Currently the DefaultBuilderProvider only detects builders if there is a static builder factory method available.
As I'm trying to use Wire (https://github.com/square/wire), and it only generates the inner builder class, I'd like to have mapstruct detect and use that builder class out of the box.

I brought us this issue at the Wire repository (square/wire#3073), but the introduction of a static factory method for the builder got rejected.

I think it's either way more appropriate to add support in mapstruct out of the box.
I created an SPI that adds this support, but I prefer to have this baked into mapstruct.
I could open a PR if needed.

an example of target that cannot be used for mapping:

public class SomeTarget {
    private final String someField;

    private SomeTarget(String someField) {
        this.someField = someField;
    }

    public static class SomeTargetBuilder {
        private String someField;

        public SomeTargetBuilder someField(String someField) {
            this.someField = someField;
            return this;
        }

        public SomeTarget build() {
            return new SomeTarget(this.someField);
        }
    }
}

Trying to generate a mapper would currently produce the following error: SomeTarget does not have an accessible constructor.