Fix derive collection after unresolved attribute fallback

crates/hir_def/src/nameres/collector.rs

@@ -367,6 +367,8 @@ impl DefCollector<'_> {
     /// This improves UX when proc macros are turned off or don't work, and replicates the behavior
     /// before we supported proc. attribute macros.
     fn reseed_with_unresolved_attributes(&mut self) -> ReachedFixedPoint {
+        cov_mark::hit!(unresolved_attribute_fallback);
+
         let mut added_items = false;
         let unexpanded_macros = std::mem::replace(&mut self.unexpanded_macros, Vec::new());
         for directive in &unexpanded_macros {
@@ -391,7 +393,9 @@ impl DefCollector<'_> {
                 added_items = true;
             }
         }
-        self.unexpanded_macros = unexpanded_macros;
+
+        // The collection above might add new unresolved macros (eg. derives), so merge the lists.
+        self.unexpanded_macros.extend(unexpanded_macros);
 
         if added_items {
             // Continue name resolution with the new data.

crates/hir_def/src/nameres/tests/macros.rs

@@ -685,6 +685,27 @@ pub trait Clone {}
     );
 }
 
+#[test]
+fn builtin_derive_with_unresolved_attributes_fall_back() {
+    // Tests that we still resolve derives after ignoring an unresolved attribute.
+    cov_mark::check!(unresolved_attribute_fallback);
+    let map = compute_crate_def_map(
+        r#"
+        //- /main.rs crate:main deps:core
+        use core::Clone;
+
+        #[derive(Clone)]
+        #[unresolved]
+        struct Foo;
+
+        //- /core.rs crate:core
+        #[rustc_builtin_macro]
+        pub macro Clone {}
+        "#,
+    );
+    assert_eq!(map.modules[map.root].scope.impls().len(), 1);
+}
+
 #[test]
 fn macro_expansion_overflow() {
     cov_mark::check!(macro_expansion_overflow);
@@ -842,7 +863,6 @@ fn collects_derive_helpers() {
 fn resolve_macro_def() {
     check(
         r#"
-//- /lib.rs
 pub macro structs($($i:ident),*) {
     $(struct $i { field: u32 } )*
 }