Design rationale

I always hear that intrusive collections can‘t be done in Rust (either "can’t be done safely" or “can't be done as performantly as C++”). Can you elaborate on this?

I would say that it is a combination of both “can't be done safely” and “can't be done as performantly as C++”. You should keep in mind that C++ libraries such as Boost.Intrusive don't exactly have a safe API and instead place a bunch of requirements on the user regarding object lifetimes.

There are several factor that a Rust implementation of intrusive collections must take into account:

  1. Elements in a collection can‘t have a pointer back to the collection itself since these would become invalid if the collection is moved. C++ doesn’t suffer from this limitation because it can adjust these pointers in the move constructor.

    (Not that I‘m complaining about Rust’s lack of move constructors. Having to deal with move constructors introduces a lot of complexity for collection code due to exception safety issues.)

    This basically means that we have to use NULL-terminated linked lists instead of circular linked lists. It's very slightly slower (depending on who you ask), but generally not a big deal. This same restriction also applies to normal collection types in Rust.

  2. Rust doesn't have any equivalent to C++'s “pointer to member” type to tell a collection type which struct field to use for the intrusive Link (basically the prev/next pointers in a linked list). Instead, we use an unsafe Adapter trait which describes how to convert between a *const T and *const Link.

    The intrusive_adapter! macro provides a safe and easy way of creating and adapter type. It uses offset_of! and container_of! internally in a way that is guaranteed to be safe. This macro only supports the case where the link is a direct field of the object type; more complicated scenarios (such as nested fields) require a manual implementation of Adapter.

  3. You can't safely give out &mut references to elements in a collection due to the aliasing rules (an object may be part of multiple intrusive collections at once) and because it allows the user to “break” the links in a collection with code like mem::replace(&mut, Link::new()).

    Since we can only give out & references to collection elements, any mutability must be done through Cell / RefCell / UnsafeCell.

  4. Objects must not be moved or destroyed while they are still linked into an intrusive collection. While this basically sounds like having the collection take a shared borrow of an object when inserting it, lifetimes aren't ideal for this since they effectively restrict the lifetime of the collection to a single function.

    The basic approach that intrusive-collections takes is that a collection takes ownership of an object, usually in the form of an owned pointer (Box<T>, Rc<T>, etc), and then returns the owned pointer back to the user when the object is removed from the collection. The owned pointer type is a generic parameter of the collection (through the Adapter trait).

    The case of Box<T> is the simplest, since the collection becomes the sole owner of the object. Rc<T> is more interesting since the ownership is shared. This enables inserting an object into multiple intrusive collections (with multiple Links in the object) without needing to worry about the object moving or dropping while still in use.

    intrusive-collection also supports using &'a T as the owned pointer type. This allows shared ownership (you can keep accessing variables that have been inserted in the collection) without the need for reference counting. Safety is ensure by restricting the lifetime of the collection such that it does not outlive any objects that have been inserted into it. As a bonus, this also works with arenas which give out &'a T references to allocated objects.

    Finally, if you really need to avoid the overhead of Rc, the crate also provides an UnsafeRef type which acts like an Rc but without the reference count: you are expected to free the object yourself once you are sure it is no longer in use.

So basically, intrusive-collections is almost as performant as C++ intrusive collections. If you want every last bit of performance, you can use UnsafeRef instead of Rc for the owned pointer and UnsafeCell instead of RefCell for the object data. However I would expect the gains to be relatively minor compared to the added unsafety.