Rollup merge of rust-lang#144605 - LorrensP-2158466:cache-extern-prelude, r=petrochenkov

Resolve: cachify `ExternPreludeEntry.binding` through a `Cell`

Provides interior mutability to the `binding` field of `ExternPreludeEntry` as this field behaves like a cache. Per [zulip thread](https://rust-lang.zulipchat.com/#narrow/channel/421156-gsoc/topic/Project.3A.20Parallel.20Macro.20Expansion/near/531390914).

A little preparatory work for batched import resolution, which is part of [#gsoc > Project: Parallel Macro Expansion](https://rust-lang.zulipchat.com/#narrow/channel/421156-gsoc/topic/Project.3A.20Parallel.20Macro.20Expansion).

r? `@petrochenkov`

Author: jhpratt

compiler/rustc_resolve/src/build_reduced_graph.rs

@@ -984,18 +984,17 @@ impl<'a, 'ra, 'tcx> BuildReducedGraphVisitor<'a, 'ra, 'tcx> {
                 // more details: https://github.com/rust-lang/rust/pull/111761
                 return;
             }
-            let entry = self
-                .r
-                .extern_prelude
-                .entry(ident)
-                .or_insert(ExternPreludeEntry { binding: None, introduced_by_item: true });
+            let entry = self.r.extern_prelude.entry(ident).or_insert(ExternPreludeEntry {
+                binding: Cell::new(None),
+                introduced_by_item: true,
+            });
             if orig_name.is_some() {
                 entry.introduced_by_item = true;
             }
             // Binding from `extern crate` item in source code can replace
             // a binding from `--extern` on command line here.
             if !entry.is_import() {
-                entry.binding = Some(imported_binding)
+                entry.binding.set(Some(imported_binding));
             } else if ident.name != kw::Underscore {
                 self.r.dcx().span_delayed_bug(
                     item.span,

compiler/rustc_resolve/src/lib.rs

@@ -1009,13 +1009,13 @@ impl<'ra> NameBindingData<'ra> {
 
 #[derive(Default, Clone)]
 struct ExternPreludeEntry<'ra> {
-    binding: Option<NameBinding<'ra>>,
+    binding: Cell<Option<NameBinding<'ra>>>,
     introduced_by_item: bool,
 }
 
 impl ExternPreludeEntry<'_> {
     fn is_import(&self) -> bool {
-        self.binding.is_some_and(|binding| binding.is_import())
+        self.binding.get().is_some_and(|binding| binding.is_import())
     }
 }
 
@@ -2006,7 +2006,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
             // but not introduce it, as used if they are accessed from lexical scope.
             if used == Used::Scope {
                 if let Some(entry) = self.extern_prelude.get(&ident.normalize_to_macros_2_0()) {
-                    if !entry.introduced_by_item && entry.binding == Some(used_binding) {
+                    if !entry.introduced_by_item && entry.binding.get() == Some(used_binding) {
                         return;
                     }
                 }
@@ -2170,7 +2170,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
 
         let norm_ident = ident.normalize_to_macros_2_0();
         let binding = self.extern_prelude.get(&norm_ident).cloned().and_then(|entry| {
-            Some(if let Some(binding) = entry.binding {
+            Some(if let Some(binding) = entry.binding.get() {
                 if finalize {
                     if !entry.is_import() {
                         self.cstore_mut().process_path_extern(self.tcx, ident.name, ident.span);
@@ -2195,8 +2195,8 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
             })
         });
 
-        if let Some(entry) = self.extern_prelude.get_mut(&norm_ident) {
-            entry.binding = binding;
+        if let Some(entry) = self.extern_prelude.get(&norm_ident) {
+            entry.binding.set(binding);
         }
 
         binding