delegation: Implement list delegation

```rust
reuse prefix::{a, b, c}
```

compiler/rustc_resolve/src/build_reduced_graph.rs

@@ -825,7 +825,9 @@ impl<'a, 'b, 'tcx> BuildReducedGraphVisitor<'a, 'b, 'tcx> {
             }
             ItemKind::Impl { .. } | ItemKind::ForeignMod(..) | ItemKind::GlobalAsm(..) => {}
 
-            ItemKind::MacroDef(..) | ItemKind::MacCall(_) => unreachable!(),
+            ItemKind::MacroDef(..) | ItemKind::MacCall(_) | ItemKind::DelegationMac(..) => {
+                unreachable!()
+            }
         }
     }
 
@@ -1381,7 +1383,7 @@ impl<'a, 'b, 'tcx> Visitor<'b> for BuildReducedGraphVisitor<'a, 'b, 'tcx> {
                 | AssocItemKind::Delegation(..)
                 | AssocItemKind::Fn(..) => ValueNS,
                 AssocItemKind::Type(..) => TypeNS,
-                AssocItemKind::MacCall(_) => bug!(), // handled above
+                AssocItemKind::MacCall(_) | AssocItemKind::DelegationMac(..) => bug!(), // handled above
             };
 
             let parent = self.parent_scope.module;

compiler/rustc_resolve/src/def_collector.rs

@@ -139,6 +139,7 @@ impl<'a, 'b, 'tcx> visit::Visitor<'a> for DefCollector<'a, 'b, 'tcx> {
             ItemKind::GlobalAsm(..) => DefKind::GlobalAsm,
             ItemKind::Use(..) => return visit::walk_item(self, i),
             ItemKind::MacCall(..) => return self.visit_macro_invoc(i.id),
+            ItemKind::DelegationMac(..) => unreachable!(),
         };
         let def_id = self.create_def(i.id, i.ident.name, def_kind, i.span);
 
@@ -278,6 +279,7 @@ impl<'a, 'b, 'tcx> visit::Visitor<'a> for DefCollector<'a, 'b, 'tcx> {
             AssocItemKind::Const(..) => DefKind::AssocConst,
             AssocItemKind::Type(..) => DefKind::AssocTy,
             AssocItemKind::MacCall(..) => return self.visit_macro_invoc(i.id),
+            AssocItemKind::DelegationMac(..) => unreachable!(),
         };
 
         let def = self.create_def(i.id, i.ident.name, def_kind, i.span);

compiler/rustc_resolve/src/effective_visibilities.rs

@@ -236,7 +236,7 @@ impl<'r, 'ast, 'tcx> Visitor<'ast> for EffectiveVisibilitiesVisitor<'ast, 'r, 't
             ast::ItemKind::Impl(..) => return,
 
             // Should be unreachable at this stage
-            ast::ItemKind::MacCall(..) => panic!(
+            ast::ItemKind::MacCall(..) | ast::ItemKind::DelegationMac(..) => panic!(
                 "ast::ItemKind::MacCall encountered, this should not anymore appear at this stage"
             ),
 

compiler/rustc_resolve/src/late.rs

@@ -2561,7 +2561,9 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
 
             ItemKind::ExternCrate(..) => {}
 
-            ItemKind::MacCall(_) => panic!("unexpanded macro in resolve!"),
+            ItemKind::MacCall(_) | ItemKind::DelegationMac(..) => {
+                panic!("unexpanded macro in resolve!")
+            }
         }
     }
 
@@ -2849,7 +2851,7 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
                     .with_lifetime_rib(LifetimeRibKind::AnonymousReportError, |this| {
                         walk_assoc_item(this, generics, LifetimeBinderKind::Item, item)
                     }),
-                AssocItemKind::MacCall(_) => {
+                AssocItemKind::MacCall(_) | AssocItemKind::DelegationMac(..) => {
                     panic!("unexpanded macro in resolve!")
                 }
             };
@@ -3116,7 +3118,7 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
                     },
                 );
             }
-            AssocItemKind::MacCall(_) => {
+            AssocItemKind::MacCall(_) | AssocItemKind::DelegationMac(..) => {
                 panic!("unexpanded macro in resolve!")
             }
         }
@@ -3218,7 +3220,9 @@ impl<'a: 'ast, 'b, 'ast, 'tcx> LateResolutionVisitor<'a, 'b, 'ast, 'tcx> {
             AssocItemKind::Fn(..) => (E0324, "method"),
             AssocItemKind::Type(..) => (E0325, "type"),
             AssocItemKind::Delegation(..) => (E0324, "method"),
-            AssocItemKind::MacCall(..) => span_bug!(span, "unexpanded macro"),
+            AssocItemKind::MacCall(..) | AssocItemKind::DelegationMac(..) => {
+                span_bug!(span, "unexpanded macro")
+            }
         };
         let trait_path = path_names_to_string(path);
         self.report_error(
@@ -4790,7 +4794,8 @@ impl<'ast> Visitor<'ast> for ItemInfoCollector<'_, '_, '_> {
             | ItemKind::ExternCrate(..)
             | ItemKind::MacroDef(..)
             | ItemKind::GlobalAsm(..)
-            | ItemKind::MacCall(..) => {}
+            | ItemKind::MacCall(..)
+            | ItemKind::DelegationMac(..) => {}
             ItemKind::Delegation(..) => {
                 // Delegated functions have lifetimes, their count is not necessarily zero.
                 // But skipping the delegation items here doesn't mean that the count will be considered zero,

compiler/rustc_resolve/src/late/diagnostics.rs

@@ -2045,7 +2045,9 @@ impl<'a: 'ast, 'ast, 'tcx> LateResolutionVisitor<'a, '_, 'ast, 'tcx> {
                             AssocSuggestion::MethodWithSelf { called }
                         }
                         ast::AssocItemKind::Delegation(..) => AssocSuggestion::AssocFn { called },
-                        ast::AssocItemKind::MacCall(_) => continue,
+                        ast::AssocItemKind::MacCall(_) | ast::AssocItemKind::DelegationMac(..) => {
+                            continue;
+                        }
                     });
                 }
             }