How to put a future in Pin<Box<>> without .boxed()?

Ploppz · February 28, 2020, 9:11am

I cannot use .boxed() because it triggers errors about "one type is more general than the other".
So I use Box::pin() instead which works fine. However, the reason I use box at all here is that I need to reduce the type size of the future because I hit the compiler limit, so I need to cast it to Box<Pin<dyn ..>> I suppose? Will that help?

But it seems I cannot escape my fate which is "one type is more general than the other": Rust Playground (edit: wrong link)

I have been wrestling with these errors almost a whole week and I'm desperate for a solution. Please help

kornel · February 28, 2020, 3:52pm

I must admit, the error message is utterly confusing. I think it's worth filing a bug about it.

Removing this line makes it compile:

.then(move |x| async move {x})

Ploppz · February 28, 2020, 4:18pm

I know that removing that line solves the problem, and I forgot to say: this is a very reduced test case deriving from a larger application where I actually need the then call, but where I do something more, namely

        .then(move |x| async move {
            delay_for(Duration::from_secs(some_time)).await;
            x
        })

najamelan · February 28, 2020, 4:21pm

There is an issue already:

github.com/rust-lang/rust

Unhelpful error "one type is more general than the other" in async code

opened 06:26AM - 21 Sep 19 UTC

Seeker14491

C-enhancement A-diagnostics T-compiler A-async-await AsyncAwait-Triaged D-confusing D-newcomer-roadblock

The following code does not compile: ```rust // Crate versions: // futures-…preview v0.3.0-alpha.18 // runtime v0.3.0-alpha.7 use futures::{prelude::*, stream::FuturesUnordered}; #[runtime::main] async fn main() { let entries: Vec<_> = vec![()] .into_iter() .map(|x| async { vec![()].into_iter().map(|_| x) }) .collect::<FuturesUnordered<_>>() .map(stream::iter) .flatten() .collect() .await; } ``` It produces this error message: ``` error[E0308]: mismatched types --> src\main.rs:3:1 | 3 | #[runtime::main] | ^^^^^^^^^^^^^^^^ one type is more general than the other | = note: expected type `std::ops::FnOnce<(std::iter::Map<std::vec::IntoIter<()>, [closure@src\main.rs:7:51: 7:56 x:&()]>,)>` found type `std::ops::FnOnce<(std::iter::Map<std::vec::IntoIter<()>, [closure@src\main.rs:7:51: 7:56 x:&()]>,)>` ``` In this case, the code can be made to compile by adding `move` in two places on this line: ```rust // ... .map(|x| async move { vec![()].into_iter().map(move |_| x) }) // ... ``` Some possibly related issues: #57362, #60658 **rustc version:** `1.39.0-nightly (97e58c0d3 2019-09-20)`

Some threads from the user forum:

It seems it's sometimes related to lifetimes of the closure. Sometimes you can work around that with a dedicated function on which you can set for<'a>, although nobody has posted an example of that I think.

In the last thread it seems to be related to Send.

system · May 28, 2020, 4:21pm

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How to put a future in Pin<Box<>> without .boxed()?

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