Add test for #51559

Closes #51559

src/test/ui/consts/issue-51559.rs

@@ -0,0 +1,7 @@
+#![feature(const_raw_ptr_to_usize_cast)]
+
+const BAR: *mut () = ((|| 3) as fn() -> i32) as *mut ();
+pub const FOO: usize = unsafe { BAR as usize };
+//~^ ERROR it is undefined behavior to use this value
+
+fn main() {}

src/test/ui/consts/issue-51559.stderr

@@ -0,0 +1,11 @@
+error[E0080]: it is undefined behavior to use this value
+  --> $DIR/issue-51559.rs:4:1
+   |
+LL | pub const FOO: usize = unsafe { BAR as usize };
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ type validation failed: encountered a pointer, but expected initialized plain (non-pointer) bytes
+   |
+   = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rust compiler repository if you believe it should not be considered undefined behavior
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0080`.