const_fn_unsize: check that the cast behaves correctly

Author: RalfJung

src/test/ui/consts/const_fn_unsize.rs

@@ -1,14 +1,21 @@
-// check-pass
+// run-pass
+#![feature(slice_ptr_len)]
 
 use std::ptr::NonNull;
 
+#[allow(unused)]
 const fn test() {
     let _x = NonNull::<[i32; 0]>::dangling() as NonNull<[i32]>;
 }
 
 // Regression test for #75118.
 pub const fn dangling_slice<T>() -> NonNull<[T]> {
-    NonNull::<[T; 0]>::dangling()
+    NonNull::<[T; 1]>::dangling()
 }
 
-fn main() {}
+const C: NonNull<[i32]> = dangling_slice();
+
+fn main() {
+    assert_eq!(C.as_ptr(), NonNull::<[i32; 1]>::dangling().as_ptr() as *mut _);
+    assert_eq!(C.as_ptr().len(), 1);
+}