print errors to stderr

src/main.rs

@@ -2,7 +2,7 @@ fn main() {
     let exit_code = match n2::run::run() {
         Ok(code) => code,
         Err(err) => {
-            println!("n2: error: {}", err);
+            eprintln!("n2: error: {}", err);
             1
         }
     };

tests/e2e/bindings.rs

@@ -42,7 +42,7 @@ build out: my_rule
     )?;
 
     let out = space.run(&mut n2_command(vec!["out"]))?;
-    assert_output_contains(&out, "unexpected variable \"my_var\"");
+    assert_stderr_contains(&out, "unexpected variable \"my_var\"");
     Ok(())
 }
 

tests/e2e/missing.rs

@@ -11,7 +11,7 @@ fn missing_input() -> anyhow::Result<()> {
     )?;
 
     let out = space.run(&mut n2_command(vec!["out"]))?;
-    assert_output_contains(&out, "input in missing");
+    assert_stderr_contains(&out, "input in missing");
 
     Ok(())
 }

tests/e2e/mod.rs

@@ -43,6 +43,16 @@ pub fn assert_output_contains(out: &std::process::Output, text: &str) {
     }
 }
 
+pub fn assert_stderr_contains(out: &std::process::Output, text: &str) {
+    let out = std::str::from_utf8(&out.stderr).unwrap();
+    if !out.contains(text) {
+        panic!(
+            "assertion failed; expected stderr to contain {:?} but got:\n{}",
+            text, out
+        );
+    }
+}
+
 pub fn assert_output_not_contains(out: &std::process::Output, text: &str) {
     let out = std::str::from_utf8(&out.stdout).unwrap();
     if out.contains(text) {