Suggest an enum as a possible option when a trait object can’t be created due to object safety

[alilleybrinker]

When you can't create a trait object due to a trait not being object safe, you have a few options:

• if you control the trait, try to make it object safe (medium difficulty)
• if you don't control the trait, try to get the upstream owner to make it object safe or find an alternative (hard)
• if the set of types you're trying to trait-objectify is closed, use an enum (easy)

Unfortunately, the error message for a trait not being object safe doesn't point you in the direction of the enum option, and unless you're pretty familiar with Rust you may not consider it.

Adjusting the error message for failure to create a trait object due to object safety issues to mention an enum as a possibility could help people resolve these issues. I have no data, but in my experience the enum path is often workable because I'm working with a finite set of variants that I know in advance.

[alilleybrinker]

For context, this was opened at the suggestion of @estebank on Twitter.

[alilleybrinker]

On further reflection, my list of options was incomplete, as exploiting type-erasure a la erased-serde is also a possibility, see the example here. This involves, rather than making the trait object safe, making an erased version of the trait which is object safe, and then implementing the non-object-safe version of that trait for trait objects of the erased trait.

So T isn't object safe, but ET is, and you can implement T for Box<dyn ET> or &dyn ET> or whatever other trait object of ET you want, and it resolves the safety issue. Fun trick!