Trouble Forwarding to Mutating Subscript

Using Swift
1

I was trying to write a wrapper around InlineArray with a borrowing & mutating subscript that forwards to the array's subscript (to get a feel for it), but I unexpectedly ended up with an error inside my mutating getter.

public struct Vec4<Element: Numeric> {
  var storage: [4 of Element]
  public init(_ x: Element, _ y: Element, _ z: Element, _ w: Element) {
    storage = [x, y, z, w]
  }
  public subscript(index: Int) -> Element {
    borrow { storage[index] }
    mutate { &storage[index] }
  }
}

The error is:

main.swift:8:14: error: invalid return value from a mutate accessor
 6 |   public subscript(index: Int) -> Element {
 7 |     borrow { storage[index] }
 8 |     mutate { &storage[index] }
   |              |- error: invalid return value from a mutate accessor
   |              `- note: mutate accessors can return stored properties, computed properties with mutate accessors or global 'let' declarations
 9 |   }
10 | }

Am I holding it wrong, or should I file a bug report?

2 Likes
2

You're not holding it wrong, per se. In Swift 6.4 InlineArray's subscript is implemented with unsafeAddress accessors, and those don't compose with anything. As of now you could implement this as a yielding mutate accessor (using -enable-experimental-feature CoroutineAccessors).

public subscript(index: Int) -> Element {
  yielding borrow { yield storage[index] }
  yielding mutate { yield &storage[index] }
}

On main currently, InlineArray's subscript is now implemented with borrow and mutate; I think your example above would work with a recent nightly.

3 Likes
3

I was trying this out on Apple Swift version 6.5-dev (LLVM 47afa53ea425e20, Swift 871a239941f3613).

4

I would have expected it to work with that nightly. Can you post an issue?

1 Like
5

Done. Unable to Forward to InlineArray's Mutate-ing Script. · Issue #91936 · swiftlang/swift · GitHub

1 Like