fix support for 12 extractors

src/extract.rs

@@ -290,16 +290,6 @@ impl FromRequest for Method {
     }
 }
 
-#[doc(hidden)]
-impl FromRequest for () {
-    type Error = Infallible;
-    type Future = Ready<Result<Self, Self::Error>>;
-
-    fn from_request(_: &HttpRequest, _: &mut Payload) -> Self::Future {
-        ok(())
-    }
-}
-
 #[doc(hidden)]
 #[allow(non_snake_case)]
 mod tuple_from_req {
@@ -388,6 +378,15 @@ mod tuple_from_req {
         }
     }
 
+    impl FromRequest for () {
+        type Error = Infallible;
+        type Future = Ready<Result<Self, Self::Error>>;
+
+        fn from_request(_: &HttpRequest, _: &mut Payload) -> Self::Future {
+            ok(())
+        }
+    }
+
     tuple_from_req! { TupleFromRequest1; A }
     tuple_from_req! { TupleFromRequest2; A, B }
     tuple_from_req! { TupleFromRequest3; A, B, C }
@@ -398,6 +397,8 @@ mod tuple_from_req {
     tuple_from_req! { TupleFromRequest8; A, B, C, D, E, F, G, H }
     tuple_from_req! { TupleFromRequest9; A, B, C, D, E, F, G, H, I }
     tuple_from_req! { TupleFromRequest10; A, B, C, D, E, F, G, H, I, J }
+    tuple_from_req! { TupleFromRequest11; A, B, C, D, E, F, G, H, I, J, K }
+    tuple_from_req! { TupleFromRequest12; A, B, C, D, E, F, G, H, I, J, K, L }
 }
 
 #[cfg(test)]

src/handler.rs

@@ -11,18 +11,17 @@ use crate::{
 ///
 /// # What Is A Request Handler
 /// A request handler has three requirements:
-/// 1. It is an async function (or a function/closure that returns an appropriate future);
-/// 1. The function accepts zero or more parameters that implement [`FromRequest`];
-/// 1. The async function (or future) resolves to a type that can be converted into an
+/// 1. It is an async function (or a function/closure that returns an appropriate future)
+/// and has no more than 12 paramters;
+/// 2. The function parameters implement [`FromRequest`];
+/// 3. The async function (or future) resolves to a type that can be converted into an
 ///   [`HttpResponse`] (i.e., it implements the [`Responder`] trait).
 ///
 /// # Compiler Errors
 /// If you get the error `the trait Handler<_> is not implemented`, then your handler does not
-/// fulfill one or more of the above requirements.
-///
-/// Unfortunately we cannot provide a better compile error message (while keeping the trait's
-/// flexibility) unless a stable alternative to [`#[rustc_on_unimplemented]`][on_unimpl] is added
-/// to Rust.
+/// fulfill the _first_ of the above requirements.
+/// Missing other requirements manifest as errors on implementing [`FromRequest`] and [`Responder`]
+/// , respectively.
 ///
 /// # How Do Handlers Receive Variable Numbers Of Arguments
 /// Rest assured there is no macro magic here; it's just traits.
@@ -76,7 +75,6 @@ use crate::{
 ///
 /// [arity]: https://en.wikipedia.org/wiki/Arity
 /// [`from_request`]: FromRequest::from_request
-/// [on_unimpl]: https://github.com/rust-lang/rust/issues/29628
 pub trait Handler<Args>: Clone + 'static {
     type Output;
     type Future: Future<Output = Self::Output>;
@@ -148,3 +146,33 @@ factory_tuple! { A B C D E F G H I }
 factory_tuple! { A B C D E F G H I J }
 factory_tuple! { A B C D E F G H I J K }
 factory_tuple! { A B C D E F G H I J K L }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn assert_impl_handler<T: FromRequest>(_: impl Handler<T>) {}
+
+    #[test]
+    fn arg_number() {
+        async fn handler_min() {}
+        async fn handler_max(
+            _01: (),
+            _02: (),
+            _03: (),
+            _04: (),
+            _05: (),
+            _06: (),
+            _07: (),
+            _08: (),
+            _09: (),
+            _10: (),
+            _11: (),
+            _12: (),
+        ) {
+        }
+
+        assert_impl_handler(handler_min);
+        assert_impl_handler(handler_max);
+    }
+}