add const test for ptr::metadata

Author: RalfJung

tests/ui/consts/const-eval/ub-wide-ptr.rs

@@ -1,7 +1,7 @@
 // ignore-tidy-linelength
 #![allow(unused)]
 
-use std::mem;
+use std::{ptr, mem};
 
 // Strip out raw byte dumps to make comparison platform-independent:
 //@ normalize-stderr-test "(the raw bytes of the constant) \(size: [0-9]*, align: [0-9]*\)" -> "$1 (size: $$SIZE, align: $$ALIGN)"
@@ -145,6 +145,8 @@ const RAW_TRAIT_OBJ_VTABLE_NULL: *const dyn Trait = unsafe { mem::transmute((&92
 const RAW_TRAIT_OBJ_VTABLE_INVALID: *const dyn Trait = unsafe { mem::transmute((&92u8, &3u64)) };
 //~^ ERROR it is undefined behavior to use this value
 const RAW_TRAIT_OBJ_CONTENT_INVALID: *const dyn Trait = unsafe { mem::transmute::<_, &bool>(&3u8) } as *const dyn Trait; // ok because raw
+// Officially blessed way to get the vtable
+const DYN_METADATA: DynMetadata<dyn Send> = ptr::metadata::<dyn Send>(ptr::null::<i32>());
 
 // Const eval fails for these, so they need to be statics to error.
 static mut RAW_TRAIT_OBJ_VTABLE_NULL_THROUGH_REF: *const dyn Trait = unsafe {