build(rust): vendor rnix parser crate

third_party/crates/rnix-0.14.0/examples/test-nixpkgs.rs

@@ -0,0 +1,48 @@
+use std::{env, error::Error, fs, path::Path};
+
+fn main() -> Result<(), Box<dyn Error>> {
+    let path = env::var("NIX_PATH")?;
+    let nixpkgs =
+        path.split(':').find(|s| s.starts_with("nixpkgs=")).ok_or("no store path found")?;
+
+    println!("Nix store path: {}", nixpkgs);
+
+    recurse(Path::new(&nixpkgs["nixpkgs=".len()..]))
+}
+fn recurse(path: &Path) -> Result<(), Box<dyn Error>> {
+    if path.metadata()?.is_file() {
+        if path.extension().and_then(|s| s.to_str()) != Some("nix") {
+            return Ok(());
+        }
+
+        println!("Checking {}", path.display());
+        let original = fs::read_to_string(path)?;
+        if original.trim().is_empty() {
+            return Ok(());
+        }
+
+        let parsed = rnix::Root::parse(&original).tree().to_string();
+        if original != parsed {
+            eprintln!("Original input does not match parsed output!");
+            println!("Input:");
+            println!("----------");
+            println!("{}", original);
+            println!("----------");
+            println!("Output:");
+            println!("----------");
+            println!("{}", parsed);
+            println!("----------");
+            return Err("parsing error".into());
+        }
+        return Ok(());
+    } else {
+        for entry in path.read_dir()? {
+            let entry = entry?;
+            if entry.file_type()?.is_symlink() {
+                continue;
+            }
+            recurse(&entry.path())?;
+        }
+    }
+    Ok(())
+}