http://acm.timus.ru/problem.aspx?num=1243 with no_std

main.rs

#![feature(lang_items)]
#![feature(start)]
#![no_std]
#![no_main]

// These functions are used by the compiler, but not
// for a bare-bones hello world. These are normally
// provided by libstd.
#[lang = "eh_personality"]
#[no_mangle]
pub extern fn rust_eh_personality() {
}

// This function may be needed based on the compilation target.
#[lang = "eh_unwind_resume"]
#[no_mangle]
pub extern fn rust_eh_unwind_resume() {
}

#[lang = "panic_fmt"]
#[no_mangle]
pub extern fn rust_begin_panic(_msg: core::fmt::Arguments,
                               _file: &'static str,
                               _line: u32) -> ! {
    loop {}
}

// Entry point for this program.
#[no_mangle] // ensure that this symbol is called `main` in the output
pub extern fn main(_argc: i32, _argv: *const *const u8) -> i32 {
    extern {
        fn printf(fmt: *const u8, ...) -> i32;
        fn scanf(format: *const u8, ...) -> i32;
    }
    let mut buffer = [0u8; 100];
    unsafe {
        scanf("%s".as_ptr(), buffer.as_mut_ptr());
        let mut remain = 0;
        for b in buffer.iter() {
            if *b >= 0x30 {
                remain = ((remain * 10) + b - 0x30) % 7;
            } else {
                break;
            }
        }
        let result = [remain + 0x30, 0];
        printf(result.as_ptr());
    }
    0
}