add test for casting pointer to union with unsized tail

tests/ui/cast/unsized-union-ice.rs

@@ -0,0 +1,14 @@
+// Regression test for https://github.com/rust-lang/rust/issues/122581
+// This used to ICE, because the union was unsized and the pointer casting code
+// assumed that non-struct ADTs must be sized.
+
+union Union {
+    val: std::mem::ManuallyDrop<[u8]>,
+    //~^ ERROR the size for values of type `[u8]` cannot be known at compilation time
+}
+
+fn cast(ptr: *const ()) -> *const Union {
+    ptr as _
+}
+
+fn main() {}

tests/ui/cast/unsized-union-ice.stderr

@@ -0,0 +1,23 @@
+error[E0277]: the size for values of type `[u8]` cannot be known at compilation time
+  --> $DIR/unsized-union-ice.rs:6:10
+   |
+LL |     val: std::mem::ManuallyDrop<[u8]>,
+   |          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ doesn't have a size known at compile-time
+   |
+   = help: within `ManuallyDrop<[u8]>`, the trait `Sized` is not implemented for `[u8]`, which is required by `ManuallyDrop<[u8]>: Sized`
+note: required because it appears within the type `ManuallyDrop<[u8]>`
+  --> $SRC_DIR/core/src/mem/manually_drop.rs:LL:COL
+   = note: no field of a union may have a dynamically sized type
+   = help: change the field's type to have a statically known size
+help: borrowed types always have a statically known size
+   |
+LL |     val: &std::mem::ManuallyDrop<[u8]>,
+   |          +
+help: the `Box` type always has a statically known size and allocates its contents in the heap
+   |
+LL |     val: Box<std::mem::ManuallyDrop<[u8]>>,
+   |          ++++                            +
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0277`.