> But with a C++-like constructor, it starts with a struct where all fields are in an invalid state, and then the struct's invariants are slowly established field by field.
AIUI, that's why MaybeUninit<T> exists. But even if you address the issue of it being unsafe to assert that a MaybeUninit has been initialized (which &out references could in principle solve) there are real problems with this; for example, MaybeUninit<T> has no niches or free-for-use padding even when T does, so you can't just "project" MaybeUninit to individual fields except in special cases. My understanding is that C++ partial initialization has the exact same issues in principle, they just don't come up as often because the standard for code correctness is a lot less rigorous.
AIUI, that's why MaybeUninit<T> exists. But even if you address the issue of it being unsafe to assert that a MaybeUninit has been initialized (which &out references could in principle solve) there are real problems with this; for example, MaybeUninit<T> has no niches or free-for-use padding even when T does, so you can't just "project" MaybeUninit to individual fields except in special cases. My understanding is that C++ partial initialization has the exact same issues in principle, they just don't come up as often because the standard for code correctness is a lot less rigorous.