Generics, traits, and lifetimes, oh my!

lifetime_hell.rs

pub trait MyTrait<'a> {
  fn foo(n: &'a i32);
}

struct Derp;

impl<'a> MyTrait<'a> for Derp {
  fn foo(n: &'a i32) {
  }
}

struct Herp<'a, T> where T: MyTrait<'a> {
  derp: T
}

fn main() {
  Herp::<Derp>::foo(42);
}

This is a rather contrived example, but I think it captures my current problem.

If I do struct Herp<T> I get:

error[E0261]: use of undeclared lifetime name `'a`
  --> example.rs:12:33
   |
12 | struct Herp<T> where T: MyTrait<'a> {
   |                                 ^^ undeclared lifetime

If I do struct Herp<'a, T> I get:

error[E0392]: parameter `'a` is never used
  --> example.rs:12:13
   |
12 | struct Herp<'a, T> where T: MyTrait<'a> {
   |             ^^ unused type parameter
   |
   = help: consider removing `'a` or using a marker such as `std::marker::PhantomData`