Can't mock trait containing function that takes an argument of type Option<&T>

[thomaswhiteway]

Seen using mockall 0.5.1 on both stable and nightly.

For example, trying to compile the following fails.

use mockall::automock;

#[automock]
trait Test<T> {
    fn test(value: Option<&T>);
}

error[E0637]: `&` without an explicit lifetime name cannot be used here
 --> src/lib.rs:5:27
  |
5 |     fn test(value: Option<&str>);
  |                           ^ explicit lifetime name needed here

error[E0106]: missing lifetime specifier
 --> src/lib.rs:5:27
  |
5 |     fn test(value: Option<&str>);
  |                           ^ expected lifetime parameter

Workaround

If T is known it's possible to work around this by specifying a lifetime parameter in the method definition, e.g. the following works

#[automock]
trait Test {
    fn test<'a>(value: Option<&'a str>);
}

If the trait is generic this fails with a different error.

#[automock]
trait Test<T> {
    fn test(value: Option<&T>);
}

error[E0261]: use of undeclared lifetime name `'a`
 --> src/lib.rs:5:32
  |
5 |     fn test<'a>(value: Option<&'a T>);
  |                                ^^ undeclared lifetime

error: aborting due to previous error

[asomers]

Yes, this is a problem. More generally, the only arguments that Mockall really supports are 'static arguments and references to 'static data.

[robin-kunzler]

Hi, is there a workaround for this issue for generic traits? If not, how could this be fixed in mockall?

[asomers]

The only workaround I know of is to change the trait's definition in test mode to take a Option<&'static T>. Enhancing Mockall to fully support this feature is non-trivial.

[robin-kunzler]

I see, thanks!

[asomers]

I'm a fool. I didn't realize that the workaround is trivial: declare an explicit generic lifetime. That's actually worked ever since 0.5.0 (PR #48). Do it like this:

#[automock]
trait Test<T> {
    fn test<'a>(value: Option<&'a T>);
}

[asomers]

I think I'm going to close this issue rather than create an enhancement for it. The cost-benefit ratio doesn't seem very good, given how easy the workaround is.