[DougBTX]

Inside a constructor you can access a partially initialised "this" value, and even call methods on it, which leads to rules like: "Do not call overridable methods in constructors"[0], as they can lead to surprising, non-local, bugs.

Rust has functions associated with types which are conventionally used like constructors, but critically the new objects must have all their fields provided all at once, so it is impossible to observe a partially initialised object.

[0] https://learn.microsoft.com/en-us/dotnet/fundamentals/code-a...

[titzer]

Virgil solved this a little differently. The initialization expressions for fields (outside of constructors) as well as implicit assignment of constructor parameters to fields happens before super constructor calls. Such initialization expressions cannot reference "this"--"this" is only available in _constructor bodies_. Initializing fields before calling super and then the chaining of super calls guarantees the whole chain of super constructor calls will finish before entering the body of a constructor, and all fields will be initialized. Thus by construction, virtual methods invoked on "this" won't see uninitialized fields.

https://github.com/titzer/virgil/blob/master/doc/tutorial/Cl...

[zozbot234]

You can most likely use session types to soundly observe a partially initialized MaybeUninit<MyObject> in Rust. The proper use of session types could ensure that the object is only assumed to be initialized after every field of it has been written to, and that no uninitialized fields are ever accessed in an unsound way. The issue though is that this is not automated in any way, it requires you to write custom code for each case of partial initialization you might be dealing with.