[Pitch] `With` functions in the standard library

[ksluder (Kyle Sluder)]

@John_McCall

Ben, to what degree would you feel better about this if it were a free function instead of a magic method? We’d lose the nice interaction with optional chaining, but on the other hand, it would be clearer that it wasn’t a mutation of the original value, and it might make available some more suggestive names.

That addresses this use case:

Button("Duplicate") {
  let newItem = with(myModel.items[selectedRowIndex]) {
      $0.name += " copy"
  }
  myModel.insert(newItem)
}

But it doesn’t address this use case:

Button("Edit").sheet(…, onDismiss:
    let newItem = with(myModel.items[selectedRowIndex]) {
      $0.name += newName
      $0.unitPrice += newPrice
      $0.upc = newUPC
  }
  myModel.items[selectedRowIndex] = newItem
})

The second accesses myModel.items[selectedRowIndex] twice and makes an unnecessary copy. Is the first pattern really so much more common than the second?