resolve: Remove an incorrect assert

src/librustc_resolve/macros.rs

@@ -773,7 +773,6 @@ impl<'a> Resolver<'a> {
                     check_consistency(self, &[seg], ident.span, kind, initial_res, res);
                 }
                 Err(..) => {
-                    assert!(initial_binding.is_none());
                     let expected = kind.descr_expected();
                     let msg = format!("cannot find {} `{}` in this scope", expected, ident);
                     let mut err = self.session.struct_span_err(ident.span, &msg);

src/test/ui/proc-macro/disappearing-resolution.rs

@@ -0,0 +1,22 @@
+// Regression test for issue #64803 (initial attribute resolution can disappear later).
+
+// aux-build:test-macros.rs
+
+#[macro_use]
+extern crate test_macros;
+
+mod m {
+    use test_macros::Empty;
+}
+use m::Empty; //~ ERROR derive macro `Empty` is private
+
+// To resolve `empty_helper` we need to resolve `Empty`.
+// During initial resolution `use m::Empty` introduces no entries, so we proceed to `macro_use`,
+// successfully resolve `Empty` from there, and then resolve `empty_helper` as its helper.
+// During validation `use m::Empty` introduces a `Res::Err` stub, so `Empty` resolves to it,
+// and `empty_helper` can no longer be resolved.
+#[empty_helper] //~ ERROR cannot find attribute `empty_helper` in this scope
+#[derive(Empty)]
+struct S;
+
+fn main() {}

src/test/ui/proc-macro/disappearing-resolution.stderr

@@ -0,0 +1,15 @@
+error: cannot find attribute `empty_helper` in this scope
+  --> $DIR/disappearing-resolution.rs:18:3
+   |
+LL | #[empty_helper]
+   |   ^^^^^^^^^^^^
+
+error[E0603]: derive macro `Empty` is private
+  --> $DIR/disappearing-resolution.rs:11:8
+   |
+LL | use m::Empty;
+   |        ^^^^^
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0603`.