add cfg(nightly) attribute

nixide/build.rs

@@ -0,0 +1,15 @@
+use std::env;
+
+fn main() {
+    // allow the `cfg(nightly)` attribute
+    println!("cargo::rustc-check-cfg=cfg(nightly)");
+
+    // NOTE: This allows nixide to conditionally compile based
+    // NOTE: whether a user has access to nightly features.
+    if let Ok(toolchain) = env::var("RUSTUP_TOOLCHAIN")
+        && toolchain.contains("nightly")
+    {
+        // enable the `nightly` flag
+        println!("cargo::rustc-cfg=nightly");
+    }
+}