Add #[rustc_never_returns_null_ptr]

And look for it in the useless_ptr_null_checks lint

compiler/rustc_lint/src/ptr_nulls.rs

@@ -49,7 +49,7 @@ fn ptr_cast_chain<'a>(cx: &'a LateContext<'_>, mut e: &'a Expr<'a>) -> Option<&'
         } else if let ExprKind::Call(path, [arg]) = e.kind
             && let ExprKind::Path(ref qpath) = path.kind
             && let Some(def_id) = cx.qpath_res(qpath, path.hir_id).opt_def_id()
-            && matches!(cx.tcx.get_diagnostic_name(def_id), Some(sym::ptr_from_ref | sym::ptr_from_mut)) {
+            && cx.tcx.has_attr(def_id, sym::rustc_never_returns_null_ptr) {
             had_at_least_one_cast = true;
             arg
         } else if had_at_least_one_cast {