Auto merge of #113316 - DrMeepster:underefer_perf, r=oli-obk

Rewrite `UnDerefer`, again

This PR is intended to improve the perf regression introduced by #112882.

`UnDerefer` has been separated out again for borrowck reasons. It was a bit overzealous to remove it in the previous PR.

r? `@oli-obk`

Author: bors