How does Actix know handler arguments?

[aunyks]

I've noticed that all route handler functions don't need to accept the same number or types of arguments.

How does Actix know what these arguments are and how to populate them when it receives a request? I assume this happens at compile time.

Thanks in advance!

[asonix]

I believe that's a result of this macro that implements Handler for a given Fn type: https://github.com/actix/actix-web/blob/master/src/handler.rs#L65

[robjtede]

It's just a trait implemented for functions of various arities. It is the same reason that fn foo(arg: impl Trait) arguments work except that we restrict all the fn arguments to implement FromRequest, too.

The only other interesting part is that we implement FromRequest for tuples up to the same arity which satisfies the T: FromRequest bound you'll see on web::to, for example.

So, we can take the fn arg types, describe them as a tuple, run from_request on the tuple, and get extracted fn args back to call your handler with.

[robjtede]

written this up in the documentation:
https://actix.rs/actix-web/actix_web/trait.Handler.html#how-do-handlers-receive-variable-numbers-of-arguments

[aunyks]

Thanks everyone!