[Feature Request] [proposal] Have the Writable trait require a function that returns an estimate of the buffer size needed.

[martinvuyk]

Review Mojo's priorities

• I have read the roadmap and priorities and I believe this request falls within the priorities.

What is your request?

Have the Writable trait require a function that returns an estimate of the buffer size needed. The function could return 0 in the case of datastructures that don't know it ahead of time.

What is your motivation for this change?

Currently many uses of the writer API look like this:

fn __str__(self) -> String:
    var res = String(capacity=some_estimate) # most of the time it's even an empty constructor
    res.write(self)
    return res

One either has to know the exact size of each component or guess sizes. These calculations aren't of any use when the type's write_to function gets called without reserving it that way.

Any other details?

This change will be useful for:

• Letting the type define its specific memory needs, without having to reserve the string size beforehand (which is not scalable for deeply nested datastructures)
• To avoid memory allocation overhead in loops
• And maybe it also opens the door to other optimizations

CC: @jackos