Trait cfx_storage::utils::wrap::WrappedLifetimeFamily
source · pub trait WrappedLifetimeFamily<'a, Constrain: ?Sized> {
type Out: 'a + Sized + ElementSatisfy<Constrain>;
}Expand description
This trait is designed for associated type in trait, such as iterator, where the type could be a borrow of self, or an independent type.
It’s difficult to offer generic trait methods implementation for all variants. One possible way is to use HRTB, however under some cases we could only implement ’_ or have implementation for ’_ instead of for<’any>, especially when we have no control over third-party code, which render some usages infeasible.
Say if we have the code below:
trait LifetimeBoundedTrait<'a> {
fn get(&'a mut self) -> T<'a>;
}
#[derive(Debug)]
struct T<'a> {
s: &'a i64,
}
struct EndUserType<TypeImplTrait> {
v: TypeImplTrait,
}
impl<T> EndUserType<T>
where for<'a> T: LifetimeBoundedTrait<'a>
{
fn use_t(&mut self) {
let ret = self.v.get();
println!("{:?}", ret);
}
}
struct ConcreteType {}
impl<'a> LifetimeBoundedTrait<'a> for ConcreteType {
fn get(&'a mut self) -> T<'a> { T { s: &0 } }
}
impl<'a: 'b, 'b> LifetimeBoundedTrait<'b> for &'a mut ConcreteType {
fn get(&'b mut self) -> T<'b> { (*self).get() }
}Ideally, we could store &mut ConcreteType in EndUserType#use_t in the same way. But It’s not possible for the EndUserType to store &mut ConcreteType as v, because &mut ConcreteType only implements LifetimeBoundedTrait for ’_, not for any lifetime.
What we have to do, is to drop the lifetime bound for ’a, which is counter-intuitive but works.
impl<’a, ’b> LifetimeBoundedTrait<’b> for &’a mut ConcreteType { fn get(&’b mut self) -> T<’b> { (*self).get() } }
In some other cases, with &’x F, rust requires F: ’x in generic type constrain, which may be harder to workaround for HRTB.