custom weight function wrapper (#4158)

* custom weight function wrapper

* dox

* Better tests.

* remove u8 encoding

* Update frame/support/src/weights.rs

Co-Authored-By: thiolliere <gui.thiolliere@gmail.com>

* fix pays fee

Co-authored-by: thiolliere <gui.thiolliere@gmail.com>

substrate/frame/support/src/weights.rs

@@ -236,6 +236,36 @@ impl SimpleDispatchInfo {
 	}
 }
 
+/// A struct to represent a weight which is a function of the input arguments. The given items have
+/// the following types:
+///
+/// - `F`: a closure with the same argument list as the dispatched, wrapped in a tuple.
+/// - `DispatchClass`: class of the dispatch.
+/// - `bool`: whether this dispatch pays fee or not.
+pub struct FunctionOf<F>(pub F, pub DispatchClass, pub bool);
+
+impl<Args, F> WeighData<Args> for FunctionOf<F>
+where
+	F : Fn(Args) -> Weight
+{
+	fn weigh_data(&self, args: Args) -> Weight {
+		(self.0)(args)
+	}
+}
+
+impl<Args, F> ClassifyDispatch<Args> for FunctionOf<F> {
+	fn classify_dispatch(&self, _: Args) -> DispatchClass {
+		self.1.clone()
+	}
+}
+
+impl<T, F> PaysFee<T> for FunctionOf<F> {
+	fn pays_fee(&self, _: T) -> bool {
+		self.2
+	}
+}
+
+
 /// Implementation for unchecked extrinsic.
 impl<Address, Call, Signature, Extra> GetDispatchInfo
 	for UncheckedExtrinsic<Address, Call, Signature, Extra>
@@ -271,3 +301,46 @@ impl<Call: Encode, Extra: Encode> GetDispatchInfo for sp_runtime::testing::TestX
 		}
 	}
 }
+
+#[cfg(test)]
+#[allow(dead_code)]
+mod tests {
+	use crate::decl_module;
+	use super::*;
+
+	pub trait Trait {
+		type Origin;
+		type Balance;
+		type BlockNumber;
+	}
+
+	pub struct TraitImpl {}
+
+	impl Trait for TraitImpl {
+		type Origin = u32;
+		type BlockNumber = u32;
+		type Balance = u32;
+	}
+
+	decl_module! {
+		pub struct Module<T: Trait> for enum Call where origin: T::Origin {
+			// no arguments, fixed weight
+			#[weight = SimpleDispatchInfo::FixedNormal(1000)]
+			fn f0(_origin) { unimplemented!(); }
+
+			// weight = a x 10 + b
+			#[weight = FunctionOf(|args: (&u32, &u32)| args.0 * 10 + args.1, DispatchClass::Normal, true)]
+			fn f11(_origin, _a: u32, _eb: u32) { unimplemented!(); }
+
+			#[weight = FunctionOf(|_: (&u32, &u32)| 0, DispatchClass::Operational, true)]
+			fn f12(_origin, _a: u32, _eb: u32) { unimplemented!(); }
+		}
+	}
+
+	#[test]
+	fn weights_are_correct() {
+		assert_eq!(Call::<TraitImpl>::f11(10, 20).get_dispatch_info().weight, 120);
+		assert_eq!(Call::<TraitImpl>::f11(10, 20).get_dispatch_info().class, DispatchClass::Normal);
+		assert_eq!(Call::<TraitImpl>::f0().get_dispatch_info().weight, 1000);
+	}
+}