As this is provably impossible, this "solution" introduces exactly one of the problems the author was trying to avoid:
> can't be validated
That's why 'conservative'. I.e. it's allowed to reject a valid, terminating program, but if it does pass the check, your program is guaranteed to terminate. This is something that's possible, the only question is the tradeoff between the subset of the syntax and how complicated is the static analysis.
- Clang (depending on warnings level) may reject a valid program -- doesn't make it less useful, you just suppress the check for the offending line and carry on
- Rust borrow checker may be seemingly picky and reject a perfectly valid program form your viewpoint. Does it make it less useful? I wouldn't say so.
My point is that the validator can't give you the safety property that is claimed as a defence against one of the inherent issues with this approach.
I mostly have a non-malicious user in mind though (i.e. end-user software, where the software and the config have same permissions)
If you do have such security concerns, you probably need a sandbox at some point. E.g. big source of my frustration are CI pipelines -- they run isolated anyway and execute arbitrary code. Having a YAML there does nothing for the security.