Future is not 'Sync' as this value is used across an await

I came across this issue a few days back and I can't seem to figure out what the cause of it is in the first place let alone how to deal with it.

I'm invoking an async method that in itself calls another async method of a struct (async-trait constrained method). It says 'future' is not Sync, and I tried with every possible dyn Trait + Send + Sync, unsafe impl Send/Sync bandage wherever possible to no avail. Could somebody give me a clue as to where this issue is coming from?

EDIT That driver method is type constrained inside

type Handler = Box<dyn Fn() -> Pin<Box<dyn Future<Output = Response<Body>> + Send + Sync + 'static>>
        + Send
        + Sync
        + 'static>;

// So, as in
Box::new(|| {
    Box::pin(driver.driver_func())
});

[async_trait::async_trait]
pub trait AsyncTrait {
    fn async_func(&self);
}

pub trait AsyncTraitObj;

[async_trait::async_trait]
impl AsyncTrait for AsyncTraitObj {
     fn async_func(&self) {}
}

struct Outer {
    trait_object: Arc<dyn AsyncTrait + Send + Sync>,
}

impl Outer {
    pub fn outer_func(&self) {
        self.trait_object.async_func().await;   // warning (cause of error): future is not 'Sync' as this value is used across an await
    }
}

struct Driver {
    pub fn driver_func(&self) {
        self.outer.outer_func().await; // Trigger method
    }
}

dtolnay / async-trait

Future is not 'Sync' as this value is used across an await #219