Manual 'Any' type erasure with setter?

I understand how to create an AnyThing type erasure like below, when I have just getters. But when I try to add the setter for field on the AnyThing I get a compiler error. I'm not sure how to proceed. Any recommendations?

The error: Cannot assign to property: 't' is a 'let' constant

protocol Thing {
    var field: String { get set }
    // ... imagine other req's that make it unusable except as a generic constraint

class Thing1: Thing {
    var field: String = "hi"

/// A type-erased Thing
struct AnyThing {
    let thing: Any
    let getField: () -> String
    let setField: (String) -> Void
    init<T: Thing>(_ t: T) {
        self.thing = t
        self.getField = { t.field }
        self.setField = { t.field = $0 } // <-- error here
    var field: String {
        get { getField() }
        set { setField(newValue) }

let thing = Thing1()
let anyThing = AnyThing(thing)
print("thing.field = \(thing.field)")
print("anyThing.field = \(anyThing.field)")
anyThing.field = "hola"
print("anyThing.field = \(anyThing.field)")

You need to add an AnyObject constraint to the Thing protocol or a nonmutating set on the field requirement.

It's nothing to do with type erasure, it's just that field is implicitly mutable and in case of a conforming type which is a class, this could allow you to reassign self in field's setter. By adding an AnyObject constraint or nonmutating set, the compiler knows you cannot do that and thus allows you to assign a new value to field.

Okay. Do you think there is a a way to make AnyThing work with both classes and structs?