Allow pallet in construct_runtime to have fixed index (#6969)

* implement index for pallet + some tests

* add test and doc

* remove deprecated and document behavior

* update internal doc

* Apply suggestions from code review

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* address review

* use index for all module, break construct_runtime

* fix line length

* implement migration helper funciton in scheduler

* fix start at index 0

* Update frame/scheduler/src/lib.rs

Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>

* Update frame/support/procedural/src/lib.rs

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* bump frame-metadata crate

* factorize

* avoid some unwrap and remove nightly join

* Update frame/support/src/event.rs

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* fix test

* add test and improve error message

* factorize test

* keep iterator, and use slice instead of vec

* refactor to avoid to have expects

* small refactor

* Test something

* Make sure we update the `Cargo.lock`

* Apply suggestions from code review

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* return 2 error

* Apply suggestions from code review

Co-authored-by: Alexander Popiak <alexander.popiak@parity.io>

* Update frame/scheduler/src/lib.rs

Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>

* fix typo

* Revert "fix typo"

This reverts commit f2de8f2db34e8ac72bc9c34437c60dca3fa4ac22.

* Revert "Update frame/scheduler/src/lib.rs"

This reverts commit 6feb4605c6f784b64591e229de7a6fec6dbffb4b.

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>
Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>
Co-authored-by: Bastian Köcher <git@kchr.de>
Co-authored-by: Alexander Popiak <alexander.popiak@parity.io>
Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>

substrate/frame/support/test/tests/construct_runtime_ui/conflicting_index.rs

@@ -0,0 +1,14 @@
+use frame_support::construct_runtime;
+
+construct_runtime! {
+	pub enum Runtime where
+		UncheckedExtrinsic = UncheckedExtrinsic,
+		Block = Block,
+		NodeBlock = Block,
+	{
+		System: system::{},
+		Pallet1: pallet1::{} = 0,
+	}
+}
+
+fn main() {}

substrate/frame/support/test/tests/construct_runtime_ui/conflicting_index.stderr

@@ -0,0 +1,11 @@
+error: Module indices are conflicting: Both modules System and Pallet1 are at index 0
+ --> $DIR/conflicting_index.rs:9:3
+  |
+9 |         System: system::{},
+  |         ^^^^^^
+
+error: Module indices are conflicting: Both modules System and Pallet1 are at index 0
+  --> $DIR/conflicting_index.rs:10:3
+   |
+10 |         Pallet1: pallet1::{} = 0,
+   |         ^^^^^^^

substrate/frame/support/test/tests/construct_runtime_ui/conflicting_index_2.rs

@@ -0,0 +1,16 @@
+use frame_support::construct_runtime;
+
+construct_runtime! {
+	pub enum Runtime where
+		UncheckedExtrinsic = UncheckedExtrinsic,
+		Block = Block,
+		NodeBlock = Block,
+	{
+		System: system::{} = 5,
+		Pallet1: pallet1::{} = 3,
+		Pallet2: pallet2::{},
+		Pallet3: pallet3::{},
+	}
+}
+
+fn main() {}

substrate/frame/support/test/tests/construct_runtime_ui/conflicting_index_2.stderr

@@ -0,0 +1,11 @@
+error: Module indices are conflicting: Both modules System and Pallet3 are at index 5
+ --> $DIR/conflicting_index_2.rs:9:3
+  |
+9 |         System: system::{} = 5,
+  |         ^^^^^^
+
+error: Module indices are conflicting: Both modules System and Pallet3 are at index 5
+  --> $DIR/conflicting_index_2.rs:12:3
+   |
+12 |         Pallet3: pallet3::{},
+   |         ^^^^^^^

substrate/frame/support/test/tests/construct_runtime_ui/more_than_256_modules.rs

@@ -0,0 +1,14 @@
+use frame_support::construct_runtime;
+
+construct_runtime! {
+	pub enum Runtime where
+		UncheckedExtrinsic = UncheckedExtrinsic,
+		Block = Block,
+		NodeBlock = Block,
+	{
+		System: system::{} = 255,
+		Pallet256: pallet256::{},
+	}
+}
+
+fn main() {}

substrate/frame/support/test/tests/construct_runtime_ui/more_than_256_modules.stderr

@@ -0,0 +1,5 @@
+error: Module index doesn't fit into u8, index is 256
+  --> $DIR/more_than_256_modules.rs:10:3
+   |
+10 |         Pallet256: pallet256::{},
+   |         ^^^^^^^^^