Disable SIMD when targeting stable

[richarddd]

Hi,

It would be great if i can still use the simd feature, but disable its functionality when not building for nightly.

The simplest way to accomplish this would be in a build script call this:

fn set_nightly_cfg() {
    let rustc = std::env::var("RUSTC").unwrap();
    let version = std::process::Command::new(rustc)
        .arg("--version")
        .output()
        .unwrap();

    assert!(version.status.success());

    let stdout = String::from_utf8(version.stdout).unwrap();
    assert!(stdout.contains("rustc"));
    let nightly = stdout.contains("nightly") || stdout.contains("dev");
    if nightly {
        println!("cargo:rustc-cfg=rust_nightly");
        println!("cargo::rustc-check-cfg=cfg(rust_nightly)");
    }
}

And simd_portable + simd featues can then be feature gated like:

#![cfg_attr(rust_nightly, feature(portable_simd))]

Unfortunately cargo has no way to enable specific features on nightly only