Unused lifetime parameter -- except it isn't
from arendjr@programming.dev to rust@programming.dev on 23 Mar 16:50
https://programming.dev/post/11838278

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

#rust

threaded - newest

jlh@lemmy.jlh.name on 23 Mar 17:08 next collapse

I think you might not be allowed to use two 'a s in one type? I could be wrong though, I’m not very familiar with lifetimes.

taladar@sh.itjust.works on 23 Mar 17:15 next collapse

Well, you aren’t using it on the right side of Func and only using it as a parameter to that in BuiltInFunction. You can probably use C: for<'a> Context<'a> instead in the parameter lines?

arendjr@programming.dev on 23 Mar 17:57 collapse

Thanks, that solves it indeed!

notriddle@programming.dev on 23 Mar 17:28 collapse

The warning is in the wrong place, but it’s real.

The actual problem is on line 5, where you have a type alias that doesn’t use the lifetime parameter. The warning ought to be there, but rustc expands type aliases in place, so it doesn’t check there.

The compiler should be able to fix it by not expanding type aliases directly in place. github.com/rust-lang/rust/issues/112792

arendjr@programming.dev on 23 Mar 17:58 collapse

Ahh, that’s good to know! Thanks, together with @taladar@sh.itjust.works fix that resolves it :+1: