Hidden lifetime in return types containing trait parameters

[mahkoh]

trait Y { fn f(&self) { } }

struct X;

impl Y for X { }

struct Z<Trait: ?Sized> {
    _data: std::marker::PhantomData<Trait>,
}

fn f<Y: ?Sized>(y: &Y) -> Z<Y> {
    Z { _data: std::marker::PhantomData }
}

fn main() {
    {
        let arc = std::sync::Arc::new(X);
        f(&*arc as &Y)
    };
}

test4.rs:18:13: 18:16 error: `arc` does not live long enough
test4.rs:18         f(&*arc as &Y)
                        ^~~

[mahkoh]

Worked on monday.

[huonw]

#22230 and/or #21972. This works:

trait Y { fn f(&self) { } }

struct X;

impl Y for X { }

struct Z<Trait: ?Sized> {
    _data: std::marker::PhantomData<Trait>,
}

fn f<Y: ?Sized>(y: &Y) -> Z<Y> {
    Z { _data: std::marker::PhantomData }
}

fn main() {
    {
        let arc = std::sync::Arc::new(X);
        f(&*arc as &(Y+'static))
    };
}

[mahkoh]

Ah, I tried this with &'a (Y+'b) but it didn't work. Using 'static seems to solve the problem.

[mahkoh]

It's a bit surprising that the lifetimes cannot be specified in the function signature, e.g.

fn f<'a, Y: ?Sized+'static>(y: &'a Y) -> Z<Y> {
    Z { _data: std::marker::PhantomData }
}

alone does not solve the problem. But once you know that you have to specify the 'static in the as it's clear.