Tasks: general system for recognizing and executing service work (#1343)

`polkadot-sdk` version of original tasks PR located here:
https://github.com/paritytech/substrate/pull/14329

Fixes #206

## Status
- [x] Generic `Task` trait
- [x] `RuntimeTask` aggregated enum, compatible with
`construct_runtime!`
- [x] Casting between `Task` and `RuntimeTask` without needing `dyn` or
`Box`
- [x] Tasks Example pallet
- [x] Runtime tests for Tasks example pallet
- [x] Parsing for task-related macros
- [x] Retrofit parsing to make macros optional
- [x] Expansion for task-related macros
- [x] Adds support for args in tasks
- [x] Retrofit tasks example pallet to use macros instead of manual
syntax
- [x] Weights
- [x] Cleanup
- [x] UI tests
- [x] Docs

## Target Syntax
Adapted from
https://github.com/paritytech/polkadot-sdk/issues/206#issue-1865172283

```rust
// NOTE: this enum is optional and is auto-generated by the other macros if not present
#[pallet::task]
pub enum Task<T: Config> {
    AddNumberIntoTotal {
        i: u32,
    }
}

/// Some running total.
#[pallet::storage]
pub(super) type Total<T: Config<I>, I: 'static = ()> =
StorageValue<_, (u32, u32), ValueQuery>;

/// Numbers to be added into the total.
#[pallet::storage]
pub(super) type Numbers<T: Config<I>, I: 'static = ()> =
StorageMap<_, Twox64Concat, u32, u32, OptionQuery>;

#[pallet::tasks_experimental]
impl<T: Config<I>, I: 'static> Pallet<T, I> {
	/// Add a pair of numbers into the totals and remove them.
	#[pallet::task_list(Numbers::<T, I>::iter_keys())]
	#[pallet::task_condition(|i| Numbers::<T, I>::contains_key(i))]
	#[pallet::task_index(0)]
	pub fn add_number_into_total(i: u32) -> DispatchResult {
		let v = Numbers::<T, I>::take(i).ok_or(Error::<T, I>::NotFound)?;
		Total::<T, I>::mutate(|(total_keys, total_values)| {
			*total_keys += i;
			*total_values += v;
		});
		Ok(())
	}
}
```

---------

Co-authored-by: Nikhil Gupta <17176722+gupnik@users.noreply.github.com>
Co-authored-by: kianenigma <kian@parity.io>
Co-authored-by: Nikhil Gupta <>
Co-authored-by: Gavin Wood <gavin@parity.io>
Co-authored-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>
Co-authored-by: gupnik <nikhilgupta.iitk@gmail.com>

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

@@ -0,0 +1,87 @@
+// This file is part of Substrate.
+
+// Copyright (C) Parity Technologies (UK) Ltd.
+// SPDX-License-Identifier: Apache-2.0
+
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// 	http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+//! Contains the [`Task`] trait, which defines a general-purpose way for defining and executing
+//! service work, and supporting types.
+
+use codec::FullCodec;
+use scale_info::TypeInfo;
+use sp_runtime::DispatchError;
+use sp_std::{fmt::Debug, iter::Iterator, vec, vec::IntoIter};
+use sp_weights::Weight;
+
+/// Contain's re-exports of all the supporting types for the [`Task`] trait. Used in the macro
+/// expansion of `RuntimeTask`.
+#[doc(hidden)]
+pub mod __private {
+	pub use codec::FullCodec;
+	pub use scale_info::TypeInfo;
+	pub use sp_runtime::DispatchError;
+	pub use sp_std::{fmt::Debug, iter::Iterator, vec, vec::IntoIter};
+	pub use sp_weights::Weight;
+}
+
+/// A general-purpose trait which defines a type of service work (i.e., work to performed by an
+/// off-chain worker) including methods for enumerating, validating, indexing, and running
+/// tasks of this type.
+pub trait Task: Sized + FullCodec + TypeInfo + Clone + Debug + PartialEq + Eq {
+	/// An [`Iterator`] over tasks of this type used as the return type for `enumerate`.
+	type Enumeration: Iterator;
+
+	/// Inspects the pallet's state and enumerates tasks of this type.
+	fn iter() -> Self::Enumeration;
+
+	/// Checks if a particular instance of this `Task` variant is a valid piece of work.
+	fn is_valid(&self) -> bool;
+
+	/// Performs the work for this particular `Task` variant.
+	fn run(&self) -> Result<(), DispatchError>;
+
+	/// Returns the weight of executing this `Task`.
+	fn weight(&self) -> Weight;
+
+	/// A unique value representing this `Task` within the current pallet. Analogous to
+	/// `call_index`, but for tasks.'
+	///
+	/// This value should be unique within the current pallet and can overlap with task indices
+	/// in other pallets.
+	fn task_index(&self) -> u32;
+}
+
+impl Task for () {
+	type Enumeration = IntoIter<Self>;
+
+	fn iter() -> Self::Enumeration {
+		vec![].into_iter()
+	}
+
+	fn is_valid(&self) -> bool {
+		true
+	}
+
+	fn run(&self) -> Result<(), DispatchError> {
+		Ok(())
+	}
+
+	fn weight(&self) -> Weight {
+		Weight::default()
+	}
+
+	fn task_index(&self) -> u32 {
+		0
+	}
+}