Introduce #[pallet::call_index] attribute to dispatchables (#11381)

* Introduce #[pallet::call_index] attribute to dispatchables

* cargo fmt

* Add more docs and prevent duplicates of call indices

* Add UI test for conflicting call indices

* cargo fmt

Co-authored-by: parity-processbot <>

substrate/frame/support/src/lib.rs

@@ -1553,6 +1553,15 @@ pub mod pallet_prelude {
 /// used using `#[pallet::compact]`, function must return `DispatchResultWithPostInfo` or
 /// `DispatchResult`.
 ///
+/// Each dispatchable may also be annotated with the `#[pallet::call_index($idx)]` attribute,
+/// which defines and sets the codec index for the dispatchable function in the `Call` enum.
+///
+/// All call indexes start from 0, until it encounters a dispatchable function with a defined
+/// call index. The dispatchable function that lexically follows the function with a defined
+/// call index will have that call index, but incremented by 1, e.g. if there are 3
+/// dispatchable functions `fn foo`, `fn bar` and `fn qux` in that order, and only `fn bar` has
+/// a call index of 10, then `fn qux` will have an index of 11, instead of 1.
+///
 /// All arguments must implement `Debug`, `PartialEq`, `Eq`, `Decode`, `Encode`, `Clone`. For
 /// ease of use, bound the trait `Member` available in frame_support::pallet_prelude.
 ///
@@ -1566,16 +1575,18 @@ pub mod pallet_prelude {
 /// **WARNING**: modifying dispatchables, changing their order, removing some must be done with
 /// care. Indeed this will change the outer runtime call type (which is an enum with one
 /// variant per pallet), this outer runtime call can be stored on-chain (e.g. in
-/// pallet-scheduler). Thus migration might be needed.
+/// pallet-scheduler). Thus migration might be needed. To mitigate against some of this, the
+/// `#[pallet::call_index($idx)]` attribute can be used to fix the order of the dispatchable so
+/// that the `Call` enum encoding does not change after modification.
 ///
 /// ### Macro expansion
 ///
-/// The macro create an enum `Call` with one variant per dispatchable. This enum implements:
+/// The macro creates an enum `Call` with one variant per dispatchable. This enum implements:
 /// `Clone`, `Eq`, `PartialEq`, `Debug` (with stripped implementation in `not("std")`),
 /// `Encode`, `Decode`, `GetDispatchInfo`, `GetCallName`, `UnfilteredDispatchable`.
 ///
-/// The macro implement on `Pallet`, the `Callable` trait and a function `call_functions` which
-/// returns the dispatchable metadatas.
+/// The macro implement the `Callable` trait on `Pallet` and a function `call_functions` which
+/// returns the dispatchable metadata.
 ///
 /// # Extra constants: `#[pallet::extra_constants]` optional
 ///