Declarative macro_rules! derive macros (currently an unimplemented, pre-feature-gated RFC)

[Kijewski]

With rust-lang/rfcs#3698 it would be possible to remove all integration features like with-axum. Instead it would be possible to implement the code generation as a proper macro that is passed as a call-back argument for our generator (untested pseudo-code):

#[doc(hidden)]
pub use {axum_core, rinja};

#[macro_export]
macro_rules! Template {
    ($($tt:tt)*) => {
        $crate::impl_template! {
            [$($tt)*]
            [$crate::rinja]
            [$crate::impl_integration] // in `rinja` itself this argument would be absent
        }
    }
}

#[doc(hidden)]
#[macro_export]
macro_rules! impl_integration {
    (
        [$ident:ident]
        [$($impl_generics:tt)*]
        [$($ty_generics:tt)*]
    ) => {
        impl $($impl_generics)* $crate::axum_core::response::IntoResponse
        for $ident $($ty_generics)* {
            #[inline]
            fn into_response(self) -> $crate::axum_core::response::Response {
                $crate::into_response(&self)
            }
        }
    }
}

This way --all-features would work fine, and everyone could easily implement their own integration crate.