How to get around lifetime issue in HRTB

[xfoxfu]

I have the following code, which fails to compile due to lifetime error.

use std::marker::PhantomData;

trait Exampler<'a> {
    fn new(d: &'a u8) -> Self;
    fn get_best<'f>(&self, vals: impl Iterator<Item = &'f u8>) -> &'f u8;
}

struct Foo<'a>(&'a u8);

impl<'a> Exampler<'a> for Foo<'a> {
    fn new(d: &'a u8) -> Self {
        Foo(d)
    }
    fn get_best<'f>(&self, mut vals: impl Iterator<Item = &'f u8>) -> &'f u8 {
        vals.next().unwrap()
    }
}

struct Bar<E>(PhantomData<E>);

impl<'a, E> Bar<E>
where E: Exampler<'a> {
    pub fn foobar(self) -> u8 {
        let i = 0u8;
        static u8s: &[u8] = &[1,2,3,4,5];
        let e = E::new(&i);
        *e.get_best(u8s.into_iter())
    }
}

pub fn main() {
    Bar::<Foo>(PhantomData).foobar();
}

error[E0597]: `i` does not live long enough
  --> src/main.rs:26:24
   |
21 | impl<'a, E> Bar<E>
   |      -- lifetime `'a` defined here
...
26 |         let e = E::new(&i);
   |                 -------^^-
   |                 |      |
   |                 |      borrowed value does not live long enough
   |                 argument requires that `i` is borrowed for `'a`
27 |         *e.get_best(u8s.into_iter())
28 |     }
   |     - `i` dropped here while still borrowed

In order to address this problem, I tried HRTB with

impl<E> Bar<E>
where E: for<'a> Exampler<'a>

which also fails to compile because of

error[E0599]: no method named `foobar` found for struct `Bar<Foo<'_>>` in the current scope
  --> src/main.rs:32:29
   |
8  | struct Foo<'a>(&'a u8);
   | ----------------------- doesn't satisfy `Foo<'_>: Exampler<'a>`
...
19 | struct Bar<E>(PhantomData<E>);
   | ------------------------------ method `foobar` not found for this
...
32 |     Bar::<Foo>(PhantomData).foobar();
   |                             ^^^^^^ method not found in `Bar<Foo<'_>>`
   |
   = note: the method `foobar` exists but the following trait bounds were not satisfied:
           `Foo<'_>: Exampler<'a>`

I think this is because actually Foo<'a> effectively uses the lifetime 'a in Exampler<'a>, so the for<'a> is not valid for Foo<'a>: Exampler<'a>.

I wonder how to get this compiled, thanks for your possible help.

[alice]

A Foo<'a> does not implement Exampler<'b> for lifetimes 'b smaller than 'a because the constructor cannot build a Foo<'a> from an &'b u8 as it doesn't live long enough. However your trait bound says it must implement it for every lifetime.

[xfoxfu]

Thanks for your explanation. Here I wonder how I could correctly describe the lifetime bound for 'a .