build.rs

// The rustc-cfg listed below are considered public API, but it is *unstable*
// and outside of the normal semver guarantees:
//
// - `crossbeam_no_atomic_cas`
//      Assume the target does *not* support atomic CAS operations.
//      This is usually detected automatically by the build script, but you may
//      need to enable it manually when building for custom targets or using
//      non-cargo build systems that don't run the build script.
//
// With the exceptions mentioned above, the rustc-cfg emitted by the build
// script are *not* public API.

#![warn(rust_2018_idioms)]

use std::env;

include!("no_atomic.rs");

fn main() {
    let target = match env::var("TARGET") {
        Ok(target) => target,
        Err(e) => {
            println!(
                "cargo:warning={}: unable to get TARGET environment variable: {}",
                env!("CARGO_PKG_NAME"),
                e
            );
            return;
        }
    };

    let cfg = match autocfg::AutoCfg::new() {
        Ok(cfg) => cfg,
        Err(e) => {
            println!(
                "cargo:warning={}: unable to determine rustc version: {}",
                env!("CARGO_PKG_NAME"),
                e
            );
            return;
        }
    };

    // Note that this is `no_`*, not `has_*`. This allows treating as the latest
    // stable rustc is used when the build script doesn't run. This is useful
    // for non-cargo build systems that don't run the build script.
    if NO_ATOMIC_CAS.contains(&&*target) {
        println!("cargo:rustc-cfg=crossbeam_no_atomic_cas");
    }

    if !cfg.probe_rustc_version(1, 61) {
        println!("cargo:rustc-cfg=crossbeam_no_const_fn_trait_bound");
    }

    println!("cargo:rerun-if-changed=no_atomic.rs");
}