Provide and use macro for impl Derive via Builder

1. There were a lot of open-coded impl Default ... FooBuilder::default().build()....

2. There were three cases where we #[derive(Default)]. As it happens none of these had non-Default default fields, so there wasn't an actual bug.

assigned to @Diziet

requested review from @eta

added 10 commits

• ae07da4b...b137be22 - 8 commits from branch tpo/core:main
• 3420fe73 - Define and use impl_default_via_builder
• 7909a51f - No longer derive Default on three structs which derive Builder

Compare with previous version

Rebased to fix another trivial conflict.

marked this merge request as draft

Rebased to:

• Make placement of the new macro calls consistent: always right after the FooConfig struct
• Add further call sites. I have now reviewed what I think are all the relevant places and I think they all use this macro now.
• Rename the macro. I plan to increase its functionality (including adding tests to it for properties that I discover are not currently true everywhere). Renaming it now will avoid having to touch most of the call sites.

marked this merge request as ready

mentioned in merge request !502 (merged)

mentioned in commit 7207b175

merged

approved this merge request

mentioned in merge request !505 (merged)