Ah, okay I thought that source generators had more limitations compared to F#. Turns out they can make arbitrary calls, including across a network, which previously I thought was only in the domain of type providers:
> For Schema.NET, we do a HTTP request to get the JSON we need to build
> For Schema.NET, we do a HTTP request to get the JSON we need to build
https://turnerj.com/blog/the-pain-points-of-csharp-source-ge...
Also phillipcarter says yes so I'll happily take his view over mine.