Helper macro for Morph impls (#11570)

* Helper macro for Morph impls

* No need to deprecate for now

* Improved macro

* Doc tests

* Grumbles

substrate/frame/support/src/traits/dispatch.rs

@@ -18,15 +18,12 @@
 //! Traits for dealing with dispatching calls and the origin from which they are dispatched.
 
 use crate::dispatch::{DispatchResultWithPostInfo, Parameter, RawOrigin};
-use sp_arithmetic::traits::{CheckedSub, Zero};
 use sp_runtime::{
 	traits::{BadOrigin, Member, Morph, TryMorph},
 	Either,
 };
 use sp_std::marker::PhantomData;
 
-use super::TypedGet;
-
 /// Some sort of check on the origin is performed by this object.
 pub trait EnsureOrigin<OuterOrigin> {
 	/// A return type.
@@ -226,27 +223,6 @@ impl<
 	}
 }
 
-/// Mutator which reduces a scalar by a particular amount.
-pub struct ReduceBy<N>(PhantomData<N>);
-impl<N: TypedGet> TryMorph<N::Type> for ReduceBy<N>
-where
-	N::Type: CheckedSub,
-{
-	type Outcome = N::Type;
-	fn try_morph(r: N::Type) -> Result<N::Type, ()> {
-		r.checked_sub(&N::get()).ok_or(())
-	}
-}
-impl<N: TypedGet> Morph<N::Type> for ReduceBy<N>
-where
-	N::Type: CheckedSub + Zero,
-{
-	type Outcome = N::Type;
-	fn morph(r: N::Type) -> N::Type {
-		r.checked_sub(&N::get()).unwrap_or(Zero::zero())
-	}
-}
-
 /// Type that can be dispatched with an origin but without checking the origin filter.
 ///
 /// Implemented for pallet dispatchable type by `decl_module` and for runtime dispatchable by