delegation: Support renaming

compiler/rustc_ast/src/ast.rs

@@ -3112,6 +3112,7 @@ pub struct Delegation {
     /// Path resolution id.
     pub id: NodeId,
     pub qself: Option<P<QSelf>>,
+    pub rename: Option<Ident>,
     pub path: Path,
     pub body: Option<P<Block>>,
 }

compiler/rustc_ast/src/mut_visit.rs

@@ -1149,10 +1149,13 @@ pub fn noop_visit_item_kind<T: MutVisitor>(kind: &mut ItemKind, vis: &mut T) {
         }
         ItemKind::MacCall(m) => vis.visit_mac_call(m),
         ItemKind::MacroDef(def) => vis.visit_macro_def(def),
-        ItemKind::Delegation(box Delegation { id, qself, path, body }) => {
+        ItemKind::Delegation(box Delegation { id, qself, path, rename, body }) => {
             vis.visit_id(id);
             vis.visit_qself(qself);
             vis.visit_path(path);
+            if let Some(rename) = rename {
+                vis.visit_ident(rename);
+            }
             if let Some(body) = body {
                 vis.visit_block(body);
             }
@@ -1195,10 +1198,13 @@ pub fn noop_flat_map_assoc_item<T: MutVisitor>(
             visit_opt(ty, |ty| visitor.visit_ty(ty));
         }
         AssocItemKind::MacCall(mac) => visitor.visit_mac_call(mac),
-        AssocItemKind::Delegation(box Delegation { id, qself, path, body }) => {
+        AssocItemKind::Delegation(box Delegation { id, qself, path, rename, body }) => {
             visitor.visit_id(id);
             visitor.visit_qself(qself);
             visitor.visit_path(path);
+            if let Some(rename) = rename {
+                visitor.visit_ident(rename);
+            }
             if let Some(body) = body {
                 visitor.visit_block(body);
             }

compiler/rustc_ast/src/visit.rs

@@ -382,11 +382,12 @@ pub fn walk_item<'a, V: Visitor<'a>>(visitor: &mut V, item: &'a Item) -> V::Resu
         }
         ItemKind::MacCall(mac) => try_visit!(visitor.visit_mac_call(mac)),
         ItemKind::MacroDef(ts) => try_visit!(visitor.visit_mac_def(ts, item.id)),
-        ItemKind::Delegation(box Delegation { id, qself, path, body }) => {
+        ItemKind::Delegation(box Delegation { id, qself, path, rename, body }) => {
             if let Some(qself) = qself {
                 try_visit!(visitor.visit_ty(&qself.ty));
             }
             try_visit!(visitor.visit_path(path, *id));
+            visit_opt!(visitor, visit_ident, *rename);
             visit_opt!(visitor, visit_block, body);
         }
     }
@@ -782,11 +783,12 @@ pub fn walk_assoc_item<'a, V: Visitor<'a>>(
         AssocItemKind::MacCall(mac) => {
             try_visit!(visitor.visit_mac_call(mac));
         }
-        AssocItemKind::Delegation(box Delegation { id, qself, path, body }) => {
+        AssocItemKind::Delegation(box Delegation { id, qself, path, rename, body }) => {
             if let Some(qself) = qself {
                 try_visit!(visitor.visit_ty(&qself.ty));
             }
             try_visit!(visitor.visit_path(path, *id));
+            visit_opt!(visitor, visit_ident, *rename);
             visit_opt!(visitor, visit_block, body);
         }
     }

compiler/rustc_parse/src/parser/item.rs

@@ -686,6 +686,8 @@ impl<'a> Parser<'a> {
             (None, self.parse_path(PathStyle::Expr)?)
         };
 
+        let rename = if self.eat_keyword(kw::As) { Some(self.parse_ident()?) } else { None };
+
         let body = if self.check(&token::OpenDelim(Delimiter::Brace)) {
             Some(self.parse_block()?)
         } else {
@@ -695,11 +697,9 @@ impl<'a> Parser<'a> {
         let span = span.to(self.prev_token.span);
         self.psess.gated_spans.gate(sym::fn_delegation, span);
 
-        let ident = path.segments.last().map(|seg| seg.ident).unwrap_or(Ident::empty());
-        Ok((
-            ident,
-            ItemKind::Delegation(Box::new(Delegation { id: DUMMY_NODE_ID, qself, path, body })),
-        ))
+        let ident = rename.unwrap_or_else(|| path.segments.last().unwrap().ident);
+        let deleg = Delegation { id: DUMMY_NODE_ID, qself, path, rename, body };
+        Ok((ident, ItemKind::Delegation(Box::new(deleg))))
     }
 
     fn parse_item_list<T>(

tests/ui/delegation/rename.rs

@@ -0,0 +1,20 @@
+//@ check-pass
+
+#![feature(fn_delegation)]
+#![allow(incomplete_features)]
+
+mod to_reuse {
+    pub fn a() {}
+}
+
+reuse to_reuse::a as b;
+
+struct S;
+impl S {
+    reuse to_reuse::a as b;
+}
+
+fn main() {
+    b();
+    S::b();
+}