2 files changed, 17 insertions, 4 deletions

diff --git a/crates/hir_def/src/data.rs b/crates/hir_def/src/data.rs
index f9bb0c6da..e976e419e 100644
--- a/crates/hir_def/src/data.rs
+++ b/crates/hir_def/src/data.rs
@@ -36,18 +36,25 @@ impl FunctionData {
     pub(crate) fn fn_data_query(db: &dyn DefDatabase, func: FunctionId) -> Arc<FunctionData> {
         let loc = func.lookup(db);
         let krate = loc.container.module(db).krate;
+        let crate_graph = db.crate_graph();
+        let cfg_options = &crate_graph[krate].cfg_options;
         let item_tree = db.item_tree(loc.id.file_id);
         let func = &item_tree[loc.id.value];
-        let is_varargs = func
+
+        let enabled_params = func
             .params
             .clone()
-            .last()
+            .filter(|&param| item_tree.attrs(db, krate, param.into()).is_cfg_enabled(cfg_options));
+
+        // If last cfg-enabled param is a `...` param, it's a varargs function.
+        let is_varargs = enabled_params
+            .clone()
+            .next_back()
             .map_or(false, |param| matches!(item_tree[param], Param::Varargs));
 
         Arc::new(FunctionData {
             name: func.name.clone(),
-            params: func
-                .params
+            params: enabled_params
                 .clone()
                 .filter_map(|id| match &item_tree[id] {
                     Param::Normal(ty) => Some(item_tree[*ty].clone()),
diff --git a/crates/hir_def/src/item_tree.rs b/crates/hir_def/src/item_tree.rs
index c14da95ab..90df3d929 100644
--- a/crates/hir_def/src/item_tree.rs
+++ b/crates/hir_def/src/item_tree.rs
@@ -824,6 +824,12 @@ impl<T> Iterator for IdRange<T> {
     }
 }
 
+impl<T> DoubleEndedIterator for IdRange<T> {
+    fn next_back(&mut self) -> Option<Self::Item> {
+        self.range.next_back().map(|raw| Idx::from_raw(raw.into()))
+    }
+}
+
 impl<T> fmt::Debug for IdRange<T> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.debug_tuple(&format!("IdRange::<{}>", type_name::<T>())).field(&self.range).finish()