Simplify dropck remark on PhantomData

[workingjubilee]

Fixes #125540

This is more similar to the original draft but there was a recommended change to the current form, along with injecting the caveat above it. I don't understand why, I found the previous phrasing more clear: "this indicates (thing) but (thing) matters only rarely".

r? @lcnr

[Noratrieb]

It still mentions the rare circumstances, so I don't think this fixes the issue. I don't like that sentence either. It should either be dropped, or explicitly mention those rare circumstances ("using the unstable may_dangle feature").

As long as we mention rare circumstances, we leave the reader puzzled what those circumstances may be.

[workingjubilee]

Hmm. My understanding is that the "rare circumstances" ambiguity comes about because the teams are hedging so they are allowed to change it.

My draft seems more acceptable to me than the status quo because it's unambiguous as to whether and when you should use PhantomData, it just is sly about whether it has any given effect.

[lcnr]

cc #103413 (comment)

"Currently, adding a field of type PhantomData<T> indicates that your type owns data of type T in very rare circumstances" is intentional. Adding PhantomData<T> to your type only impacts whether your type 'owns' T in rare circumstances: if your type already has another field which requires drop:

struct DoesNotOwnT<T> {
   field: *const T,
   phantom_data: PhantomData<T>,
}

struct DoesOwnT<T> {
   field: *const T,
   other_field: String,
   phantom_data: PhantomData<T>,
}

[lcnr]

I've opened an RFC to fix this behavior rust-lang/rfcs#3417, but I currently don't have the capacity to push this over the finish line. See the examples in that RFC for more details

[workingjubilee]

oic, thank you for explaining!

hmm... the definition provided here still feels abstruse, but I understand why you phrased it that way now.

[lcnr]

yeah 🤔 I think for this to make sense you'd have to actually go into the difference between needs_drop and dropck_outlives (PhantomData is only considered for dropck_outlives, as there shouldn't be any types for which needs_drop and Copy both hold)

Ideally the way we make this more understandable is by just merging rust-lang/rfcs#3417, though I guess we can also go more in-depth in the comment if you think that's more achievable.

[workingjubilee]

Hmm, I currently do not have an idea of how to proceed!