Can't mock struct with lifetime

[leo-vital]

Hello. Currently I'm having to mock something like this:

struct StructWithLifetime<'a> {
    field: &'a str,
}

trait TraitWithAssociatedType {
    type Type: 'static;

    fn method(&self, input: Self::Type) -> String;
}

impl TraitWithAssociatedType for StructWithLifetime<'_> {
    type Type = String;

    fn method(&self, input: Self::Type) -> String {
        format!("{}: {}", self.field, input)
    }
}

#[cfg(test)]
mockall::mock! {
    pub StructWithLifetime<'a> {}

    impl TraitWithAssociatedType for StructWithLifetime<'_> {
        type Type = String;
        fn method(&self, input: Self::Type) -> String;
    }
}

But I keep getting an error related to the lifetime:

error[E0107]: struct takes 0 lifetime arguments but 1 lifetime argument was supplied
  --> src/main.rs:20:1
   |
20 | /  mockall::mock! {
21 | |      pub StructWithLifetime<'a> {}
22 | |
23 | |      impl TraitWithAssociatedType for StructWithLifetime<'_> {
   | | ____________________________________________________________-
24 | ||         type Type = String;
25 | ||         fn method(&self, input: Self::Type) -> String;
26 | ||     }
27 | || }
   | || ^
   | ||_|
   |  |_expected 0 lifetime arguments
   |    help: remove the unnecessary generics

Strangely enough, when I inline the macro, the error appears to be here:

#[doc = r" Expectation type for methods that return a `'static` type."]
#[doc = r" This is the type returned by the `expect_*` methods."]
pub struct Expectation {
    common:Common,rfunc:Mutex<Rfunc> ,
}
// some other code
impl MockStructWithLifetime<'_>{
    #[must_use = "Must set return value when not using the \"nightly\" feature"]
    #[doc = "Create an [`Expectation`](__mock_MockStructWithLifetime_TraitWithAssociatedType/__method/struct.Expectation.html) for mocking the `method` method"]
    pub fn expect_method(&mut self) ->  &mut __mock_MockStructWithLifetime_TraitWithAssociatedType::__method::Expectation<'_>{
        self.TraitWithAssociatedType_expectations.method.expect()
    }

    }

Specifically, the struct Expectation has no lifetime parameter, but the expect_method function returns Expectation<'_>.

I tried using automock on the implementation of the trait on the struct but got a similar error. And I do not think mocking the trait works for my case, because my understanding is that for that to work I have to define concrete types for the associated types, and in my tests I need to have a different mock for each associated type I have.

Is there a way around this? The struct needs the lifetime parameter because it contains a struct that needs it, so I cannot avoid having it.

[asomers]

Yes, there is an easy workaround. Use mock! instead of #[automock], and leave out the lifetime parameter. Any consumer that can accept a lifetime-bound struct can also accept a 'static one. But that workaround might fail if a consumer expects the mock object to have a lifetime parameter. In that case, what happens if you do this?

#[cfg(test)]
mockall::mock! {
    pub StructWithLifetime<'a> {}

    impl TraitWithAssociatedType for StructWithLifetime<'static> {
        type Type = String;
        fn method(&self, input: Self::Type) -> String;
    }
}

[leo-vital]

Yes, there is an easy workaround. Use mock! instead of #[automock], and leave out the lifetime parameter. Any consumer that can accept a lifetime-bound struct can also accept a 'static one. But that workaround might fail if a consumer expects the mock object to have a lifetime parameter. In that case, what happens if you do this?

#[cfg(test)]
mockall::mock! {
    pub StructWithLifetime<'a> {}

    impl TraitWithAssociatedType for StructWithLifetime<'static> {
        type Type = String;
        fn method(&self, input: Self::Type) -> String;
    }
}

Thanks for the response!

Removing the lifetime parameter from the mock does not work for my case, there are sections of the code that do something like:

#[mockall_double::double]
use crate::StructWithLifetime;
fn do_something_with_struct(s: StructWithLifetime<'_>)...

I tried your other suggestion but I keep getting the same error, even in the simple example I provided. :/