Auto merge of #51612 - ashtneoi:51515-missing-first-char, r=pnkfelix

NLL: fix E0594 "change to mutable ref" suggestion

Fix #51515.
Fix #51879.

Questions:
- [x] Is this the right place to fix this? It feels brittle, being so close to the frontend. **It's probably fine.**
- [ ] Have I missed any other cases that trigger this behavior?
- [x] Is it okay to use HELP and SUGGESTION in the UI test? **Yes.**
- [x] Do I need more tests for this? **No.**