FRAME: Simplify and extend pallets config definition by using the stabilized feature: associated type bounds

[gui1117]

Associated type bounds is stabilized: rust-lang/rust#122055

So we can do code like this:

#![feature(associated_type_bounds)]

trait Foo: Bar<B: Default> {
}

trait Bar {
	type B;
}

fn f<T: Foo>() {
	<<T as Bar>::B as Default>::default();
}

Instead of doing this:

trait Foo: Bar<B> {
	type B: IsType<Bar::B> + Default;
}

trait Bar {
	type B;
}

fn f<T: Foo>() {
	// Do back and forth conversion between <T as Foo>::B and <T as Bar>::B
}

1: simplify Config definition

So for pallets other than frame_system we no longer need the associated types: RuntimeEvent, RuntimeOrigin, RuntimeCall, etc...

Instead we should be able to modify the pallet Config trait definition like this:

        #[pallet::config]
-       pub trait Config: frame_system::Config {
-               /// Overarching event type.
-               type RuntimeEvent: From<Event<Self>> + IsType<<Self as frame_system::Config>::RuntimeEvent>;
-
+       pub trait Config: frame_system::Config<RuntimeEvent: From<Event<Self>>> {
                /// The type in which the assets for swapping are measured.
                type Balance: Balance;
 

2: extend Config definition

We should be able to add constraint on pallet dependencies way more easily without having to write where ... for each implementation block.

To illustrate we could have a pallet which depends on pallet_balances but requires at least u128 as currency type

#[pallet::config]
pub trait Config: pallet_balances::Config<Balance: From<u128>> {
}

[MrishoLukamba]

This looks like a good issue to tackle and get crazy on rust types and traits.
So essentially , we would have this syntax where we tight couple every pallet? @ggwpez @thiolliere

[ggwpez]

So essentially , we would have this syntax where we tight couple every pallet?

I think a good start would be to use it for one or two pallets as a proof-of-concept. For example as gui pointed out, in the RuntimeEvent or RuntimeCall types of a pallet config.
Best to start small with this so that we can quickly review it.