Relax default entitlement mapping for dictionary/array elements

[SupunS]

Issue to be solved

Currently, there is no way for dictionaries/arrays to define the entitlement mapping for their children. So the default behavior is to have no entitlements for references when a child is accessed through a reference to the parent.
This 'default behavior' was chosen due to some safety concerns: #2587 (comment).

However, this can be restrictive sometimes, if one gets a mutable reference to a dictionary auth(Mutate) {String: {String: AnyStruct}} and wants to mutate the inner dictionary, then this is a bit of a hassle. In order to mutate the inner one, they would need to either:

• Get a dereferenced copy of the child, update it, and insert it again.
• Remove the child, update it, and insert it again.

Both of these are not only expensive but also involve a lot of boilerplate code.

e.g: https://discord.com/channels/613813861610684416/1176642288986894366/1176949623638732912

Suggested Solution

An alternative is to give Identity entitlement mapping for children. So children would get the same entitlement as the parent.

[SupunS]

This is not a breaking change, since the proposal is to relax an existing restriction.

[SupunS]

cc: @joshuahannan

[turbolent]

@SupunS Could you please add the concerns to the description or as a comment?

[turbolent]

@dsainati1 Can you please add your concerns?

[turbolent]

Example:

let xs = {"a": {"b": "c"}}
let x: ??? = xs[1]

[dsainati1]

My concerns here are primarily about safety; I don't think it is necessarily the case that the entitlements I have to an outer collection should automatically propagate to the inner collections. E.g. just because I have Mutate permissions to an array of NFTs, allowing me to insert or remove NFTs from the array, does not mean I should necessarily have permissions to modify the NFTs themselves.

[turbolent]

Would it would be possible to expose this functionality, getting an entitled reference to an inner container/element of the outer container, through a getter function right?

For example:

resource Collection {
  var nfts: @{UInt64: NFT}

  fun getMutableNFTRef(id: UInt64): auth(Mutate) &NFT {
    return &nfts[id] auth(Mutate) &NFT
  }
}

[dsainati1]

Yeah I think this should be possible. If it isn't currently, that seems like the right way to solve this.

[SupunS]

For composites, this is already working through entitlement mappings. One doesn't even have to define a getter.

The problem is for the dictionaries and arrays that are within another dictionary/array, where there is no way for a user to define custom getters/setters or entitlement mappings.