Implementation of Send is missing when calling FutureExt::boxed() - async-await

Question

I'm experimenting with futures with WASM, using wasm-bindgen-futures and the rust-webpack-template; the full working code and problem code is available.
My experiment is to call an async fn run(), wrapped by a fn run_js() -> js_sys::Promise, from JavaScript. The following is working:
pub async fn run() -> Result<(), JsValue> {
Ok(())
}
// Called by our JS entry point to run the example.
#[wasm_bindgen(js_name = run)]
pub fn run_js() -> js_sys::Promise {
use crate::compat::future_to_promise;
use futures::future::FutureExt;
future_to_promise(async move {
run().await?;
Ok(JsValue::UNDEFINED)
}.boxed())
}
The next step was to add a sleep function and call it from run():
// in run:
sleep(500).await?;
pub async fn sleep(millis: i32) -> Result<(), JsValue> {
use crate::compat::promise_to_future;
let promise = js_sys::Promise::new(&mut move |resolve, _| {
let window = web_sys::window().expect("should have a Window");
window.set_timeout_with_callback_and_timeout_and_arguments_0(
&resolve, millis
).expect("don't expect error on setTimeout()");
});
promise_to_future(promise).await?;
Ok(())
}
crate::compat converts Future 0.3 -> Future 0.1 -> Promise, and back. For completeness, here is promise_to_future:
pub fn promise_to_future(promise: Promise) -> impl Future<Output=Result<JsValue, JsValue>> {
// promise to 0.1
let future01 = JsFuture::from(promise);
// 0.1 to 0.3
Compat01As03::new(future01)
}
Adding this, I get a compilation error here:
error[E0277]: `*mut u8` cannot be sent between threads safely
--> src/lib.rs:45:7
|
42 | future_to_promise(async move {
43 | run().await?;
44 | Ok(JsValue::UNDEFINED)
45 | }.boxed())
| ^^^^^ `*mut u8` cannot be sent between threads safely
|
= help: within `impl core::future::future::Future`, the trait `std::marker::Send` is not implemented for `*mut u8`
error[E0277]: `(dyn std::ops::FnMut(wasm_bindgen::JsValue) + 'static)` cannot be sent between threads safely
--> src/lib.rs:45:7
|
42 | future_to_promise(async move {
43 | run().await?;
44 | Ok(JsValue::UNDEFINED)
45 | }.boxed())
| ^^^^^ `(dyn std::ops::FnMut(wasm_bindgen::JsValue) + 'static)` cannot be sent between threads safely
|
= help: the trait `std::marker::Send` is not implemented for `(dyn std::ops::FnMut(wasm_bindgen::JsValue) + 'static)`
(more source code context added, and abridged; full version below)
I can't make out what the error is; something here is not Send, but that's about as much as I understand from this.
Shouldn't this be possible in principle? Or how should I write my sleep function to work correctly?
Full Compiler output:
Compiling rust-webpack v0.1.0 (/data/Documents/Programmieren/rust/hello-web/crate)
error[E0277]: `*mut u8` cannot be sent between threads safely
--> src/lib.rs:45:7
|
45 | }.boxed())
| ^^^^^ `*mut u8` cannot be sent between threads safely
|
= help: within `impl core::future::future::Future`, the trait `std::marker::Send` is not implemented for `*mut u8`
= note: required because it appears within the type `std::marker::PhantomData<*mut u8>`
= note: required because it appears within the type `wasm_bindgen::JsValue`
= note: required because it appears within the type `js_sys::Object`
= note: required because it appears within the type `js_sys::Promise`
= note: required because it appears within the type `{i32, js_sys::Promise, fn(std::result::Result<wasm_bindgen::JsValue, wasm_bindgen::JsValue>) -> std::result::Result<<std::result::Result<wasm_bindgen::JsValue, wasm_bindgen::JsValue> as std::ops::Try>::Ok, <std::result::Result<wasm_bindgen::JsValue, wasm_bindgen::JsValue> as std::ops::Try>::Error> {<std::result::Result<wasm_bindgen::JsValue, wasm_bindgen::JsValue> as std::ops::Try>::into_result}, impl core::future::future::Future, ()}`
= note: required because it appears within the type `[static generator#src/sleep.rs:4:56: 16:2 millis:i32 {i32, js_sys::Promise, fn(std::result::Result<wasm_bindgen::JsValue, wasm_bindgen::JsValue>) -> std::result::Result<<std::result::Result<wasm_bindgen::JsValue, wasm_bindgen::JsValue> as std::ops::Try>::Ok, <std::result::Result<wasm_bindgen::JsValue, wasm_bindgen::JsValue> as std::ops::Try>::Error> {<std::result::Result<wasm_bindgen::JsValue, wasm_bindgen::JsValue> as std::ops::Try>::into_result}, impl core::future::future::Future, ()}]`
= note: required because it appears within the type `std::future::GenFuture<[static generator#src/sleep.rs:4:56: 16:2 millis:i32 {i32, js_sys::Promise, fn(std::result::Result<wasm_bindgen::JsValue, wasm_bindgen::JsValue>) -> std::result::Result<<std::result::Result<wasm_bindgen::JsValue, wasm_bindgen::JsValue> as std::ops::Try>::Ok, <std::result::Result<wasm_bindgen::JsValue, wasm_bindgen::JsValue> as std::ops::Try>::Error> {<std::result::Result<wasm_bindgen::JsValue, wasm_bindgen::JsValue> as std::ops::Try>::into_result}, impl core::future::future::Future, ()}]>`
= note: required because it appears within the type `impl core::future::future::Future`
= note: required because it appears within the type `impl core::future::future::Future`
= note: required because it appears within the type `{fn(std::result::Result<(), wasm_bindgen::JsValue>) -> std::result::Result<<std::result::Result<(), wasm_bindgen::JsValue> as std::ops::Try>::Ok, <std::result::Result<(), wasm_bindgen::JsValue> as std::ops::Try>::Error> {<std::result::Result<(), wasm_bindgen::JsValue> as std::ops::Try>::into_result}, impl core::future::future::Future, ()}`
= note: required because it appears within the type `[static generator#src/lib.rs:16:43: 34:2 {fn(std::result::Result<(), wasm_bindgen::JsValue>) -> std::result::Result<<std::result::Result<(), wasm_bindgen::JsValue> as std::ops::Try>::Ok, <std::result::Result<(), wasm_bindgen::JsValue> as std::ops::Try>::Error> {<std::result::Result<(), wasm_bindgen::JsValue> as std::ops::Try>::into_result}, impl core::future::future::Future, ()}]`
= note: required because it appears within the type `std::future::GenFuture<[static generator#src/lib.rs:16:43: 34:2 {fn(std::result::Result<(), wasm_bindgen::JsValue>) -> std::result::Result<<std::result::Result<(), wasm_bindgen::JsValue> as std::ops::Try>::Ok, <std::result::Result<(), wasm_bindgen::JsValue> as std::ops::Try>::Error> {<std::result::Result<(), wasm_bindgen::JsValue> as std::ops::Try>::into_result}, impl core::future::future::Future, ()}]>`
= note: required because it appears within the type `impl core::future::future::Future`
= note: required because it appears within the type `impl core::future::future::Future`
= note: required because it appears within the type `{fn(std::result::Result<(), wasm_bindgen::JsValue>) -> std::result::Result<<std::result::Result<(), wasm_bindgen::JsValue> as std::ops::Try>::Ok, <std::result::Result<(), wasm_bindgen::JsValue> as std::ops::Try>::Error> {<std::result::Result<(), wasm_bindgen::JsValue> as std::ops::Try>::into_result}, impl core::future::future::Future, ()}`
= note: required because it appears within the type `[static generator#src/lib.rs:42:34: 45:6 {fn(std::result::Result<(), wasm_bindgen::JsValue>) -> std::result::Result<<std::result::Result<(), wasm_bindgen::JsValue> as std::ops::Try>::Ok, <std::result::Result<(), wasm_bindgen::JsValue> as std::ops::Try>::Error> {<std::result::Result<(), wasm_bindgen::JsValue> as std::ops::Try>::into_result}, impl core::future::future::Future, ()}]`
= note: required because it appears within the type `std::future::GenFuture<[static generator#src/lib.rs:42:34: 45:6 {fn(std::result::Result<(), wasm_bindgen::JsValue>) -> std::result::Result<<std::result::Result<(), wasm_bindgen::JsValue> as std::ops::Try>::Ok, <std::result::Result<(), wasm_bindgen::JsValue> as std::ops::Try>::Error> {<std::result::Result<(), wasm_bindgen::JsValue> as std::ops::Try>::into_result}, impl core::future::future::Future, ()}]>`
= note: required because it appears within the type `impl core::future::future::Future`
error[E0277]: `(dyn std::ops::FnMut(wasm_bindgen::JsValue) + 'static)` cannot be sent between threads safely
--> src/lib.rs:45:7
|
45 | }.boxed())
| ^^^^^ `(dyn std::ops::FnMut(wasm_bindgen::JsValue) + 'static)` cannot be sent between threads safely
|
= help: the trait `std::marker::Send` is not implemented for `(dyn std::ops::FnMut(wasm_bindgen::JsValue) + 'static)`
= note: required because of the requirements on the impl of `std::marker::Send` for `std::ptr::Unique<(dyn std::ops::FnMut(wasm_bindgen::JsValue) + 'static)>`
= note: required because it appears within the type `std::boxed::Box<(dyn std::ops::FnMut(wasm_bindgen::JsValue) + 'static)>`
= note: required because it appears within the type `std::mem::ManuallyDrop<std::boxed::Box<(dyn std::ops::FnMut(wasm_bindgen::JsValue) + 'static)>>`
= note: required because it appears within the type `wasm_bindgen::closure::Closure<(dyn std::ops::FnMut(wasm_bindgen::JsValue) + 'static)>`
= note: required because it appears within the type `(wasm_bindgen::closure::Closure<(dyn std::ops::FnMut(wasm_bindgen::JsValue) + 'static)>, wasm_bindgen::closure::Closure<(dyn std::ops::FnMut(wasm_bindgen::JsValue) + 'static)>)`
= note: required because it appears within the type `std::option::Option<(wasm_bindgen::closure::Closure<(dyn std::ops::FnMut(wasm_bindgen::JsValue) + 'static)>, wasm_bindgen::closure::Closure<(dyn std::ops::FnMut(wasm_bindgen::JsValue) + 'static)>)>`
= note: required because it appears within the type `wasm_bindgen_futures::JsFuture`
= note: required because it appears within the type `futures::task_impl::Spawn<wasm_bindgen_futures::JsFuture>`
= note: required because it appears within the type `futures_util::compat::compat01as03::Compat01As03<wasm_bindgen_futures::JsFuture>`
= note: required because it appears within the type `impl core::future::future::Future`
= note: required because it appears within the type `{i32, js_sys::Promise, fn(std::result::Result<wasm_bindgen::JsValue, wasm_bindgen::JsValue>) -> std::result::Result<<std::result::Result<wasm_bindgen::JsValue, wasm_bindgen::JsValue> as std::ops::Try>::Ok, <std::result::Result<wasm_bindgen::JsValue, wasm_bindgen::JsValue> as std::ops::Try>::Error> {<std::result::Result<wasm_bindgen::JsValue, wasm_bindgen::JsValue> as std::ops::Try>::into_result}, impl core::future::future::Future, ()}`
= note: required because it appears within the type `[static generator#src/sleep.rs:4:56: 16:2 millis:i32 {i32, js_sys::Promise, fn(std::result::Result<wasm_bindgen::JsValue, wasm_bindgen::JsValue>) -> std::result::Result<<std::result::Result<wasm_bindgen::JsValue, wasm_bindgen::JsValue> as std::ops::Try>::Ok, <std::result::Result<wasm_bindgen::JsValue, wasm_bindgen::JsValue> as std::ops::Try>::Error> {<std::result::Result<wasm_bindgen::JsValue, wasm_bindgen::JsValue> as std::ops::Try>::into_result}, impl core::future::future::Future, ()}]`
= note: required because it appears within the type `std::future::GenFuture<[static generator#src/sleep.rs:4:56: 16:2 millis:i32 {i32, js_sys::Promise, fn(std::result::Result<wasm_bindgen::JsValue, wasm_bindgen::JsValue>) -> std::result::Result<<std::result::Result<wasm_bindgen::JsValue, wasm_bindgen::JsValue> as std::ops::Try>::Ok, <std::result::Result<wasm_bindgen::JsValue, wasm_bindgen::JsValue> as std::ops::Try>::Error> {<std::result::Result<wasm_bindgen::JsValue, wasm_bindgen::JsValue> as std::ops::Try>::into_result}, impl core::future::future::Future, ()}]>`
= note: required because it appears within the type `impl core::future::future::Future`
= note: required because it appears within the type `impl core::future::future::Future`
= note: required because it appears within the type `{fn(std::result::Result<(), wasm_bindgen::JsValue>) -> std::result::Result<<std::result::Result<(), wasm_bindgen::JsValue> as std::ops::Try>::Ok, <std::result::Result<(), wasm_bindgen::JsValue> as std::ops::Try>::Error> {<std::result::Result<(), wasm_bindgen::JsValue> as std::ops::Try>::into_result}, impl core::future::future::Future, ()}`
= note: required because it appears within the type `[static generator#src/lib.rs:16:43: 34:2 {fn(std::result::Result<(), wasm_bindgen::JsValue>) -> std::result::Result<<std::result::Result<(), wasm_bindgen::JsValue> as std::ops::Try>::Ok, <std::result::Result<(), wasm_bindgen::JsValue> as std::ops::Try>::Error> {<std::result::Result<(), wasm_bindgen::JsValue> as std::ops::Try>::into_result}, impl core::future::future::Future, ()}]`
= note: required because it appears within the type `std::future::GenFuture<[static generator#src/lib.rs:16:43: 34:2 {fn(std::result::Result<(), wasm_bindgen::JsValue>) -> std::result::Result<<std::result::Result<(), wasm_bindgen::JsValue> as std::ops::Try>::Ok, <std::result::Result<(), wasm_bindgen::JsValue> as std::ops::Try>::Error> {<std::result::Result<(), wasm_bindgen::JsValue> as std::ops::Try>::into_result}, impl core::future::future::Future, ()}]>`
= note: required because it appears within the type `impl core::future::future::Future`
= note: required because it appears within the type `impl core::future::future::Future`
= note: required because it appears within the type `{fn(std::result::Result<(), wasm_bindgen::JsValue>) -> std::result::Result<<std::result::Result<(), wasm_bindgen::JsValue> as std::ops::Try>::Ok, <std::result::Result<(), wasm_bindgen::JsValue> as std::ops::Try>::Error> {<std::result::Result<(), wasm_bindgen::JsValue> as std::ops::Try>::into_result}, impl core::future::future::Future, ()}`
= note: required because it appears within the type `[static generator#src/lib.rs:42:34: 45:6 {fn(std::result::Result<(), wasm_bindgen::JsValue>) -> std::result::Result<<std::result::Result<(), wasm_bindgen::JsValue> as std::ops::Try>::Ok, <std::result::Result<(), wasm_bindgen::JsValue> as std::ops::Try>::Error> {<std::result::Result<(), wasm_bindgen::JsValue> as std::ops::Try>::into_result}, impl core::future::future::Future, ()}]`
= note: required because it appears within the type `std::future::GenFuture<[static generator#src/lib.rs:42:34: 45:6 {fn(std::result::Result<(), wasm_bindgen::JsValue>) -> std::result::Result<<std::result::Result<(), wasm_bindgen::JsValue> as std::ops::Try>::Ok, <std::result::Result<(), wasm_bindgen::JsValue> as std::ops::Try>::Error> {<std::result::Result<(), wasm_bindgen::JsValue> as std::ops::Try>::into_result}, impl core::future::future::Future, ()}]>`
= note: required because it appears within the type `impl core::future::future::Future`

Answer 1

The .boxed() extension function requires the Future to be Send since a recent change in futures. Apparently your future doesn't fulfill that constraint - likely because JS futures are only valid on the main JS thread.
If .boxed() happens inside the scope of your library you can use Box::pin(future) instead of future.boxed() in order to get a type erased boxed future without the Send requirement.