Make struct layout not depend on unsizeable tail

tests/ui/layout/issue-112048-unsizing-field-order.rs

@@ -0,0 +1,25 @@
+// run-pass
+
+// Check that unsizing doesn't reorder fields.
+
+#![allow(dead_code)]
+
+use std::fmt::Debug;
+
+#[derive(Debug)]
+struct GcNode<T: ?Sized> {
+    gets_swapped_with_next: usize,
+    next: Option<&'static GcNode<dyn Debug>>,
+    tail: T,
+}
+
+fn main() {
+    let node: Box<GcNode<dyn Debug>> = Box::new(GcNode {
+        gets_swapped_with_next: 42,
+        next: None,
+        tail: Box::new(1),
+    });
+
+    assert_eq!(node.gets_swapped_with_next, 42);
+    assert!(node.next.is_none());
+}