[Variant] Check pending before VariantObject::insert

[friendlymatthew]

Rationale for this change

A follow up from #7778, we should make sure to check for pending fields before calling ObjectBuilder::insert

[alamb]

Makes sense to me -- thank you @friendlymatthew and @harshmotw-db

[alamb]

I wonder if there is some sort of RAAI mechanism we can use to ensure that the pending field is completed automatically when the child ListBuilder or ObjectBuilder is dropped

That way the compiler can ensure this type of bug is not possible

Something like

/// If this is a builder for a nested object or list, on `Drop` this object will finish the
/// in progress field for the parent
enum PendingParent {
...
}

impl Drop for PendingParent {
 List(...),
Object {
  field_name: &str,
  offset: usize
 }
}

Then we could create a field on the builder like this

struct ObjectBuilder {
...
  pending: Option<PendingParent>,
}

I think the biggest challenge would be sorting out the type lifetimes

[friendlymatthew]

Interesting, I can take a stab at this

[alamb]

• Filed [Variant] Make it harder to forget to finish a pending parent i n ObjectBuilder #7798