So... I've kinda considered this problem many times, #265 contains some of the previous discussion and my head contains even more, but I feel like we're still not at an optimum.

Currently, objc2 contains a lot of complex machinery to make classes able to opt-in to being &mut, and icrate uses this on NSMutableString, NSMutableAttributedString, NSMutableArray<T>, NSMutableSet<T>, NSMutableDictionary<K, V> and NSEnumerator<T> to make accessing the inner state of those types safe.

A few examples of current issues in icrate with this approach:

• NSMutableAttributedString::mutableString must return a reference bound to the attributed string, as otherwise you'd be able to create two Id<NSMutableString> to the same string. But this doesn't work with Id<T>, so we'd need a lifetime annotation in there somehow (or maybe resort to autorelease pools).
• NSTreeNode::mutableChildNodes has to remain unsafe, as we have no way of preventing calling that multiple times. Or maybe NSTreeNode also has to be mutable? But that in turn would push NSTreeController to being mutable, which in turn needs us to change NSEditor.
• While NSThread::threadDictionary has to remain unsafe because of thread-safety concerns, it cannot even be used from the owning thread safely.
• In user code, storing one of these mutable objects in a custom object needs to be done using a RefCell.

So while &mut does work for the select few types we've introduced it on, my main problem is that this is not enough! &mut is infectious, and we need to introduce it consistently, everywhere!

And that, in turn, is just not feasible (time-wise, Apple has a lot of API surface)! Especially so with our goal of making header-translator a user-accessible tool, which means user-controlled code, and in turn means users would also have to add extra configuration to handle this for their mutable types, which is a tough ask.

Honestly, maybe we should abandon this endeavour, and instead go down the beaten path Swift has taken; do not try to do anything about mutability, i.e. mark NSString, NSMutableString and so on as InteriorMutable, and have all methods accept &self?

All of this is not to say that &mut doesn't have advantages; it does, very much so! And removing it does have several performance-related disadvantages, namely:

• NSString::as_str would have to be unsafe, since the internal storage for the string could now be changed through &self. This has a perf-impact on the impl Display for NSString, as the &mut Formatter<'_> in there could contain a &NSMutableString internally that pointed to the same string, and hence we'll probably need some sort of allocation.
• NSData::bytes would have to be unsafe, which in turn means that the nice impls of Deref, Index, Write and so would be gone.
• Other methods using NS_RETURNS_INNER_POINTER would have to be unsafe.
• Getters on collections would have to retain the object (this is also what they do in Swift), whereas before we could avoid this since they're guaranteed to point into internal storage, and we knew we weren't changing that storage.
• Iterators would have to retain the element being iterated over, to avoid problems if the iterator is invalidated - see here for an example of the issue.
  • Even then, are they sound? Is the fast iterator mutation detection enough? What about if the iterator is passed in and out of an autorelease pool?
  • Will have to be researched (and fuzzed), but a safe (inefficient) fallback is always to just use array.objectAtIndex(i).
• NSString, NSMutableString, NSArray, and so on would loose their Send + Sync-ness.

Some of these performance implications could be resolved by making extra unsafe methods with a safety precondition that the object is not mutated for the lifetime of the return value, although that is still a worse user-experience.