RustFuture: Clarify Send bound and switch to RawWaker #2574
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bendk
commented
Jun 19, 2025
| // | ||
| // In addition, `WrappedFuture::result` can be a raw pointer which is `!Send`. | ||
| // This `!Send` bound is because Rust can't uphold safety guarantees for raw pointers. | ||
| // However, this code does in fact treat the pointers correctly, for example we only return them across the FFI once. |
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@jhugman Does this seem correct to you?
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// If the `wasm-unstable-single-threaded` feature is enabled, then the wrapped future may not be `Send`, and thus `WrappedFuture` is not Send either.
// This seems okay for now, since if `wasm-unstable-single-threaded` then futures should stay on one thread.
This seems correct.
I'm not sure I can comment on the rest of the comment though.
Updated the `unsafe impl `Send` for WrappedFuture` docstring. I believe the new docs describe the situation more accurately. Moved back to using `RawWaker` because I want to drop the need for WrappedFuture to be `Send`. `WrappedFuture` will not always be `Send` when `wasm-unstable-single-threaded` is enabled. The current waker creation depends on `RustFuture` being `Send` which depends on `WrappedFuture` being `Send` (https://doc.rust-lang.org/std/task/struct.Waker.html#impl-From%3CArc%3CW%3E%3E-for-Waker). As a side bonus, I think the tracing printouts will improve.
mhammond
approved these changes
Jun 22, 2025
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Updated the
unsafe implSendfor WrappedFuturedocstring. I believe the new docs describe the situation more accurately.Moved back to using
RawWakerbecause I want to drop the need for WrappedFuture to beSend.WrappedFuturewill not always beSendwhenwasm-unstable-single-threadedis enabled. The current waker creation depends onRustFuturebeingSendwhich depends onWrappedFuturebeingSend(https://doc.rust-lang.org/std/task/struct.Waker.html#impl-From%3CArc%3CW%3E%3E-for-Waker).As a side bonus, I think the tracing printouts will improve.