I'd say "pretty low" on the idiomatic scale, however a very desirable thing would be for them to manage to make most of the generated code not unsafe. Limiting the scope of unsafe would make what would remain easier to convert to safe rust, and then you're off to the refactoring race with much more confidence.

A pretty good go would be the ability to recognise wide-spread high-level C patterns and be able to convert them to idiomatic Rust pattern, but even that would be difficult, generally speaking idiomatic C and Rust would likely be structured quite differently, especially on the data(-structure) side.