Allow removing type declarations from contracts

[SupunS]

Issue to be solved

Currently, the contract update validator prevents removing type declarations from a contract, to prevent it from getting re-added with a different structure. However, it would be useful to have a way to remove type definitions and force users of those contracts to start using a new type instead. This could be even more useful with cadence 1.0.

Suggested Solution

Suggestion I:

One possible solution is to introduce a pragma to tombstone a type definition. e.g: #removedType(T).
One can remove a type and add this pragma to indicate that this type was removed. It will prevent re-adding a new type with the same name in future. The pragma itself is not removable once it is aded.

Cons:

• The problem with just removing a type is that, any stored value belongs to this type would become unusable. This does not cause any harm to the network though (which was the original intention of preventing type removals).
• How would migrations handle these missing types?

Suggestion II:

Alternatively, it is possible to provide a way to remove and replace a type with another, particularly during the Cadence 1.0 upgrades. e.g: #replacedType(T, R), where R is the new type to replace with. Similar to the suggested removedType pragma, this will prevent any re-additions of a new type with the same name.

• During the state migrations, all usages of T would be replaced with type R, including static types of values, conformances, etc.
• This is in a way, providing the user also the ability to define type-change rules similar to what we have done for the system contracts.
• Migrations will first have to collect all these replacements pairs first, before do the actual migrations

This would be only supported if T and R satisfy particular conditions, such as:

• T and R has to be of the same kind. i.e: both are resources / both are resource-interfaces / etc.
• The new type R should have at-most the set of fields as the old type T. It's ok to have less number of fields, but no any additional fields. The filed names and types of the fields should be the same. (This is similar to the existing field update restrictions during contract updates).
• So this is basically similar to allowing renaming a type (in addition to what is already supported in existing contract updates)
• Anything else??

[j1010001]

Not breaking, does not make sense to do this post C1.0 migration,
might be needed for some contracts to complete migration to C1.0, not a blocker, nice to have (workaround - keeping old types, NFT standard also keep some old type interfaces, if people start to use the old types, it can cause problems).

[j1010001]

Implementing Suggestion 1 would unblock staging of the FiatToken contract, which is now a blocker for Crescendo launch. Increasing priority to urgent.

[turbolent]

@dsainati1 Could you please document this? Both in the language reference, as well as a short update to the forum post https://forum.flow.com/t/update-on-cadence-1-0/5197