4 ms·

I don't see why it may fail to integrate. Declare Pin as !Move and... thats all? I mean, there will be issues, edge-cases because it is just how these things ha

• Georgelemental 2mo ago
  Pin applies to the pointer, !Move applies to the pointee.
  • ordu 2mo ago
    So... Pin should be defined as Pin<T: !Move>?
    • mcherm 2mo ago
      No, the point of Pin is to wrap types that CAN move. If the type were !Move then Pin wouldn't be needed.
      • simonask 2mo ago
        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.
        • stymaar 2mo ago
          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 https://doc.rust-lang.org/std/pin/index.html#unpin).
          • simonask 2mo ago
            Right, of course. Yeah, it's an API riddled with so many double negatives it's hard to keep track.
      • Dagonfly 2mo ago
        > 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.
        • Mond_ 2mo ago
          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?
          • Dagonfly 2mo ago
            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.
            • yoshuaw 2mo ago
              You're right that this is an important issue. I wrote a post explaining this in some detail, so at the very least we avoid having the same problem with generator functions: https://blog.yoshuawuyts.com/gen-auto-trait-problem https://blog.yoshuawuyts.com/gen-auto-trait-problem
              • stymaar 2mo ago
                Thanks for the explanation (though I have to admit I liked your old blog theme more). Since it's just a matter of how async get desugared, can it be changed through an edition?
• mahirsaid 2mo ago
  Give it some time and see how people will use it and problems emerging. I do feel that this will stay.
• afdbcreid 2mo ago
  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).