add tests for const NonNull::as_ref

tests/ui/consts/const-eval/nonnull_as_ref.rs

@@ -0,0 +1,8 @@
+// check-pass
+
+use std::ptr::NonNull;
+
+const NON_NULL: NonNull<u8> = unsafe { NonNull::new_unchecked((&42u8 as *const u8).cast_mut()) };
+const _: () = assert!(42 == *unsafe { NON_NULL.as_ref() });
+
+fn main() {}

tests/ui/consts/const-eval/nonnull_as_ref_ub.rs

@@ -0,0 +1,6 @@
+use std::ptr::NonNull;
+
+const NON_NULL: NonNull<u8> = unsafe { NonNull::dangling() };
+const _: () = assert!(42 == *unsafe { NON_NULL.as_ref() });
+
+fn main() {}

tests/ui/consts/const-eval/nonnull_as_ref_ub.stderr

@@ -0,0 +1,16 @@
+error[E0080]: evaluation of constant value failed
+  --> $SRC_DIR/core/src/ptr/non_null.rs:LL:COL
+   |
+   = note: dereferencing pointer failed: 0x1[noalloc] is a dangling pointer (it has no provenance)
+   |
+note: inside `NonNull::<u8>::as_ref::<'_>`
+  --> $SRC_DIR/core/src/ptr/non_null.rs:LL:COL
+note: inside `_`
+  --> $DIR/nonnull_as_ref_ub.rs:4:39
+   |
+LL | const _: () = assert!(42 == *unsafe { NON_NULL.as_ref() });
+   |                                       ^^^^^^^^^^^^^^^^^
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0080`.