public Builder input(ToShapeId inputShape) {
            input = inputShape == null ? UnitTypeTrait.UNIT : inputShape.toShapeId();
            return this;
        }
Sets the input shape ID of the operation.

@param inputShape Shape ID that MUST reference a structure.
@return Returns the builder.
smithy-model/src/main/java/software/amazon/smithy/model/shapes/OperationShape.java