no terrible cloning!

src/main.rs

@@ -1,3 +1,10 @@
-fn main() {
-    println!("Hello, world!");
+use std::error::Error;
+
+fn main() -> Result<(), Box<dyn Error>> {
+    let input_file = std::env::args().nth(1).expect("first argument");
+    let src = std::fs::read_to_string(input_file)?;
+
+    parser::parse_file(src);
+
+    Ok(())
 }