I was even considering if we had to modify BuilderGCConfig. Enabled to be a pointer, or if we'd only need to modify this temporary struct (unmarshal to xx and if it's nil, then we set the default to true, which we already do below), but that would require New() to set the default (for cases where the UnmarshalJSON is not called, and looks like we'd have to patch other places to start with New() (default config) instead of starting with an empty config.

Modifying the temporary struct doesn't work because UnmarshalJSON only gets called if there's a gc section. So doing the change in the temporary struct works for this:

"builder": {"gc": {}}

But it doesn't work when "gc" is missing or if the entire configuration is missing. At the same time, doing it this way ensures that "enabled": null isn't accidentally an acceptable value. If the temporary struct has a pointer, then it would be ok for someone to set null as the value. If we set Enabled to true before running unmarshal then it will be set to true if the attribute isn't present otherwise it'll be overridden and null will cause it to error.

Yeah, I came to that conclusion as well; I had a bit of a brain-fart and thought it would be called unconditionally, but that's that didn't really make sense;

for cases where the UnmarshalJSON is not called