Add regression test for #72394

Author: ecstatic-morse

src/test/ui/unsafe/unsafe-unstable-const-fn.rs

@@ -0,0 +1,13 @@
+#![stable(feature = "foo", since = "1.33.0")]
+#![feature(staged_api)]
+#![feature(const_compare_raw_pointers)]
+#![feature(const_fn)]
+
+#[stable(feature = "foo", since = "1.33.0")]
+#[rustc_const_unstable(feature = "const_foo", issue = "none")]
+const fn unstable(a: *const i32, b: *const i32) -> bool {
+    a == b
+    //~^ pointer operation is unsafe
+}
+
+fn main() {}

src/test/ui/unsafe/unsafe-unstable-const-fn.stderr

@@ -0,0 +1,11 @@
+error[E0133]: pointer operation is unsafe and requires unsafe function or block
+  --> $DIR/unsafe-unstable-const-fn.rs:9:5
+   |
+LL |     a == b
+   |     ^^^^^^ pointer operation
+   |
+   = note: operations on pointers in constants
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0133`.