Fix build with no-default-features

Author: saethlin

src/stacked_borrows/stack.rs

@@ -41,7 +41,6 @@ pub struct Stack {
 
 impl Stack {
     pub fn retain(&mut self, tags: &FxHashSet<SbTag>) {
-        #[cfg(feature = "stack-cache")]
         let mut first_removed = None;
 
         let mut read_idx = 1;
@@ -73,6 +72,9 @@ impl Stack {
         }
         self.borrows.truncate(write_idx);
 
+        #[cfg(not(feature = "stack-cache"))]
+        drop(first_removed); // This is only needed for the stack-cache
+
         #[cfg(feature = "stack-cache")]
         if let Some(first_removed) = first_removed {
             // Either end of unique_range may have shifted, all we really know is that we can't