Can't have generics on app state marked with FromRef?

[ghost]

I was trying to get dependency injection to work more or less nicely with generics, but I got kinda stomped before I could even begin. Let's say I have this

#[derive(Clone, FromRef)]
struct AppState<T: ApiClient> {
    thing: T,
}

trait ApiClient {
    fn do_the_thing(&self);
}

Rust then complains with the following error:

error[E0107]: missing generics for struct `AppState`
  --> src/main.rs:15:8
   |
15 | struct AppState<T: ApiClient> {
   |        ^^^^^^^^ expected 1 generic argument
   |
note: struct defined here, with 1 generic parameter: `T`
  --> src/main.rs:15:8
   |
15 | struct AppState<T: ApiClient> {
   |        ^^^^^^^^ -
help: add missing generic argument
   |
15 | struct AppState<T><T: ApiClient> {
   |                +++

Obviously struct AppState<T><T: ApiClient> is not valid syntax either. What the heck is going on?

[davidpdrsn]

Try writing a manual implementation.

[ghost]

Ohhhhh I see how it works now. I still get a different error if I try to implement it for T: ApiClient, but it works for a concrete implementation.

Thank you!

[davidpdrsn]

Yeah I think it conflicts with https://docs.rs/axum/latest/axum/extract/trait.FromRef.html#impl-FromRef%3CT%3E-for-T

[jplatte]

Maybe we should raise a more useful error when the derive is used on a generic type.

[davidpdrsn]

Yeah that would be good! I'll do a quick PR for that.