`Borrow` and `Inout` types for safe, first-class references

[Alejandro (Alejandro Alonso)]

@OneSadCookie

If you start with these types, then then you don't need the accessors; you can just use get/mutating get accessors that return Borrow/Inout references. Obviously at that point they can't have value properties, but then the language can offer to transparently dereference these types, which will result in more ergonomic syntax.

While this is true, you will very quickly realize you run into ambiguity issues if you didn’t have the accessors:

struct Array<Element: ~Copyable>: ~Copyable {
  subscript(i: Int) -> Borrow<Element> {
    get { ... }
  }

  subscript(I: Int) -> Inout<Element> {
    mutating get { ... }
  }
}

let a = Array<Atomic<Int>>(...)
let b = a[0] // error: ambiguous use of subscript what type is 'B'?

Of course some languages have this problem solved, but our language’s type checker does not work like that. The borrow/mutate accessors help alleviate this issue.