chore: add new test to handle RUSTFLAGS env var case

Author: not-matthias

crates/cargo-codspeed/tests/cargo_config.in/benches/cargo_config_bench.rs

@@ -8,6 +8,9 @@ fn bench_failing_without_custom_flag(c: &mut Criterion) {
             compile_error!(
                 "custom_feature_flag is not enabled - .cargo/config.toml rustflags not applied"
             );
+
+            #[cfg(rustflags_feature_flag)]
+            compile_error!("rustflags_feature_flag was set");
         })
     });
 }

crates/cargo-codspeed/tests/cargo_config.in/build.rs

@@ -1,3 +1,4 @@
 fn main() {
     println!("cargo::rustc-check-cfg=cfg(custom_feature_flag)");
+    println!("cargo::rustc-check-cfg=cfg(rustflags_feature_flag)");
 }

crates/cargo-codspeed/tests/cargo_config.rs

@@ -33,3 +33,37 @@ fn test_cargo_config_rustflags() {
 
     teardown(tmp_dir);
 }
+
+#[test]
+fn test_cargo_config_with_rustflags_env() {
+    let tmp_dir = setup("tests/cargo_config.in", Project::Simple);
+
+    std::env::set_var("RUSTFLAGS", "--cfg rustflags_feature_flag");
+
+    let output = Command::new("cargo")
+        .arg("bench")
+        .arg("--no-run")
+        .current_dir(&tmp_dir)
+        .output()
+        .expect("Failed to execute cargo bench");
+    assert!(!output.status.success());
+    let stdout = String::from_utf8_lossy(&output.stderr);
+    assert!(
+        stdout.contains("rustflags_feature_flag was set"),
+        "Incorrect output: {stdout}"
+    );
+
+    // Test that cargo codspeed build also works with the custom flag
+    let output = cargo_codspeed(&tmp_dir)
+        .arg("build")
+        .output()
+        .expect("Failed to execute cargo codspeed build");
+    assert!(!output.status.success());
+    let stdout = String::from_utf8_lossy(&output.stderr);
+    assert!(
+        stdout.contains("rustflags_feature_flag was set"),
+        "Incorrect output: {stdout}"
+    );
+
+    teardown(tmp_dir);
+}