Attribute Macro entry 

#[entry]

Creates an unsafe program entry point (i.e. a kmain function).

It’s unsafe because you are not supposed to call it - it should only be called from the start-up code once initialisation is complete.

When placed on a function like:

#[entry]
fn foo() -> ! {
    panic!("On no")
}

You get something like:

#[doc(hidden)]
#[export_name = "kmain"]
pub unsafe extern "C" fn __aarch32_rt_kmain() -> ! {
    foo()
}

fn foo() -> ! {
    panic!("On no")
}

The symbol kmain is what the assembly code in aarch32-rt start-up code will jump to, and the extern "C" makes it sound to call from assembly.