Non 'static higher-rank trait bound

help
1

in the code below, I want TraitA::get_b to return some generic that may refrence self. the function dofoo needs to be able to accept anything with TraitA where the type returned by get_b has an arbitrary constraint (in this case, TraitB).

this, from what I can tell, can not be done without making dofoo require that T be static. this is becouse the trait bounds for<'b> T::BT<'b>: TraitB means that T::BT needs to have TraitB for all lifetimes, including 'static. because T must outlive 'T::BT' and T::BT must be valid for 'static, T needs to be valid for 'static.

is there any way to constrain T::BT without makeing T static? this code compiles if you remove the call to dofoo from main or constrain BT from within TraitA (eg define BT with type BT<'b>: TraitB where Self: 'b). neither of these solutions work for my use case.

if this really cant be done yet in rust, is there an issue open I can fallow or link to so I can be notified when support is added?

trait TraitA {
    type BT<'b>
    where Self: 'b;

    fn get_b<'b>(&'b self) -> Self::BT<'b>;
}

trait TraitB {
    fn foo(&self) {}
}


struct DataA<'a>(&'a i32);
impl TraitA for DataA<'_> {
    type BT<'b> = DataB<'b>
    where Self: 'b;

    fn get_b<'b>(&'b self) -> Self::BT<'b> {
        DataB(&self.0)        
    }
}

struct DataB<'b>(&'b i32);
impl TraitB for DataB<'_> {}


fn dofoo<T>(a: T) 
where T: TraitA,
      for<'b> T::BT<'b>: TraitB {
    let b = a.get_b();
    b.foo();
    a.get_b().foo();
}

fn main() {
    let data: i32 = 8;
    let a = DataA(&data);
    let b = a.get_b();
    b.foo();
    a.get_b().foo();
    //dofoo(a);
}
2

What are you trying to do? Ie., can you tell us about the specific problem instead of a generic "dofoo" function?

3

Also, do you control TraitA? (If so, preview.)

2 Likes
4

this is exactly what i need thank you!

1 Like
5

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