What's the purpose of T in Handler?

[dominickpastore]

Summary

Handler has this generic type parameter T. I'm looking at the docs and code, and it seems to be related to the extractors used by the handler, but I can't figure out why the trait needs it as one of its type parameters. The call function doesn't refer to T at all. The Layered and HandlerService structs do refer to T, but I can't figure out why they would need it either, except because the Handler they wrap needs it.

All in all, it seems like nothing would be lost if Handler (and HandlerService and Layered) didn't include T in their generic type parameters. I assume it's there for a reason though, so what am I missing here?

(The reason I ask is I'm trying to write a trait with a function that will return a Handler, but this T parameter is giving me a lot of trouble, and I don't understand its purpose well enough to figure out what to do about it.)

axum version

0.7.7

[mladedav]

Feel free to play around with the code. You can try deleting it and see what happens. And honestly, I'd recommend doing just that because I'm not sure how much the following will make sense without you playing with the code.

The main reason is that we're creating multiple blanket implementations for the trait, one for F: Fn(), one for F: Fn(T1), one for F: Fn(T1, T2), etc. Normally, that wouldn't be possible because rustc would see that as conflicting implementations so we need to convince it it's actually slightly different traits. See this example, it might better show you what I'm trying to get across, the commented out code wouldn't work.

The second part of that parameter is usually named M which is the second parameter of FromRequest which exists for blanked implementation of impl<T> FromRequestParts for T where T: FromRequest, again solving issues with conflicting implementations.

[dominickpastore]

Thanks for the insight. I have a feeling I'm going to have some follow up questions about how to fix my trait later, but I'm going to try experimenting with the code like you suggested and see what I can learn from that first. Thanks!