Adds ability to provide defaults for types provided by construct_runtime

[gupnik]

Step in paritytech/polkadot-sdk#171

As described in #14453 (comment), this PR attempts to provide defaults for types provided by construct_runtime.

This is achieved by the combination of two macros:

1. #[pallet::no_default_bounds] that adds the ability to remove trait bounds while creating DefaultConfig. This allows us to set types like RuntimeCall as ().

trait Config {
    #[pallet::no_default_bounds]
    type RuntimeCall: ....; //bounds
}

becomes

trait DefaultConfig {
    type RuntimeCall;
}

2. #[inject_runtime_type] that can be added to types generated by construct_runtime while registering the default impl. When this is later combined with the user-provided impl, it's replaced with the respective types.

impl DefaultConfig for TestDefaultConfig {
    #[inject_runtime_type]
    type RuntimeCall = ();
}

becomes

impl frame_system::Config for Runtime {
    type RuntimeCall = RuntimeCall;
}

Todo:

• Docs
• UI Tests

[sam0x17]

this is looking good to me just needs docs in a few places and some integration tests etc

[sam0x17]

nice making this extendible 💯

[kianenigma]

Seems like the right direction to have a fully elided impl.

Can you elaborate a bit about why you chose the word verbatim?

[sam0x17]

Can you elaborate a bit about why you chose the word verbatim?

that's my fault. Its a term syn uses internally a lot to refer to tokenstreams that are left as-is without any further parsing.

in general it just means "as-is"

[juangirini]

Can we use an enum instead of a boolean in here?

[gupnik]

I am not sure if it adds a lot of value here. What are the enum variants that you would suggest?

[bkchr]

Already looks much better!

[gupnik]

bot merge