Changes for 1.99

- [x] Rebase once the beta is out
- [x] Verify https://github.com/rust-lang/rust/pull/160820 works/is correct on r9x
- [x] Go through commits and squash later fixes to the same areas as earlier ones
- [x] Verify https://github.com/rust-lang/rust/pull/150679 -- all good, creating Dir fails on systems that don't support it