Stop eagerly creating AnonConst defs

Sometimes they end up lowering into a ConstArgKind::Path, which has no
def associated with it. So we now create the defs only when we're sure
the lowered AnonConst will be in the HIR.

Author: camelid

compiler/rustc_ast_lowering/src/lib.rs

@@ -2418,8 +2418,17 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
     }
 
     fn lower_anon_const(&mut self, c: &AnonConst) -> &'hir hir::AnonConst {
+        // Some ast::AnonConst's turn into ConstArgKind::Path's, so we create their def
+        // only when we're sure they're going to stay as an AnonConst.
+        let def_id = self.create_def(
+            self.current_hir_id_owner.def_id,
+            c.id,
+            kw::Empty,
+            DefKind::AnonConst,
+            c.value.span,
+        );
         self.arena.alloc(self.with_new_scopes(c.value.span, |this| hir::AnonConst {
-            def_id: this.local_def_id(c.id),
+            def_id,
             hir_id: this.lower_node_id(c.id),
             body: this.lower_const_body(c.value.span, Some(&c.value)),
             span: this.lower_span(c.value.span),

compiler/rustc_resolve/src/def_collector.rs

@@ -309,11 +309,6 @@ impl<'a, 'b, 'tcx> visit::Visitor<'a> for DefCollector<'a, 'b, 'tcx> {
         }
     }
 
-    fn visit_anon_const(&mut self, constant: &'a AnonConst) {
-        let def = self.create_def(constant.id, kw::Empty, DefKind::AnonConst, constant.value.span);
-        self.with_parent(def, |this| visit::walk_anon_const(this, constant));
-    }
-
     fn visit_expr(&mut self, expr: &'a Expr) {
         let parent_def = match expr.kind {
             ExprKind::MacCall(..) => return self.visit_macro_invoc(expr.id),