[Go] Support ForceDefaults

[chriscraws]

Go doesn't support the ForceDefaults command, so as far as I know, there's no way to add default values to a vtable (or even zero-values for vtable entries that have no default).

Currently, the VTable methods such as PrependUint64Slot take a value to set, as well as a default value. In cases where there is no default value, the type's zero value is used:

func (b *Builder) PrependUint64Slot(o int, x, d uint64) {
	if x != d {
		b.PrependUint64(x)
		b.Slot(o)
	}
}

The generated code has no way of not providing a default value d.

My proposal is to modify the condition from x != d to b.forceDefaults || x != d and to add a ForceDefaults(b bool) method to the Builder struct to set the new variable. Happy to provide a pull request.

[aardappel]

force_defaults as in C++ has always been a bit of a hack, and not the way FlatBuffers is intended to be used.

We now have a much better way of doing this on a per-field basis: #6014
So the best way forward would be to implement this functionality in Go.
@CasperN

[CasperN]

Yep, I agree. This looks like the optional scalars feature in #6014. It hasn't been implemented in Go since the main maintainer has been unavailable. I recommend implementing that design if you choose to you PR. I'll be happy to review it. The issue is super long but I tried to keep a summary in the top comment along with links to implementations.

[github-actions]

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