AnyStruct and AnyResource

AnyStruct is the top type of all non-resource types, i.e., all non-resource types are a subtype of it.

AnyResource is the top type of all resource types.

// Declare a variable that has the type `AnyStruct`.
// Any non-resource typed value can be assigned to it, for example an integer,
// but not resource-typed values.
//
var someStruct: AnyStruct = 1

// Assign a value with a different non-resource type, `Bool`.
someStruct = true

// Declare a structure named `TestStruct`, create an instance of it,
// and assign it to the `AnyStruct`-typed variable
//
struct TestStruct {}

let testStruct = TestStruct()

someStruct = testStruct

// Declare a resource named `TestResource`

resource Test {}

// Declare a variable that has the type `AnyResource`.
// Any resource-typed value can be assigned to it,
// but not non-resource typed values.
//
var someResource: @AnyResource <- create Test()

// Invalid: Resource-typed values can not be assigned
// to `AnyStruct`-typed variables
//
someStruct <- create Test()

// Invalid: Non-resource typed values can not be assigned
// to `AnyResource`-typed variables
//
someResource = 1

However, using AnyStruct and AnyResource does not opt-out of type checking. It is invalid to access fields and call functions on these types, as they have no fields and functions.

// Declare a variable that has the type `AnyStruct`.
// The initial value is an integer,
// but the variable still has the explicit type `AnyStruct`.
//
let a: AnyStruct = 1

// Invalid: Operator cannot be used for an `AnyStruct` value (`a`, left-hand side)
// and an `Int` value (`2`, right-hand side).
//
a + 2

AnyStruct and AnyResource may be used like other types, for example, they may be the element type of arrays or be the element type of an optional type.

// Declare a variable that has the type `[AnyStruct]`,
// i.e. an array of elements of any non-resource type.
//
let anyValues: [AnyStruct] = [1, "2", true]

// Declare a variable that has the type `AnyStruct?`,
// i.e. an optional type of any non-resource type.
//
var maybeSomething: AnyStruct? = 42

maybeSomething = "twenty-four"

maybeSomething = nil

AnyStruct is also the super-type of all non-resource optional types, and AnyResource is the super-type of all resource optional types.

let maybeInt: Int? = 1
let anything: AnyStruct = maybeInt

Conditional downcasting allows coercing a value which has the type AnyStruct or AnyResource back to its original type.