test(cargo-rustc): show the current precedence

Author: weihanglo

tests/testsuite/rustc.rs

@@ -795,3 +795,37 @@ windows
         )
         .run();
 }
+
+#[cargo_test]
+fn precedence() {
+    // Ensure that the precedence of cargo-rustc is only lower than RUSTFLAGS,
+    // but higher than most flags set by cargo.
+    //
+    // See rust-lang/cargo#14346
+    let p = project()
+        .file(
+            "Cargo.toml",
+            r#"
+            [package]
+            name = "foo"
+            edition = "2021"
+
+            [lints.rust]
+            unexpected_cfgs = "allow"
+            "#,
+        )
+        .file("src/lib.rs", "")
+        .build();
+
+    p.cargo("rustc --release -v -- --cfg cargo_rustc -C strip=symbols")
+        .env("RUSTFLAGS", "--cfg from_rustflags")
+        .with_stderr_data(
+            str![[r#"
+[COMPILING] foo v0.0.0 ([ROOT]/foo)
+[RUNNING] `rustc [..]--cfg cargo_rustc -C strip=symbols [..]-C strip=debuginfo [..]--cfg from_rustflags`
+[FINISHED] `release` profile [optimized] target(s) in [ELAPSED]s
+
+"#]]
+        )
+        .run();
+}