[codegen/go] Fix accessors on struct ptr outputs

[lukehoban]

The accesor methods on nestred struct Ptr outputs were previously not accepting pointer typed inputs to their applier callbacks as they should, and would thus always panic if used.

The "simple" fix would be to just accept the pointer type and blindly dereference it. But this doesn't seem like the right experience - it would make these accessors very unsafe to use in practice.

Instead, this PR implements the accessors on pointer-typed outputs as nil-coaslescing, always lifting the output type into a pointer type and flowing a nil value into the result type. This ensures the accessor will not nil-deref, and that user code can handle the nil value itself (or use .Apply directly to implement more specialized behaviour).

Before:

// Name of your S3 bucket.
func (o BuildStorageLocationPtrOutput) Bucket() pulumi.StringOutput {
	return o.ApplyT(func(v BuildStorageLocation) string { return v.Bucket }).(pulumi.StringOutput)
}

After:

// Name of your S3 bucket.
func (o BuildStorageLocationPtrOutput) Bucket() pulumi.StringPtrOutput {
	return o.ApplyT(func(v *BuildStorageLocation) *string {
		if v == nil {
			return nil
		}
		return &v.Bucket
	}).(pulumi.StringPtrOutput)
}

However, due to the decision to have this more usable behaviour, this is a breaking change, as some/many accessors now return a pointer type when they previously did not.

Fixes pulumi/pulumi-azure#530.

[EvanBoyle]

Quick fix! 🚢

[elsesiy]

Thanks @lukehoban, this must be a new record 🥇

[lukehoban]

However, due to the decision to have this more usable behaviour, this is a breaking change, as some/many accessors now return a pointer type when they previously did not.

Notably though, the existing methods were guaranteed to panic if called, so I don’t think this is actually a breaking change in practice.

[lukehoban]

Here's an example of codegen differences in AWS.

https://github.com/pulumi/pulumi-aws/compare/lukehoban/goaccessor#diff-1f1b5ecc107eb8868597adfb597e7e16

Manually confirmed that this builds cleanly there.