Improve UX/docs of OnGenesis

[mrcnski]

Attempting to implement OnGenesis gives this error:

error[E0119]: conflicting implementations of trait `frame_support::traits::OnGenesis` for type `coretime::pallet::Pallet<_>`
   --> polkadot/runtime/parachains/src/coretime/mod.rs:86:12
    |
86  |     #[pallet::pallet]
    |               ^^^^^^ conflicting implementation for `coretime::pallet::Pallet<_>`
...
123 |     impl<T: Config> OnGenesis for Pallet<T> {
    |     --------------------------------------- first implementation here

The docs are sparse and there aren't any examples of usage in the repo.

If it's not intended to be implemented directly, this should be documented, with an alternative provided.

Also, the Hooks trait could expose on_genesis (it's kind of weird that it does not).

[gui1117]

In FRAME, OnGenesis is generated by the pallet macro unconditionally:

polkadot-sdk/substrate/frame/support/procedural/src/pallet/expand/pallet_struct.rs

Lines 217 to 226 in 9a27b53

	// Implement `OnGenesis` for `Pallet`
	impl<#type_impl_gen> #frame_support::traits::OnGenesis
	for #pallet_ident<#type_use_gen>
	#config_where_clause
	{
	fn on_genesis() {
	let storage_version: #frame_support::traits::StorageVersion = #storage_version;
	storage_version.put::<Self>();
	}
	}

You may want to use GenesisBuild: https://docs.rs/frame-support/latest/frame_support/attr.pallet.html#genesis-build-palletgenesis_build-optional

That said, I agree with your complains on docs.

[bkchr]

No we don't want GenesisBuild ;) Actually having a way to run something after having setup the state would be nice.