What is the best way to inject a list with google-guice?

We can do something like this:-

@Override
protected void configure() {
    bind(ShapeBuilder.class).annotatedWith(Names.named("Circle")).to(CircleBuilder.class);
    bind(ShapeBuilder.class).annotatedWith(Names.named("Triangle")).to(TriangleBuilder.class);
    bind(ShapeBuilder.class).annotatedWith(Names.named("Square")).to(SquareBuilder.class);
}

In your FormBuilder Class:-

class FormBuilder{
   List<ShapeBuilder> shapeBuilderList;
   @Inject
   public FormBuilder(@Named("Circle")ShapeBuilder circle, @Named("Triangle")ShapeBuilder triangle,@Named("Square")ShapeBuilder square){
       shapeBuilderList = new ArrayList<>();
       //add all of them
       shapeBuilderList.add(circle);
    }
}

Consider Multibindings, which will collect bindings very much like your code snippet. There is no provision for lists through Multibinder, because Multibinder is designed for binding to the same collection in multiple modules, and the element order of a Multibinder list would depend on the order that your Modules were evaluated.

Multibinder<ShapeBuilder> shapeBinder =
    Multibinder.newSetBinder(binder(), ShapeBuilder.class);
shapeBinder.addBinding().to(CircleBuilder.class);
shapeBinder.addBinding().to(TriangleBuilder.class);
shapeBinder.addBinding().to(SquareBuilder.class);

// Now you can inject Set<ShapeBuilder>.

Alternatively, your @Provides method can take in parameters (e.g. CircleBuilder or Provider<CircleBuilder>) so you can have Guice create everything except the List itself. Not only will Guice bind Providers of all bound types automatically, but it will also inject every parameter in any @Provides method.

@Provides List<ShapeBuilder> provideShapeBuilders(
    CircleBuilder circleBuilder,
    SquareBuilder squareBuilder,
    TriangleBuilder triangleBuilder,
    Provider<TrapezoidBuilder> trapezoidBuilderProvider) {
  return new ArrayList<ShapeBuilder>(
      circleBuilder,
      squareBuilder,
      triangleBuilder,
      trapezoidBuilderProvider.get(),
      trapezoidBuilderProvider.get());
}