Add check for existing generated file

build.rs

@@ -77,9 +77,11 @@ fn main() {
             ),
         )
         .unwrap();
+        println!("cargo:rerun-if-changed={}", xlap);
+    } else if !dest_path.exists() {
+        panic!("Specify the absolute path to your `xlap.yml` using the `XLAP` environment variable on the first build");
+    }
     println!("cargo:rerun-if-changed=build.rs");
     println!("cargo:rerun-if-changed=src/lib.rs");
-        println!("cargo:rerun-if-changed={}", xlap);
     println!("cargo:rerun-if-env-changed=XLAP");
-    }
 }