runtime: Set b:undo_indent where missing (#12944)

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/indent/rust.vim b/runtime/indent/rust.vim
index 2b544f4..b27d93c 100644
--- a/runtime/indent/rust.vim
+++ b/runtime/indent/rust.vim
@@ -2,6 +2,7 @@
 " Language:         Rust
 " Author:           Chris Morgan <me@chrismorgan.info>
 " Last Change:      2017 Jun 13
+"                   2023 Aug 28 by Vim Project (undo_indent)
 " For bugs, patches and license go to https://github.com/rust-lang/rust.vim
 
 " Only load this indent file when no other was loaded.
@@ -24,6 +25,8 @@
 
 setlocal indentexpr=GetRustIndent(v:lnum)
 
+let b:undo_indent = "setlocal cindent< cinoptions< cinkeys< cinwords< lisp< autoindent< indentkeys< indentexpr<"
+
 " Only define the function once.
 if exists("*GetRustIndent")
 	finish