Custom parametrized extractor

[czocher]

Is there some way to create an extractor which would take additional parameters? What I had in mind is some kind of extractor to get a user only if it has a certain role assigned, something like this:

fn some_handler(RequireRole(admin): RequireRole(Role::Admin)) -> IntoResponse {
    ...
}

where the RequireRole would use a different extractor to get a user and then verify if user.role == Role::Admin. I see that the FromRequest or FromRequestParts traits don't provide a self param which kinda limits their use in this manner, but maybe there's some way to overcome that, which I'm not aware of?

[jplatte]

This is possible on Nightly where you can declare a const-generic RequireRole<const R: Role> with the right unstable feature (I don't know the name off the top of my head but the compiler error message should mention it). It would then be used as RequireRole(admin): RequireRole<Role::Admin>.

On stable, you can declare a RequireRole<R: Role> with Admin being one of several zero-sized types that implement the Role trait. However, in that case you need to add a PhantomData field to RequireRole and so the pattern match will have to be RequireRole(admin, _): RequireRole<Admin> if you still want to use destructuring.

[czocher]

Thank you for confirming my conclusion. I understand that there are no other options for implementing this as an extractor? It's either one of your proposals or a middleware?

[jplatte]

Yes, fundamentally you can only pass arguments to extractors using generics. At runtime, an extractor only exists as a thin wrapper around the inner value, so it doesn't make sense for there to be a self argument in FromRequest – that self value only exists once the extractor has already done its work.