Summary

This PR introduces a new check_components! macro that allows easy checking of whether a context implements a CGP component, with proper error highlighting on the failing component.

Example

Given the following code:

check_components! {
    CanUseMyApp for MyApp
    {
        FooComponent,
        BarComponent: String,
        [
            BazAComponent,
            BazBComponent
        ]:
            (u64, bool),
    }
}

The following will be generated:

trait CanUseMyApp<Component, Params>: CanUseComponent<Component, Params> {}

impl CanUseMyApp<FooComponent, ()> for MyApp {}
impl CanUseMyApp<BarComponent, String> for MyApp {}
impl CanUseMyApp<BazAComponent, (u64, bool)> for MyApp {}
impl CanUseMyApp<BazBComponent, (u64, bool)> for MyApp {}

The macro allows a list syntax similar to delegate_components!, which would multiplex the key/value entries for each item inside the list.

Background

The trait CanUseComponent<Component, Params> is implemented if a context implements the consumer for Component with the generic parameters Params.

A local trait like CanUseMyApp is declared by the macro to check for specific implementations of CanUseComponents. For example, CanUseMyApp<FooComponent, ()> is used to check for MyApp: CanUseFoo, while CanUseMyApp<BazAComponent, (u64, bool)> is used to check to MyApp: CanUseBazA<u64, bool>.