tidy: Re-enable check for inline unit tests

compiler/rustc_feature/src/tests.rs

@@ -0,0 +1,23 @@
+use super::UnstableFeatures;
+
+#[test]
+fn rustc_bootstrap_parsing() {
+    let is_bootstrap = |env, krate| {
+        std::env::set_var("RUSTC_BOOTSTRAP", env);
+        matches!(UnstableFeatures::from_environment(krate), UnstableFeatures::Cheat)
+    };
+    assert!(is_bootstrap("1", None));
+    assert!(is_bootstrap("1", Some("x")));
+    // RUSTC_BOOTSTRAP allows specifying a specific crate
+    assert!(is_bootstrap("x", Some("x")));
+    // RUSTC_BOOTSTRAP allows multiple comma-delimited crates
+    assert!(is_bootstrap("x,y,z", Some("x")));
+    assert!(is_bootstrap("x,y,z", Some("y")));
+    // Crate that aren't specified do not get unstable features
+    assert!(!is_bootstrap("x", Some("a")));
+    assert!(!is_bootstrap("x,y,z", Some("a")));
+    assert!(!is_bootstrap("x,y,z", None));
+
+    // this is technically a breaking change, but there are no stability guarantees for RUSTC_BOOTSTRAP
+    assert!(!is_bootstrap("0", None));
+}