SE-0389: Attached Macros

[s-k (Stephen Kockentiedt)]

My overall opinion is very positive and hasn't changed much since the pitch:

@s-k

Implementing this macro reinforced my impressions that the proposed macro system is very powerful but what it misses most is type information. For this macro, it would have sufficed to provide a list of stored properties to the macro as proposed for witness macros. However, other macros may need other types of type information.

Here are two things I want to specifically point out:

• The problem of shadowing the default memberwise initializer:

  @s-k

  Because the macro has to generate init(from:), the default memberwise initializer is no longer automatically generated. I think this is a problem. My macro could have used a peer macro for this if they had already been implemented. However, the canonical macro type for Codable would be witness macros, which are from my understanding not able to generate a peer. We definitely need a solution for this problem!

  I ended up needing to generate the memberwise initializer myself. Here, again, the problem were properties that have a default value and use type inference because the macro didn't know the types of these. I decided to only add initializer parameters for the properties with explicit types and generate warnings for those properties without. Generating the warnings was easy.

  @s-k

  One idea I had was to introduce an attribute:

  @doesNotSuppressDefaultInitializer
  init(customArgument: String) {
    ...
  }
  

  Maybe this attribute could be helpful in other circumstances as well?

• A peer macro currently can either specify that it generates a prefixed name or a suffixed name. I can see situations where a macro would want to add both a prefix and a suffix. For example, a macro attached to the type APIClient could generate a type called _APIClientMock. It would have to use arbitrary, which would lead to longer compilation times.

  Maybe, instead of having prefixed() and suffixed(), we could have something like modified(prefix:suffix:) where either argument could be omitted (i.e. modified(prefix:) or modified(suffix:)).