Rollup merge of #147805 - LorrensP-2158466:extern-mod-disamb, r=petrochenkov

use module_child index as disambiguator for external items

When defining the items of an external module, if that item is an underscore we use it's index as the disambiguator. This is needed for parallel import resolution, which is being worked on in #145108.

r? `@petrochenkov`

Author: matthiaskrgr

compiler/rustc_resolve/src/build_reduced_graph.rs

@@ -77,6 +77,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
         parent: Module<'ra>,
         ident: Ident,
         ns: Namespace,
+        child_index: usize,
         res: Res,
         vis: Visibility<DefId>,
         span: Span,
@@ -86,10 +87,8 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
         // 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.underscore_disambiguator.update_unchecked(|d| d + 1);
-            parent.underscore_disambiguator.get()
-        });
+        let key =
+            BindingKey::new_disambiguated(ident, ns, || (child_index + 1).try_into().unwrap()); // 0 indicates no underscore
         if self
             .resolution_or_default(parent, key)
             .borrow_mut_unchecked()
@@ -233,9 +232,9 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
     }
 
     pub(crate) fn build_reduced_graph_external(&self, module: Module<'ra>) {
-        for child in self.tcx.module_children(module.def_id()) {
+        for (i, child) in self.tcx.module_children(module.def_id()).into_iter().enumerate() {
             let parent_scope = ParentScope::module(module, self.arenas);
-            self.build_reduced_graph_for_external_crate_res(child, parent_scope)
+            self.build_reduced_graph_for_external_crate_res(child, parent_scope, i)
         }
     }
 
@@ -244,6 +243,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
         &self,
         child: &ModChild,
         parent_scope: ParentScope<'ra>,
+        child_index: usize,
     ) {
         let parent = parent_scope.module;
         let ModChild { ident, res, vis, ref reexport_chain } = *child;
@@ -272,7 +272,9 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                 _,
             )
             | Res::PrimTy(..)
-            | Res::ToolMod => self.define_extern(parent, ident, TypeNS, res, vis, span, expansion),
+            | Res::ToolMod => {
+                self.define_extern(parent, ident, TypeNS, child_index, res, vis, span, expansion)
+            }
             Res::Def(
                 DefKind::Fn
                 | DefKind::AssocFn
@@ -281,9 +283,9 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                 | DefKind::AssocConst
                 | DefKind::Ctor(..),
                 _,
-            ) => self.define_extern(parent, ident, ValueNS, res, vis, span, expansion),
+            ) => self.define_extern(parent, ident, ValueNS, child_index, res, vis, span, expansion),
             Res::Def(DefKind::Macro(..), _) | Res::NonMacroAttr(..) => {
-                self.define_extern(parent, ident, MacroNS, res, vis, span, expansion)
+                self.define_extern(parent, ident, MacroNS, child_index, res, vis, span, expansion)
             }
             Res::Def(
                 DefKind::TyParam