Mutation methods as a type, allowing easy copies to be made

Mutation.swift

struct Person {
    var firstName, lastName: String
    
    mutating func makeScottishClan() {
      lastName = "mc\(lastName)"
    }
}

// Person.Mutation gets automatically created (like an enum)
//   .firstName(String)
//   .lastName(String)
//   .makeScottishClan

let john = Person(firstName: "John", lastName: "Doe")

// Make copy with mutations (Person.Mutation are written in full to show what is happening)
let carol = john mutating [
  Person.Mutation.firstName("Carol"),
  Person.Mutation.makeScottishClan
]

// Or can shorten to:
let alice = john mutating [
  .firstName = "Alice",
  .makeScottishClan
]

// Can shorten to one line when just a single mutation:
let robert = alice mutating .firstName = "Robert"

I wonder if you did have something like this with a = , could it complicate parsing? Or not read as declarativel?

let robert = alice mutating .firstName = "Robert"

vs the less natural:

let robert = alice mutating .firstName("Robert")

The array passed to mutating could be dynamic, allowing a series of mutations to be built up, stored, or applied to multiple values.