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/procedural/src/lib.rs

@@ -646,7 +646,6 @@ pub fn storage_alias(attributes: TokenStream, input: TokenStream) -> TokenStream
 /// ```
 ///
 /// where `TestDefaultConfig` was defined and registered as follows:
-///
 /// ```ignore
 /// pub struct TestDefaultConfig;
 ///
@@ -673,7 +672,6 @@ pub fn storage_alias(attributes: TokenStream, input: TokenStream) -> TokenStream
 /// ```
 ///
 /// The above call to `derive_impl` would expand to roughly the following:
-///
 /// ```ignore
 /// impl frame_system::Config for Test {
 ///     use frame_system::config_preludes::TestDefaultConfig;
@@ -881,6 +879,7 @@ pub fn inject_runtime_type(_: TokenStream, tokens: TokenStream) -> TokenStream {
 	let item = syn::parse_macro_input!(item as TraitItemType);
 	if item.ident != "RuntimeCall" &&
 		item.ident != "RuntimeEvent" &&
+		item.ident != "RuntimeTask" &&
 		item.ident != "RuntimeOrigin" &&
 		item.ident != "RuntimeHoldReason" &&
 		item.ident != "RuntimeFreezeReason" &&
@@ -888,10 +887,11 @@ pub fn inject_runtime_type(_: TokenStream, tokens: TokenStream) -> TokenStream {
 	{
 		return syn::Error::new_spanned(
 			item,
-			"`#[inject_runtime_type]` can only be attached to `RuntimeCall`, `RuntimeEvent`, `RuntimeOrigin` or `PalletInfo`",
+			"`#[inject_runtime_type]` can only be attached to `RuntimeCall`, `RuntimeEvent`, \
+			`RuntimeTask`, `RuntimeOrigin` or `PalletInfo`",
 		)
 		.to_compile_error()
-		.into();
+		.into()
 	}
 	tokens
 }
@@ -1518,6 +1518,56 @@ pub fn composite_enum(_: TokenStream, _: TokenStream) -> TokenStream {
 	pallet_macro_stub()
 }
 
+///
+/// ---
+///
+/// **Rust-Analyzer users**: See the documentation of the Rust item in
+/// `frame_support::pallet_macros::tasks_experimental`.
+#[proc_macro_attribute]
+pub fn tasks_experimental(_: TokenStream, _: TokenStream) -> TokenStream {
+	pallet_macro_stub()
+}
+
+///
+/// ---
+///
+/// **Rust-Analyzer users**: See the documentation of the Rust item in
+/// `frame_support::pallet_macros::task_list`.
+#[proc_macro_attribute]
+pub fn task_list(_: TokenStream, _: TokenStream) -> TokenStream {
+	pallet_macro_stub()
+}
+
+///
+/// ---
+///
+/// **Rust-Analyzer users**: See the documentation of the Rust item in
+/// `frame_support::pallet_macros::task_condition`.
+#[proc_macro_attribute]
+pub fn task_condition(_: TokenStream, _: TokenStream) -> TokenStream {
+	pallet_macro_stub()
+}
+
+///
+/// ---
+///
+/// **Rust-Analyzer users**: See the documentation of the Rust item in
+/// `frame_support::pallet_macros::task_weight`.
+#[proc_macro_attribute]
+pub fn task_weight(_: TokenStream, _: TokenStream) -> TokenStream {
+	pallet_macro_stub()
+}
+
+///
+/// ---
+///
+/// **Rust-Analyzer users**: See the documentation of the Rust item in
+/// `frame_support::pallet_macros::task_index`.
+#[proc_macro_attribute]
+pub fn task_index(_: TokenStream, _: TokenStream) -> TokenStream {
+	pallet_macro_stub()
+}
+
 /// Can be attached to a module. Doing so will declare that module as importable into a pallet
 /// via [`#[import_section]`](`macro@import_section`).
 ///