You would need at least a bridging layer (of Objective-C) to so that your app code could use the system frameworks and respond to events (user input, network, sensors, etc).
Objective-C shares calling conventions and struct layout with C, so theoretically you could directly call the system frameworks from Rust via extern "C" functions.
It'd be super cool to see a 100% Rust (with extern "C" calls) Hello, world! iOS compatible app.
Not even theoretically, the bindings for the runtime are out there [1] with some nifty helper macros.