I don't know if you can toggle it from the front-end, but the Polonius borrow checker includes a "location insensitive" mode which is faster but accepts fewer programs.
I think the GP had something else in mind. You're mentioning a mode where the rules are more stringent, allowing for a faster check and consequently faster compile times. The GP is pictuting a mode where the checks are skipped altogether, only the necessary transformations occur with an assumption that everything is already correct. I'm weary of having something like that outside of nightly, unless cargo publish had more checks than it does today.