add macros for generating Default impls

[telcharr]

Description

Adds impl_default!, s_with_default!, and s_no_extra_traits_with_default! as a start for #4975. Unions get no implicit Default so a union field would need to supply one via #[custom_default(unsafe { mem::zeroed::<U>() })]. As discussed, custom_default needs to be the first attribute on a field since the macro matches it literally.

Checklist

• Relevant tests in libc-test/semver have been updated
• No placeholder or unstable values like *LAST or *MAX are
  included (see #3131)
• Tested locally (cd libc-test && cargo test --target mytarget);
  especially relevant for platforms that may not be checked in CI

@rustbot label +stable-nominated

[tgross35]

Thank you for putting this together! It looks very workable. I have a handful of requests but all style/maintainability.

Would you be able to add a commit that makes use of this (replacing s!) in a few small places so we make sure it works in situ? Fine to do that in a separate PR, if you prefer, as long as it's coming soon.

View changes since this review

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[telcharr]

Added a commit converting new/linux_uapi/linux/can.rs since it hits most cases. The only things it doesn't cover is #[cfg] and doc comments. If you'd like me to add a new commit implementing the macros that covers those cases let me know.

@rustbot ready