Attribute no_std 

Prevents automatically linking the standard library.

Written as an inner attribute at the top of the crate root, the no_std attribute stops the compiler from linking std into the crate, and only links core.

The attribute also swaps which prelude gets inserted, with the core prelude replacing the std prelude. Items like Option, Result, and the primitive types remain available from core without any imports needed:

#![no_std]

fn halve(x: u32) -> Option<u32> {
    if x % 2 == 0 { Some(x / 2) } else { None }
}

Anything that requires heap memory allocations is not part of core. Linking the alloc crate explicitly can be used to include these types:

#![no_std]

extern crate alloc;

use alloc::vec::Vec;

A no_std binary also removes the startup routine and default panic handler std normally provides. It must define its own #[panic_handler], and typically its own entry point by additionally using #![no_main]:

#![no_std]
#![no_main]

use core::panic::PanicInfo;

#[panic_handler]
fn on_panic(_info: &PanicInfo) -> ! {
    loop {}
}

For more information, see the Reference on the no_std attribute.