there's unstable feature no_core

[bingmatv]

There's unstable feature

#![no_core]

Is it useful for developing OS?

[bjorn3]

No, it is not useful at all. It is completely useless for any code that a user of rust will write. #![no_core] is super unstable and requires defining a ton of language items and compiler intrinsics with any mistake potentially causing the compiler itself to crash. The only reason it exists at all is because libcore needs it to avoid depending on itself.