private void writeMixins(Shape shape) {
            if (shape.getMixins().size() == 1) {
                codeWriter.writeInline("with [$I] ", shape.getMixins().iterator().next());
            } else if (shape.getMixins().size() > 1) {
                codeWriter.write("with [").indent();
                for (ShapeId id : shape.getMixins()) {
                    // Trailing spaces are trimmed.
                    codeWriter.write("$I", id);
                }
                codeWriter.dedent().writeInline("] ");
            }
        }
Serializes shapes in the IDL format.
smithy-model/src/main/java/software/amazon/smithy/model/shapes/SmithyIdlModelSerializer.java