Add test case for unsizing with niche

tests/ui/layout/issue-112048-unsizing-niche.rs

@@ -0,0 +1,30 @@
+// run-pass
+
+// Check that unsizing does not change which field is considered for niche layout.
+
+#![feature(offset_of)]
+#![allow(dead_code)]
+
+#[derive(Clone)]
+struct WideptrField<T: ?Sized> {
+    first: usize,
+    second: usize,
+    niche: NicheAtEnd,
+    tail: T,
+}
+
+#[derive(Clone)]
+#[repr(C)]
+struct NicheAtEnd {
+    arr: [u8; 7],
+    b: bool,
+}
+
+type Tail = [bool; 8];
+
+fn main() {
+    assert_eq!(
+        core::mem::offset_of!(WideptrField<Tail>, niche),
+        core::mem::offset_of!(WideptrField<dyn Send>, niche)
+    );
+}