How to project an indirect enum value?

[stephencelis]

A previous discussion mentions projectValue() as a means of unpacking an enum case's associated values, but this doesn't seem to work for indirect/recursive enum cases.

Given the following enum:

enum Foo: Equatable {
  indirect case foo(Foo)
  case bar(Int)
}

It's easy enough to extract an Int from the bar case:

let val = Foo.bar(42)
var box = container(for: val)
let expected = 42
let actual = box.projectValue().load(as: Int.self)
XCTAssertEqual(actual, expected)

But attempting to extract a Foo from the foo case fails (because it returns identity):

let val = Foo.foo(.bar(42))
var box = container(for: val)
let expected = Foo.bar(42)
let actual = box.projectValue().load(as: Foo.self)
XCTAssertEqual(actual, expected)
// ❌ XCTAssertEqual failed: ("foo(.bar(42))") is not equal to ("bar(42)")

Is there a way to properly extract such a case?

[iampatbrown]

@stephencelis How'd you end up going with this? I'm keen to get something working. The below works on your example but there's probably a much better way to go about it. Was hoping you could point me in the right direction.

extension AnyExistentialContainer {
  public mutating func projectEnumValue() -> UnsafeRawPointer {
    let projectedValue = self.projectValue()
    guard let enumMetadata = metadata as? EnumMetadata else { return projectedValue }
    let tag = enumMetadata.enumVwt.getEnumTag(for: projectedValue)
    let field = enumMetadata.descriptor.fields.records[Int(tag)]
    guard field.flags.isIndirectCase else { return projectedValue }
    let alignMask = UInt(metadata.vwt.flags.alignmentMask)
    let heapObjSize = UInt(MemoryLayout<HeapObject>.size)
    let byteOffset = (heapObjSize + alignMask) & ~alignMask
    let bytePtr = withUnsafePointer(to: &self) {
      $0.withMemoryRebound(to: UnsafePointer<HeapObject>.self, capacity: 1) {
        $0.pointee.raw
      }
    }
    return bytePtr + Int(byteOffset)
  }
}

[stephencelis]

There is some information in this Swift blog post about how the Mirror API is implemented, and this implementation using Echo. We tried to port the logic here, but were unable to get something that works robustly, and we were beginning to use a lot more of the runtime than feels necessary:

https://github.com/pointfreeco/swift-case-paths/blob/d1af52f98304706156351bb3052e556c871051a2/Sources/CasePaths/EnumReflection.swift#L83

[iampatbrown]

Thats awesome :) Didn't think to look at the other branches :P Yeah it started to feel like reimplementing a trimmed down version just for CasePaths would be tough. Thanks for letting me know.