Lazy Array

LazyArray is a data structure that contains a fixed number of entries that are indexed by a usize from 0 up to 1 before its size. All entries start by being undefined, that means that trying to get them will return None. Once for each entry, they can be set and once set they can never change.

Example Use Case

I had the idea of this library as I was parsing a binary format that contains a sequence of record that may reference each other to form an acyclic graph. The problem is that I can't a priori know in which order to load the objects as any object can reference objects before and after itself. With LazyArray I can lazily load objects as I discover dependencies between them and store them so they are only loaded once.

Why I Think It Might Be Safe

LazyArray is implemented by storing a Vec inside an UnsafeCell. I think it is fine for the following reasons.

• The number of entries is allocated as once when it is created, which means that the storage of entries should never move.
• If an entry is undefined, it is impossible to reference the None from outside.
• If an entry is defined, there may exist references to it, but its value can never change.
• It is not Sync.