(zilch lang rust cargo): parse cfg values at runtime

lang/rust/cfg-fetch.rs

@@ -0,0 +1,10 @@
+use std::{env::var_os, fs::{create_dir, File}, os::unix::process::CommandExt, process::Command};
+
+fn main() {
+    Err(
+        Command::new(var_os("rustc").unwrap())
+        .arg("--print=cfg")
+        .stdout(File::create(var_os("out").unwrap()).unwrap())
+        .exec())
+    .unwrap()
+}