Fix Clippy warnings for Rust 1.92 (#1435)

Clippy warned "this assertion has a constant value" in a `debug_assert`.
We replaced the `cfg!` expression with the `#[cfg]` attribute.

Clippy treated a tuple constructor as double parenthesis by mistake. We
extracted the tuple constructor into an explicit let-binding.

Author: wks

src/util/alloc/free_list_allocator.rs

@@ -422,7 +422,8 @@ impl<VM: VMBinding> FreeListAllocator<VM> {
             let unswept = self.unswept_blocks.get_mut(bin).unwrap();
 
             // If we do eager sweeping, we should have no unswept blocks.
-            debug_assert!(!cfg!(feature = "eager_sweeping") || unswept.is_empty());
+            #[cfg(feature = "eager_sweeping")]
+            debug_assert!(unswept.is_empty());
 
             let mut sweep_later = |list: &mut BlockList| {
                 list.release_blocks(self.space);

src/util/test_util/mock_vm.rs

@@ -56,13 +56,19 @@ macro_rules! lifetime {
 /// Call `MockMethod`.
 macro_rules! mock {
     ($fn: ident($($arg:expr),*)) => {
-        write_mockvm(|mock| mock.$fn.call(($($arg),*)))
+        {
+            let arg_tuple = ($($arg),*);
+            write_mockvm(|mock| mock.$fn.call(arg_tuple))
+        }
     };
 }
 /// Call `MockAny`.
 macro_rules! mock_any {
     ($fn: ident($($arg:expr),*)) => {
-        *write_mockvm(|mock| mock.$fn.call_any(Box::new(($($arg),*)))).downcast().unwrap()
+        {
+            let arg_tuple = ($($arg),*);
+            *write_mockvm(|mock| mock.$fn.call_any(Box::new(arg_tuple))).downcast().unwrap()
+        }
     };
 }