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/lib.rs

@@ -849,7 +849,7 @@ pub mod pallet_prelude {
 		},
 		traits::{
 			BuildGenesisConfig, ConstU32, EnsureOrigin, Get, GetDefault, GetStorageVersion, Hooks,
-			IsType, PalletInfoAccess, StorageInfoTrait, StorageVersion, TypedGet,
+			IsType, PalletInfoAccess, StorageInfoTrait, StorageVersion, Task, TypedGet,
 		},
 		Blake2_128, Blake2_128Concat, Blake2_256, CloneNoBound, DebugNoBound, EqNoBound, Identity,
 		PartialEqNoBound, RuntimeDebugNoBound, Twox128, Twox256, Twox64Concat,
@@ -2674,6 +2674,61 @@ pub mod pallet_macros {
 	/// }
 	/// ```
 	pub use frame_support_procedural::storage;
+	/// This attribute is attached to a function inside an `impl` block annoated with
+	/// [`pallet::tasks_experimental`](`tasks_experimental`) to define the conditions for a
+	/// given work item to be valid.
+	///
+	/// It takes a closure as input, which is then used to define the condition. The closure
+	/// should have the same signature as the function it is attached to, except that it should
+	/// return a `bool` instead.
+	pub use frame_support_procedural::task_condition;
+	/// This attribute is attached to a function inside an `impl` block annoated with
+	/// [`pallet::tasks_experimental`](`tasks_experimental`) to define the index of a given
+	/// work item.
+	///
+	/// It takes an integer literal as input, which is then used to define the index. This
+	/// index should be unique for each function in the `impl` block.
+	pub use frame_support_procedural::task_index;
+	/// This attribute is attached to a function inside an `impl` block annoated with
+	/// [`pallet::tasks_experimental`](`tasks_experimental`) to define an iterator over the
+	/// available work items for a task.
+	///
+	/// It takes an iterator as input that yields a tuple with same types as the function
+	/// arguments.
+	pub use frame_support_procedural::task_list;
+	/// This attribute is attached to a function inside an `impl` block annoated with
+	/// [`pallet::tasks_experimental`](`tasks_experimental`) define the weight of a given work
+	/// item.
+	///
+	/// It takes a closure as input, which should return a `Weight` value.
+	pub use frame_support_procedural::task_weight;
+	/// Allows you to define some service work that can be recognized by a script or an
+	/// off-chain worker. Such a script can then create and submit all such work items at any
+	/// given time.
+	///
+	/// These work items are defined as instances of the [`Task`](frame_support::traits::Task)
+	/// trait. [`pallet:tasks_experimental`](`tasks_experimental`) when attached to an `impl`
+	/// block inside a pallet, will generate an enum `Task<T>` whose variants are mapped to
+	/// functions inside this `impl` block.
+	///
+	/// Each such function must have the following set of attributes:
+	///
+	/// * [`pallet::task_list`](`task_list`)
+	/// * [`pallet::task_condition`](`task_condition`)
+	/// * [`pallet::task_weight`](`task_weight`)
+	/// * [`pallet::task_index`](`task_index`)
+	///
+	/// All of such Tasks are then aggregated into a `RuntimeTask` by
+	/// [`construct_runtime`](frame_support::construct_runtime).
+	///
+	/// Finally, the `RuntimeTask` can then used by a script or off-chain worker to create and
+	/// submit such tasks via an extrinsic defined in `frame_system` called `do_task`.
+	///
+	/// ## Example
+	#[doc = docify::embed!("src/tests/tasks.rs", tasks_example)]
+	/// Now, this can be executed as follows:
+	#[doc = docify::embed!("src/tests/tasks.rs", tasks_work)]
+	pub use frame_support_procedural::tasks_experimental;
 }
 
 #[deprecated(note = "Will be removed after July 2023; Use `sp_runtime::traits` directly instead.")]