Add explanation

Author: camelid

compiler/rustc_resolve/src/def_collector.rs

@@ -35,6 +35,12 @@ pub(crate) fn collect_definitions(
 struct DefCollector<'a, 'b, 'tcx> {
     resolver: &'a mut Resolver<'b, 'tcx>,
     parent_def: LocalDefId,
+    /// If we have an anon const that consists of a macro invocation, e.g. `Foo<{ m!() }>`,
+    /// we need to wait until we know what the macro expands to before we create the def for
+    /// the anon const. That's because we lower some anon consts into `hir::ConstArgKind::Path`,
+    /// which don't have defs.
+    /// 
+    /// See [`Self::visit_anon_const`].
     pending_anon_const_info: Option<PendingAnonConstInfo>,
     impl_trait_context: ImplTraitContext,
     in_attr: bool,
@@ -335,7 +341,9 @@ impl<'a, 'b, 'tcx> visit::Visitor<'a> for DefCollector<'a, 'b, 'tcx> {
         // def. we'll then create a def later in ast lowering in this case. the parent of nested
         // items will be messed up, but that's ok because there can't be any if we're just looking
         // for bare idents.
+
         if matches!(constant.value.maybe_unwrap_block().kind, ExprKind::MacCall(..)) {
+            // See self.pending_anon_const_info for explanation
             self.pending_anon_const_info =
                 Some(PendingAnonConstInfo { id: constant.id, span: constant.value.span });
             visit::walk_anon_const(self, constant)
@@ -353,8 +361,8 @@ impl<'a, 'b, 'tcx> visit::Visitor<'a> for DefCollector<'a, 'b, 'tcx> {
             return self.visit_macro_invoc(expr.id);
         }
 
-        let grandparent_def = if let Some(pending_anon) = self.pending_anon_const_info {
-            self.pending_anon_const_info = None;
+        let grandparent_def = if let Some(pending_anon) = self.pending_anon_const_info.take() {
+            // See self.pending_anon_const_info for explanation
             if !expr.is_potential_trivial_const_arg() {
                 self.create_def(pending_anon.id, kw::Empty, DefKind::AnonConst, pending_anon.span)
             } else {