Unused lifetime parameter -- except it isn't

I have a fun one, where the compiler says I have an unused lifetime parameter, except it's clearly used. It feels almost like a compiler error, though I'm probably overlooking something? Who can see the mistake?

main.rs

trait Context<'a> {
    fn name(&'a self) -> &'a str;
}

type Func<'a, C: Context<'a>> = dyn Fn(C);

pub struct BuiltInFunction<'a, C: Context<'a>> {
    pub(crate) func: Box<Func<'a, C>>,
}

error[E0392]: parameter `'a` is never used
 --> src/main.rs:7:28
  |
7 | pub struct BuiltInFunction<'a, C: Context<'a>> {
  |                            ^^ unused parameter
  |
  = help: consider removing `'a`, referring to it in a field, or using a marker such as `PhantomData`

For more information about this error, try `rustc --explain E0392`.
error: could not compile `lifetime-test` (bin "lifetime-test") due to 1 previous error