Ideas like this come up semi-regularly. My current understanding is that:
Most recent threads about this seem to be focused on sandboxing build scripts in particular, though from what little I know the issues there seem very similar to runtime sandboxing.
The Rust Secure Code Working Group has seemingly active issues on Build-time sandboxing, Safety-oriented static analysis tooling, Reduce the use of unsafe in the ecosystem and so on, which imo are more cost-effective/less ecosystem-splitting solutions for improving our overall security than a language-level feature.