alt Hacker Newsbuild.rs changes nothing about how easy it is to integrate with C. What does simplify: figuring out how to supply library you need at build time.
Which is the result of how bad dependency managment is outside (i.e. DLL-hell).
Pretty much all other use cases of build.rs can be sandboxed. Well, there is sqlx that wants to connect to database at expansion time to compile check-queries (yew).
sqlx at least has the (optional) offline mode, where you "cargo sqlx prepare" once (which wants access to a db) and then you can build in offline mode which typechecks your queries against local files.
Although I suppose that's still doing a lot of shenanigans at compile time. It could be sandboxed pretty well (theoretically). I'd hate to give it up completely though, getting a compile time error when SELECT query params or return values have type mismatches is extremely nice.