Safety of storing mutable objects in collections?

[madsmtm]

I just happened to read this

Storing mutable objects in collection objects can cause problems. Certain collections can become invalid or even corrupt if objects they contain mutate because, by mutating, these objects can affect the way they are placed in the collection. First, the properties of objects that are keys in hashing collections such as NSDictionary objects or NSSet objects will, if changed, corrupt the collection if the changed properties affect the results of the object’s hash or isEqual: methods. (If the hash method of the objects in the collection does not depend on their internal state, corruption is less likely.) Second, if an object in an ordered collection such as a sorted array has its properties changed, this might affect how the object compares to other objects in the array, thus rendering the ordering invalid.

Which is just... Ugh!

So yeah, we may need to manually define which types are safe to use as hash keys - though it might not be that bad, since you usually just want to use NSString or NSNumber anyways.

[madsmtm]

Something difficult about this is that we want to (unsafely?) allow NSObject/AnyObject as the key, while still having get, iteration and such (because you need access to the elements to be able to later use isKindOfClass: to determine that something was actually a NSString).

Maybe we could just allow NSObject/AnyObject specially?

Or maybe have a kind of "initialization invariant": If you hold a NSDictionary<NSObject, ...>, then you know (?) it was constructed from immutable types, since otherwise it'd be unsound? That at least helps with getting the elements out, though I'm unsure of how to encode that in the type-system?

[madsmtm]

Maybe it's actually just insertion that should be blocked on IsImmutable (or similar), although that may make it difficult to construct property lists? Though then again, they're actually NSDictionary<NSString, NSObject>, i.e. the key is always a string (at least I'm fairly certain).

Linking information about plists:

• Wikipedia
• Apple's Introduction to Property Lists
• About Info.plist Keys and Values

[madsmtm]

I've reverted the original solution for this (adding a trait HasStableHash that hashing collections require on their parameters) in #626, since it's not actually required for soundness, and prevents a lot of real-world use cases for hashing collections.