DIP Proposal: @manualScoped to prevent automatic field destruction

[FeepingCreature]

On Friday, 27 July 2018 at 11:44:10 UTC, aliak wrote:
> A) I'd suggest "@nodestruct" instead, since it sounds like that 
> what it's supposed to do?
Yes-ish, but it's also supposed to fill the hole in the 
typesystem created by T.init, and "you can only assign T.init to 
types marked @nodestruct" sounds kind of magic.

> B) is this basically for the case of invariants being run 
> before destructors where T.init are not valid runtime instances 
> of T?
Yep, that's the reason I'm looking at it.

> C) If it is, then this seems to me that this is something that 
> should just work without a programmer needing to know about how 
> T.init and invariants are implemented, so an implementation 
> that doesn't call invariants before a destructor only if an 
> instance was never constructed at runtime is maybe the way to 
> go? Though I have no idea how possible that is.
>
Basically impossible without giving every type a hidden "bool 
initialized" field. So, basically impossible.
The advantage of doing it with @manualScoped is twofold. First, 
it also covers the case of opAssign methods taking parameters 
that don't need to be destroyed at scope end. Second, even a 
constructor may return a T.init (and, for instance, increment a 
static variable), so if being T.init skipped the destructor we'd 
again get a constructor/destructor mismatch. @manualScoped makes 
it clear this is a variable that contains a value with a manually 
managed lifetime, so users take responsibility to call 
moveEmplace/destroy as required to make constructor/destructor 
calls match up, which is the goal.

Basically, think of a @manualScoped variable as a "weak value" in 
analogy to weak references.

> Cheers,
> - Ali
>
> maybe your PR where invariants is not called before a 
> destructor if an instance is a T.init is maybe the way to go? 
> [0]

The PR itself was just a way to hack around this. The problem 
isn't the invariant check on destruction, the problem is the 
destruction without the matching construction.