Avoid emptiness check in PeekMut::pop

alloc/src/collections/binary_heap/mod.rs

@@ -374,7 +374,10 @@ impl<'a, T: Ord, A: Allocator> PeekMut<'a, T, A> {
             // the caller could've mutated the element. It is removed from the
             // heap on the next line and pop() is not sensitive to its value.
         }
-        this.heap.pop().unwrap()
+
+        // SAFETY: Have a `PeekMut` element proves that the associated binary heap being non-empty,
+        // so the `pop` operation will not fail.
+        unsafe { this.heap.pop().unwrap_unchecked() }
     }
 }