[POC] @Builder.Exclude for a field in cases where @Builder/@SuperBuilder is on a type

[a1shadows]

relates to #2307 , #3283 , #3713

Motivation

I know that #2307 has been closed but from the discussion thread I realized that many others were having the same issues I was: that some fields need not be part of a builder (or whatever creation pattern the class is using).

I understand that there is the option of creating a static method/constructor and annotating it with @Builder to achieve exclusion of field(s). However, I would argue that a similar argument could have been made for default values as well. Yet, we have a @Builder.Default which is a super useful feature to have!

My use cases specifically was with regards to JPA Entity classes where I wanted to hide certain non-insertable/non-updatable fields in a JPA Entity from users attempting to build an Entity object. While using @Setter, this was simple enough with @Setter(AccessLevel.PRIVATE), but I wanted to use @Builder since the entities were large and complex. There I had no options with regards to excluding these fields. Not excluding them would mislead the users since such fields are not persisted in the database even when initialized so I was forced to use Setters.

I did not want to badger the maintainers with excessive argumentation so I thought a "POC" might better demonstrate what such an idea would like and would take the discussion forward more constructively. I would greatly appreciate any and all feedback.

Implementation

The basic idea is that if the @Builder/@SuperBuilder annotation is on a type, no setter methods will be generated in the builder for the fields marked with @Builder.Exclude. This means that although the builder class will contain all the fields(so as not to break any existing logic of mapping the builder class to the annotated class through a constructor, generated for builder or maybe through @AllAgsConstructor). However, it will not be possible to set the value of the excluded field before calling the build() method.
It is somewhat similar in use(not implementation) to @Builder.Default:

@Builder
public class User {

    // field setter method will not be generated in builder
    @Builder.Exclude
    private int id;

    private String name;

}

The builder for this class will not contain a User.UserBuilder id(int id) method.

I've added a warning similar to the one for @Builder.Default for when the annotation is not on type. As is the case with @Builder.Default, the @Builder.Exclude annotation holds no meaning when used without a @Builder on the type. if the @Builder is on a constructor or a static method, the exclusion will not take place

I've added a few test cases to demonstrate its working.

I'll add more test cases to the PR if the direction looks promising!

[a1shadows]

@rspilker @rzwitserloot could you please take a look at this whenever possible?