borrow_mut(r).unwrap()
         }
 
         #[track_caller]