Disables the A.x syntax for fields marked @private when defining a struct.

The only way to access these fields is through the (much less ergonomic) getfield function, or through a getter or accessor method you define.

Also allows privileged access to fields through another macro on method definitions. This lets us use the dot syntax, but only within the scope of that method and only for particular types we specially annotate

# @private denotes the private fields
@private_struct struct Foo{X,Y}
    @private x::X
    y::Y
end

foo1 = Foo(3.0, 4.0)
foo2 = Foo(5, 6)

# 4-colon syntax denotes that only f1 should allow private access to fields
@private_method f(f1::::Foo, f2::Foo) = f1.x + f2.y
g(f1::Foo, f2::Foo) = f1.x + f2.y

f(foo1, foo2) # 9.0

g(foo1, foo2) # ERROR: PrivacyError: Attempted to directly access private field Foo.x outside private context

JuliaLang #12064 discusses privacy of type fields

This Discourse post discusses a similar approach to making private properties, although it uses a keyword argument.