Why is declaring self not required in structures where it's required in classes?

The purpose of including self when using properties inside an escaping closure (whether optional closure or one explicitly marked as @escaping) with reference types is to make the capture semantics explicit. As the compiler warns us if we remove self reference:

Reference to property 'someProperty' in closure requires explicit use of 'self' to make capture semantics explicit.

But there are no ambiguous capture semantics with structs. You are always dealing with a copy inside the escaping closure. It is only ambiguous with reference types, where you need self to make clear where the strong reference cycle might be introduced, which instance you are referencing, etc.

By the way, with class types, referencing self in conjunction with the property is not the only way to make the capture semantics explicit. For example, you can make your intent explicit with a “capture list”, either:

• Capture the property only:

   class SomeClass {
       var someProperty = 1
  
       func someMethod(completion: @escaping () -> Void) { ... }
  
       func anotherMethod() {
           someMethod { [someProperty] in    // this captures the property, but not `self`
               print(someProperty)
           }
       }
   }
  

• Or capture self:

   class SomeClass {
       var someProperty = 1
  
       func someMethod(completion: @escaping () -> Void) { ... }
  
       func anotherMethod() {
           someMethod { [self] in            // this explicitly captures `self`
               print(someProperty)
           }
       }
   }
  

Both of these approaches also make it explicit what you are capturing.

For classes, closures provide a mechanism to increment a reference count, thus "keeping the object alive".

Maybe you're okay with just capturing someProperty. Maybe not! The compiler doesn't know if you're using a closure in order to increment the reference, so it makes you be explicit about your intentions.

Not only is that a non-issue with structs, but so is the possibility of mutation, which is strictly disallowed.

Let's say you wanted anotherMethod to allow mutation of any kind, in a struct. You could start by marking it as mutating…

struct SomeStruct {
  func someMethod(completion: (() -> Void)?) {}

  mutating func anotherMethod() {
    someMethod {
      self
    }
  }
}

…but no, that's an error:

Escaping closure captures mutating 'self' parameter

Capture self, though…

  mutating func anotherMethod() {
    someMethod { [self] in
      self
    }
  }

…and that's fine.

And it's also the only option Swift allows. When you use an escaping closure from within a struct, you can only use an immutable capture of an instance. i.e. [self] in is implicit, for nonmutating methods.

This can yield unexpected results. Be careful.

struct SomeStruct {
  var someProperty = 1

  func anotherMethod() {
    DispatchQueue.global().asyncAfter(deadline: .now() + 1) {
      print(someProperty)
    }
  }
}

var s = SomeStruct()
s.anotherMethod() // 1, even though it's printed after the following reassignment 
s.someProperty = 2
s.anotherMethod() // 2

I think it helps to think about what method syntax is shorthand for.

s.anotherMethod()

is really

SomeStruct.anotherMethod(s)()

You can visualize the immutability there, because there's no &.