Do we always need to download snapshot and targets?

I'm updating rust-tuf to work with TUF-1.0.30. As part of reading through the spec, I that @rdimitrov changed the spec in #209 to allow us to exit the update-cycle early in update-timestamp 5.4.3.1. Should we apply this same logic to snapshots and targets?

rust-tuf currently implements this, where it will exit early if:

• When updating the snapshot metadata, check if the trusted timestamp role's snapshot version matches the trusted snapshot version, and has the correct hash value, if present.
• When updating the targets metadata, check if the trusted snapshot role's targets version matches the trusted targets version, and has the correct hash value, if present.
• When updating a delegated targets metadata, check if the delegating targets role's version matches the trusted delegated targets version, and has the correct hash value, if present.

Is this optimization allowed? If so, should we describe it in the spec? If not, why is this dangerous?

Start with the linked TUF specification section update-timestamp 5.4.3.1 and issue #209, then compare the snapshot, targets, and delegated-targets checks described here. Done means determining whether these early exits are permitted and whether the specification should document them, including any safety rationale.