@Override
    public Void memberShape(MemberShape shape) {
        if (shape.hasTrait(IdRefTrait.ID)) {
            writer.write("$T.fromNode($L)", ShapeId.class, varName);
        } else {
            model.expectShape(shape.getTarget()).accept(this);
        }
        return null;
    }
Determines how to map a node to a shape.
smithy-trait-codegen/src/main/java/software/amazon/smithy/traitcodegen/generators/FromNodeMapperVisitor.java