fix: cfg nighyly for integration test for cargo-dist to work on stable

tests/integration/build.rs

@@ -0,0 +1,8 @@
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    let output = std::process::Command::new("rustc").arg("-V").output()?;
+    // set nightly cfg if current toolchain is nightly
+    if std::str::from_utf8(&output.stdout)?.contains("nightly") {
+        println!("cargo:rustc-cfg=nightly");
+    }
+    Ok(())
+}

tests/integration/src/lib.rs

@@ -1,4 +1,4 @@
-#![feature(c_variadic)]
+#![cfg_attr(nightly, feature(c_variadic))]
 /// Documentation for struct AUnitStruct.
 pub struct AUnitStruct;
 
@@ -108,8 +108,10 @@ where
 {
 }
 /// # Safety
+#[cfg(nightly)]
 pub unsafe extern "C" fn variadic(_: *const (), _name: ...) {}
 /// # Safety
+#[cfg(nightly)]
 pub unsafe extern "C" fn variadic_multiline(_: *const (), _: *mut (), _name: ...) {}
 
 pub trait ATraitWithGAT {