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Unused lifetime parameter -- except it isn't (programming.dev)

submitted 7 months ago by [email protected] to c/[email protected]

5 comments fedilink hide all child comments

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

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[–] [email protected] 11 points 7 months ago* (last edited 7 months ago) (1 children)

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?

[–] [email protected] 6 points 7 months ago

Thanks, that solves it indeed!