Function entity_merge_guard_compared_values 

pub fn entity_merge_guard_compared_values(
    guard: EntityMergeGuard,
    into: &Entity,
    from: &Entity,
) -> (Value, Value)

The two values guard compared, for a caller-facing preview of the refusal.

The project side returns whatever that property holds, or null when the record has none. projects_are_disjoint refuses only on two non-empty arrays, so what a caller is shown here is what the guard read.