Enable iteration over resource arrays

[turbolent]

Issue To Be Solved

Currently the for-in loop is only supported for struct arrays, not resource arrays.

Enable for-in loops to iterate over arrays of resources, binding an authorized reference:

pub resource R {
    pub let name: String

    init(name: String) {
        self.name = name
    }
}

let rs <- [
    <-create R(name: "bar"), 
    <-create R(name: "foo")
]

for ref in rs {
	// ref has type `&R`
    log(ref.name)
}

for ref: auth &R in rs {
	// ref has type `auth &R`
    log(ref.name)
}

destroy rs

Definition of Done

• Parser:
  • [ ] Allow optional type annotation for value variable of for-loop
• Checker:
  • [ ] Check optional type annotation for value variable of for-loop
  • [ ] Bind references when looping over resource arrays. Use optional type annotation to determine if authorized or not. Default to unauthorized to prevent security issues. Store type in elaboration
• Interpreter:
  • [ ] Bind reference when looping over resource arrays. Use type of elaboration
• [ ] Tests
• [ ] Documentation

[ceelo777]

Hey @turbolent I'd also be interested in working on this one as well with some guidance (since the other issue was pretty fun to tackle). Any other code snippets I could look at for reference?

[turbolent]

@ceelo777 Nice! 🎉

Here are some first pointers:

• The checker currently forbids iterating over resources of any kind:

  https://github.com/onflow/cadence/blob/3bbc8c8a1a85f729a6a7ae1537769f6cb8de4964/runtime/sema/check_for.go#L43-L48

  This needs to be removed.

• Now that the value that is iterated over may be a resource, checkUnusedExpressionResourceLoss must be called:

  https://github.com/onflow/cadence/blob/3bbc8c8a1a85f729a6a7ae1537769f6cb8de4964/runtime/sema/checker.go#L1607-L1624

• The checker declares the iteration variable as the element type:

  https://github.com/onflow/cadence/blob/3bbc8c8a1a85f729a6a7ae1537769f6cb8de4964/runtime/sema/check_for.go#L66-L66

  This needs to be changed to an authorized reference of the element type, if the iterated over value is an array of resources.

• The interpreter sets the iteration value to each element of the array:

https://github.com/onflow/cadence/blob/3bbc8c8a1a85f729a6a7ae1537769f6cb8de4964/runtime/interpreter/interpreter_statement.go#L240-L240

This needs to be changed to an authorized ephemeral reference to the element.

[turbolent]

Iterating over a resource array is already possible, but requires some boilerplate code (Playground: https://play.onflow.org/58c1529b-7df0-4a8c-8cbe-3c78d72514b2?type=script&id=9bb82938-eedf-4ae6-b914-f266d2837016&storage=none):

pub contract Test {

    pub resource R {}

    pub fun createR(): @R {
        return <-create Test.R()
    }
}

import Test from 0x1

pub fun main() {
    let rs <- [
        <- Test.createR(),
        <- Test.createR(),
        <- Test.createR()
    ]


    var i = 0
    while i < rs.length {
       let ref = &rs[i] as &Test.R
     
       log(ref.uuid)

       i = i + 1
    }

    destroy rs
}