Check whether compiler is nightly at compile time

That is it: how do I know whether the project is building by nightly or stable compiler? Something like this:

#[cfg(nightly_build)]
use std::ptr::Shared; // on nightly use system's version

#[cfg(not(nightly_build))]
use myshared::Shared; // on stable use my unoptimized Shared

You can do it with the rustc_version crate and a build.rs script:

extern crate rustc_version;
use rustc_version::{version_meta, Channel};

fn main() {
    // Set cfg flags depending on release channel
    match version_meta().unwrap().channel {
        Channel::Stable => {
            println!("cargo:rustc-cfg=RUSTC_IS_STABLE");
        }
        Channel::Beta => {
            println!("cargo:rustc-cfg=RUSTC_IS_BETA");
        }
        Channel::Nightly => {
            println!("cargo:rustc-cfg=RUSTC_IS_NIGHTLY");
        }
        Channel::Dev => {
            println!("cargo:rustc-cfg=RUSTC_IS_DEV");
        }
    }
}

Then check it with #[cfg(feature = "RUSTC_IS_STABLE")], etc in the Rust code.