delegation: Implement multi-item delegation

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

Author: petrochenkov

compiler/rustc_ast/src/ast.rs

@@ -2950,6 +2950,7 @@ impl Item {
             | ItemKind::GlobalAsm(_)
             | ItemKind::MacCall(_)
             | ItemKind::Delegation(_)
+            | ItemKind::DelegationList(_)
             | ItemKind::MacroDef(_) => None,
             ItemKind::Static(_) => None,
             ItemKind::Const(i) => Some(&i.generics),
@@ -3116,6 +3117,14 @@ pub struct Delegation {
     pub body: Option<P<Block>>,
 }
 
+#[derive(Clone, Encodable, Decodable, Debug)]
+pub struct DelegationList {
+    pub qself: Option<P<QSelf>>,
+    pub prefix: Path,
+    pub suffixes: ThinVec<Ident>,
+    pub body: Option<P<Block>>,
+}
+
 #[derive(Clone, Encodable, Decodable, Debug)]
 pub struct StaticItem {
     pub ty: P<Ty>,
@@ -3202,10 +3211,13 @@ pub enum ItemKind {
     /// A macro definition.
     MacroDef(MacroDef),
 
-    /// A delegation item (`reuse`).
+    /// A single delegation item (`reuse`).
     ///
     /// E.g. `reuse <Type as Trait>::name { target_expr_template }`.
     Delegation(Box<Delegation>),
+    /// A list delegation item (`reuse prefix::{a, b, c}`).
+    /// Treated similarly to a macro call and expanded early.
+    DelegationList(Box<DelegationList>),
 }
 
 impl ItemKind {
@@ -3214,7 +3226,7 @@ impl ItemKind {
         match self {
             Use(..) | Static(..) | Const(..) | Fn(..) | Mod(..) | GlobalAsm(..) | TyAlias(..)
             | Struct(..) | Union(..) | Trait(..) | TraitAlias(..) | MacroDef(..)
-            | Delegation(..) => "a",
+            | Delegation(..) | DelegationList(..) => "a",
             ExternCrate(..) | ForeignMod(..) | MacCall(..) | Enum(..) | Impl { .. } => "an",
         }
     }
@@ -3239,6 +3251,7 @@ impl ItemKind {
             ItemKind::MacroDef(..) => "macro definition",
             ItemKind::Impl { .. } => "implementation",
             ItemKind::Delegation(..) => "delegated function",
+            ItemKind::DelegationList(..) => "delegation list",
         }
     }
 
@@ -3282,6 +3295,8 @@ pub enum AssocItemKind {
     MacCall(P<MacCall>),
     /// An associated delegation item.
     Delegation(Box<Delegation>),
+    /// An associated delegation item list.
+    DelegationList(Box<DelegationList>),
 }
 
 impl AssocItemKind {
@@ -3290,7 +3305,9 @@ impl AssocItemKind {
             Self::Const(box ConstItem { defaultness, .. })
             | Self::Fn(box Fn { defaultness, .. })
             | Self::Type(box TyAlias { defaultness, .. }) => defaultness,
-            Self::MacCall(..) | Self::Delegation(..) => Defaultness::Final,
+            Self::MacCall(..) | Self::Delegation(..) | Self::DelegationList(..) => {
+                Defaultness::Final
+            }
         }
     }
 }
@@ -3303,6 +3320,7 @@ impl From<AssocItemKind> for ItemKind {
             AssocItemKind::Type(ty_alias_kind) => ItemKind::TyAlias(ty_alias_kind),
             AssocItemKind::MacCall(a) => ItemKind::MacCall(a),
             AssocItemKind::Delegation(delegation) => ItemKind::Delegation(delegation),
+            AssocItemKind::DelegationList(delegation) => ItemKind::DelegationList(delegation),
         }
     }
 }
@@ -3317,6 +3335,7 @@ impl TryFrom<ItemKind> for AssocItemKind {
             ItemKind::TyAlias(ty_kind) => AssocItemKind::Type(ty_kind),
             ItemKind::MacCall(a) => AssocItemKind::MacCall(a),
             ItemKind::Delegation(d) => AssocItemKind::Delegation(d),
+            ItemKind::DelegationList(d) => AssocItemKind::DelegationList(d),
             _ => return Err(item_kind),
         })
     }

compiler/rustc_ast/src/mut_visit.rs

@@ -1157,6 +1157,16 @@ pub fn noop_visit_item_kind<T: MutVisitor>(kind: &mut ItemKind, vis: &mut T) {
                 vis.visit_block(body);
             }
         }
+        ItemKind::DelegationList(box DelegationList { qself, prefix, suffixes, body }) => {
+            vis.visit_qself(qself);
+            vis.visit_path(prefix);
+            for ident in suffixes {
+                vis.visit_ident(ident);
+            }
+            if let Some(body) = body {
+                vis.visit_block(body);
+            }
+        }
     }
 }
 
@@ -1203,6 +1213,17 @@ pub fn noop_flat_map_assoc_item<T: MutVisitor>(
                 visitor.visit_block(body);
             }
         }
+        AssocItemKind::DelegationList(box DelegationList { qself, prefix, suffixes, body }) => {
+            visitor.visit_id(id);
+            visitor.visit_qself(qself);
+            visitor.visit_path(prefix);
+            for ident in suffixes {
+                visitor.visit_ident(ident);
+            }
+            if let Some(body) = body {
+                visitor.visit_block(body);
+            }
+        }
     }
     visitor.visit_span(span);
     visit_lazy_tts(tokens, visitor);

compiler/rustc_ast/src/visit.rs

@@ -389,6 +389,16 @@ pub fn walk_item<'a, V: Visitor<'a>>(visitor: &mut V, item: &'a Item) -> V::Resu
             try_visit!(visitor.visit_path(path, *id));
             visit_opt!(visitor, visit_block, body);
         }
+        ItemKind::DelegationList(box DelegationList { qself, prefix, suffixes, body }) => {
+            if let Some(qself) = qself {
+                try_visit!(visitor.visit_ty(&qself.ty));
+            }
+            try_visit!(visitor.visit_path(prefix, item.id));
+            for suffix in suffixes {
+                visitor.visit_ident(*suffix);
+            }
+            visit_opt!(visitor, visit_block, body);
+        }
     }
     walk_list!(visitor, visit_attribute, &item.attrs);
     V::Result::output()
@@ -789,6 +799,16 @@ pub fn walk_assoc_item<'a, V: Visitor<'a>>(
             try_visit!(visitor.visit_path(path, *id));
             visit_opt!(visitor, visit_block, body);
         }
+        AssocItemKind::DelegationList(box DelegationList { qself, prefix, suffixes, body }) => {
+            if let Some(qself) = qself {
+                try_visit!(visitor.visit_ty(&qself.ty));
+            }
+            try_visit!(visitor.visit_path(prefix, item.id));
+            for suffix in suffixes {
+                visitor.visit_ident(*suffix);
+            }
+            visit_opt!(visitor, visit_block, body);
+        }
     }
     V::Result::output()
 }

compiler/rustc_ast_lowering/src/item.rs

@@ -459,8 +459,8 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     delegation_results.body_id,
                 )
             }
-            ItemKind::MacCall(..) => {
-                panic!("`TyMac` should have been expanded by now")
+            ItemKind::MacCall(..) | ItemKind::DelegationList(..) => {
+                panic!("macros should have been expanded by now")
             }
         }
     }
@@ -844,7 +844,9 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 );
                 (delegation_results.generics, item_kind, true)
             }
-            AssocItemKind::MacCall(..) => panic!("macro item shouldn't exist at this point"),
+            AssocItemKind::MacCall(..) | AssocItemKind::DelegationList(..) => {
+                panic!("macro item shouldn't exist at this point")
+            }
         };
 
         let item = hir::TraitItem {
@@ -868,7 +870,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
             AssocItemKind::Delegation(box delegation) => hir::AssocItemKind::Fn {
                 has_self: self.delegation_has_self(i.id, delegation.id, i.span),
             },
-            AssocItemKind::MacCall(..) => unimplemented!(),
+            AssocItemKind::MacCall(..) | AssocItemKind::DelegationList(..) => unreachable!(),
         };
         let id = hir::TraitItemId { owner_id: hir::OwnerId { def_id: self.local_def_id(i.id) } };
         hir::TraitItemRef {
@@ -963,7 +965,9 @@ impl<'hir> LoweringContext<'_, 'hir> {
                     hir::ImplItemKind::Fn(delegation_results.sig, delegation_results.body_id),
                 )
             }
-            AssocItemKind::MacCall(..) => panic!("`TyMac` should have been expanded by now"),
+            AssocItemKind::MacCall(..) | AssocItemKind::DelegationList(..) => {
+                panic!("macros should have been expanded by now")
+            }
         };
 
         let item = hir::ImplItem {
@@ -992,7 +996,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 AssocItemKind::Delegation(box delegation) => hir::AssocItemKind::Fn {
                     has_self: self.delegation_has_self(i.id, delegation.id, i.span),
                 },
-                AssocItemKind::MacCall(..) => unimplemented!(),
+                AssocItemKind::MacCall(..) | AssocItemKind::DelegationList(..) => unreachable!(),
             },
             trait_item_def_id: self
                 .resolver

compiler/rustc_ast_pretty/src/pprust/state/item.rs

@@ -5,6 +5,7 @@ use crate::pprust::state::{AnnNode, PrintState, State, INDENT_UNIT};
 use ast::StaticItem;
 use itertools::{Itertools, Position};
 use rustc_ast as ast;
+use rustc_ast::ptr::P;
 use rustc_ast::ModKind;
 use rustc_span::symbol::Ident;
 
@@ -371,9 +372,22 @@ impl<'a> State<'a> {
                     state.print_visibility(&item.vis)
                 });
             }
-            ast::ItemKind::Delegation(box delegation) => {
-                self.print_delegation(delegation, &item.vis, &item.attrs)
-            }
+            ast::ItemKind::Delegation(deleg) => self.print_delegation(
+                &item.attrs,
+                &item.vis,
+                &deleg.qself,
+                &deleg.path,
+                None,
+                &deleg.body,
+            ),
+            ast::ItemKind::DelegationList(deleg) => self.print_delegation(
+                &item.attrs,
+                &item.vis,
+                &deleg.qself,
+                &deleg.prefix,
+                Some(&deleg.suffixes),
+                &deleg.body,
+            ),
         }
         self.ann.post(self, AnnNode::Item(item))
     }
@@ -550,31 +564,58 @@ impl<'a> State<'a> {
                     self.word(";");
                 }
             }
-            ast::AssocItemKind::Delegation(box delegation) => {
-                self.print_delegation(delegation, vis, &item.attrs)
-            }
+            ast::AssocItemKind::Delegation(deleg) => self.print_delegation(
+                &item.attrs,
+                vis,
+                &deleg.qself,
+                &deleg.path,
+                None,
+                &deleg.body,
+            ),
+            ast::AssocItemKind::DelegationList(deleg) => self.print_delegation(
+                &item.attrs,
+                vis,
+                &deleg.qself,
+                &deleg.prefix,
+                Some(&deleg.suffixes),
+                &deleg.body,
+            ),
         }
         self.ann.post(self, AnnNode::SubItem(id))
     }
 
     pub(crate) fn print_delegation(
         &mut self,
-        delegation: &ast::Delegation,
-        vis: &ast::Visibility,
         attrs: &[ast::Attribute],
+        vis: &ast::Visibility,
+        qself: &Option<P<ast::QSelf>>,
+        path: &ast::Path,
+        suffixes: Option<&[Ident]>,
+        body: &Option<P<ast::Block>>,
     ) {
-        if delegation.body.is_some() {
+        if body.is_some() {
             self.head("");
         }
         self.print_visibility(vis);
         self.word_space("reuse");
 
-        if let Some(qself) = &delegation.qself {
-            self.print_qpath(&delegation.path, qself, false);
+        if let Some(qself) = qself {
+            self.print_qpath(path, qself, false);
         } else {
-            self.print_path(&delegation.path, false, 0);
+            self.print_path(path, false, 0);
         }
-        if let Some(body) = &delegation.body {
+        if let Some(suffixes) = suffixes {
+            self.word("::");
+            self.word("{");
+            for (i, suffix) in suffixes.iter().enumerate() {
+                self.print_ident(*suffix);
+                if i != suffixes.len() {
+                    self.word(",");
+                }
+            }
+            self.word("}");
+        }
+        if let Some(body) = body {
             self.nbsp();
             self.print_block_with_attrs(body, attrs);
         } else {

compiler/rustc_expand/messages.ftl

@@ -30,6 +30,9 @@ expand_duplicate_matcher_binding = duplicate matcher binding
     .label = duplicate binding
     .label2 = previous binding
 
+expand_empty_delegation_list =
+    empty list delegation is not supported
+
 expand_expected_comma_in_list =
     expected token: `,`
 

compiler/rustc_expand/src/errors.rs

@@ -456,3 +456,10 @@ pub struct ExpectedParenOrBrace<'a> {
     pub span: Span,
     pub token: Cow<'a, str>,
 }
+
+#[derive(Diagnostic)]
+#[diag(expand_empty_delegation_list)]
+pub(crate) struct EmptyDelegationList {
+    #[primary_span]
+    pub span: Span,
+}