format

    fn format(data: FormatData, w: *Writer) Writer.Error!void {
        switch (data.value.unwrap()) {
            .instruction => |instruction| try Function.Instruction.Index.format(.{
                .instruction = instruction,
                .function = data.function,
                .builder = data.builder,
                .flags = data.flags,
            }, w),
            .constant => |constant| try Constant.format(.{
                .constant = constant,
                .builder = data.builder,
                .flags = data.flags,
            }, w),
            .metadata => unreachable,
        }
    }