Fix some new clippy lints

This changes the clippy tests to use the nightly toolchain, in the hopes of catching things earlier.

Along the way, it fixes some new lints that haven't made it to stable yet. See individual commit messages for details.

Checklist

General Contributing

[x] I have read the Code of Conduct and signed the CLA.
[x] I have added an entry to the RELEASE_NOTES.md file, or believe it's not necessary for this change.

Validation

For Default derive, I inspected the changes manually. Only a small handful of non-trivial impls remain now.