This seems to be an omission in both the evaluator implementation and in the C API for it. A virtual destructor does exist for ExternalValueBase, but since Value relies entirely on GC and mkExternal doesn't register a finalizer, it seems that this destructor is never called.

It's worth noting that the GC used by Nix can not guarantee that finalizers are called in the first place, as it is a conservative GC, so it is best not to rely on finalization. (Also GC will not run before the process exits, unless e.g. it's the Nix CLI and NIX_SHOW_STATS=1)

Nonetheless, it should be added, because release a resource most of the time, or even sometimes, is better than never.

I'm a bit wary of adding it without ever calling it, because that will cause untested code to start running in the future, which could be a huge surprise when it starts causing crashes etc.
So either the whole thing needs to be implemented correctly and/or we add a test utility that forces the finalization, just for the purpose of unit testing.

also, I noticed the equality function does not take a value descriptor for other. I guess that's because most programs won't have multiple external types. I also happen to be using Rust, so ultimately my void* v will be something like dyn Any or a wrapper containing an Any; so naturally I can use that to check type identity. but it is also possible to load multiple plugins, right? what if several add external types? then I have no idea what the other value has for a type.

is the value descriptor checked for equality/identity first? that's the only reasonable way this API makes sense, and it's consistent with the documented behavior of not being equal if the other value is of a primitive type. but it's not documented that the equality does any such check before invoking the callback.