liballoc: VecDeque: Simplify binary_search_by()

library/alloc/src/collections/vec_deque.rs

@@ -2516,25 +2516,14 @@ impl<T> VecDeque<T> {
     where
         F: FnMut(&'a T) -> Ordering,
     {
-        if self.is_empty() {
-            return Err(0);
-        }
-
         let (front, back) = self.as_slices();
 
-        match back.first().map(|elem| f(elem)) {
+        if let Some(Ordering::Less | Ordering::Equal) = back.first().map(|elem| f(elem)) {
-            Some(Ordering::Equal) => return Ok(front.len()),
+            back.binary_search_by(f).map(|idx| idx + front.len()).map_err(|idx| idx + front.len())
-            Some(Ordering::Less) => {
+        } else {
-                return back[1..]
-                    .binary_search_by(f)
-                    .map(|idx| idx + front.len() + 1)
-                    .map_err(|idx| idx + front.len() + 1);
-            }
-            _ => {}
-        }
-
             front.binary_search_by(f)
         }
+    }
 
     /// Binary searches this sorted `VecDeque` with a key extraction function.
     ///