Yes, sorry, that was unclear. I was also thinking of the feature implicitly implementing Default or related traits. I think that would be covered by your "a struct that supports Struct {..} initialization (i.e. without specifying any field) implements Default automatically" suggestion.
Note that an automatic Default implementation would be a breaking change for any fieldless struct that manually implements Default and doesn't specify #[non_exhaustive], unless the automatic implementation is "smarter" than #[derive] and avoids creating a overlapping implementation.