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by ordu10 hours ago |
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upvoteby afdbcreid1 hours ago|
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Like others said, Pin is incompatible with these semantics. Some people argue, though, that we need both (basically because pinned types can be moved before being pinned).
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upvoteby Georgelemental9 hours ago|
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Pin applies to the pointer, !Move applies to the pointee.
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upvoteby ordu9 hours ago|
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So... Pin should be defined as Pin<T: !Move>?
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upvoteby mcherm8 hours ago|
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No, the point of Pin is to wrap types that CAN move. If the type were !Move then Pin wouldn't be needed.
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upvoteby Dagonfly7 hours ago|
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> the point of Pin is to wrap types that CAN move.

I would highlight that there are many cases where you CAN move an object safely until a certain operation requires the object to "stay put" in place.

Pin allows for that by tying the object to the place only when required. That's why Pin relates to both the object and the place.

Meanwhile, !Move types can't ever move. The object has to remain in the inital place it was constructed in. !Move requires in-place construction and emplacement to be ergonomic at all.

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upvoteby 10000truths2 hours ago|
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> I would highlight that there are many cases where you CAN move an object safely until a certain operation requires the object to "stay put" in place.

You could model this with a state machine enum where the "stay put" phase is a variant that accepts a !Move, like so:

  enum StateMachine<PinnedState: ?Move> {
      InitialState,
      State1(String),
      State2(Box<PinnedState>),
      TerminalState,
  }
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upvoteby Mond_3 hours ago|
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Stupid question, can't this trivially be solved by having a movable constructor / builder type that then gets turned into a non-movable type when built?
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upvoteby Dagonfly2 hours ago|
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Sure, thats one reason why IntoFuture and Future exist. Imo, in hindsight this is also main mistake in aysnc Rust: The whole async system should be build around IntoFuture rather than Future (async fn should return impl IntoFuture).

That way you could pass around IntoFutures without being affected by auto traits leaking. Only when you actually call .await() or .poll() would the immovable Future materialize.

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upvoteby simonask8 hours ago|
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I guess `!Move` is largely equivalent to `Unpin` for the purposes of `Pin`, so for example Pin's safe constructor `Pin::new()` can be re-expressed in terms of `!Move` instead of `Unpin`. Today you need unsafe code to pin a `!Unpin` (i.e. "movable") type.

But I also suspect there are important differences between `!Move` and `Unpin` that I'm not sure about.

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upvoteby stymaar2 hours ago|
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This whole thread summarizes what's wrong with Pin: it's so confusing everyone in here got something wrong. (Just to address your mistake in particular Unpin is almost the opposite of !Move: it's the trait that represents things that can be un-pinned, that is: moved despite having been pinned. See https://doc.rust-lang.org/std/pin/index.html#unpin).
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upvoteby mahirsaid3 hours ago|
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Give it some time and see how people will use it and problems emerging. I do feel that this will stay.
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