Sendable type cannot be sent between threads safely

[vklachkov]

I tried this code:

use std::rc::Rc;

struct Unsendable(Rc<()>);

struct Sendable<T>(T);

unsafe impl<T> Send for Sendable<T> {}

impl<T> Sendable<T> {
    #[inline(always)]
    fn into_inner(self) -> T {
        self.0
    }
}

fn main() {
    tokio::task::spawn({
        let foo = Unsendable(Rc::new(()));
        let foo = Sendable(foo);
        async move {
            let Sendable(foo) = foo;
            // let foo = foo.into_inner();
        }
    });
}

If I replace line let Sendable(foo) = foo; with let foo = foo.into_inner(); the error disappears.

I create bigger example, without tokio. Rust Playground

I expected to see successful compilation

Instead I got error[E0277]: Rc<()> cannot be sent between threads safely

[theemathas]

This compiles in edition 2018, therefore it's probably related to https://doc.rust-lang.org/edition-guide/rust-2021/disjoint-capture-in-closures.html

Adding a "dummy let" as described there also makes the code compile in edition 2021.

[vklachkov]

Oh, that's right, I forgot that the 2021 edition changed the rules for capturing in closures. Thanks a lot!

Then it seems that some diagnostics are missing for this, if self is Send, but self.0 is not