Add RUSTC_BOOTSTRAP=-1 to make rustc pretend as stable compiler

compiler/rustc_feature/src/tests.rs

@@ -18,6 +18,16 @@ fn rustc_bootstrap_parsing() {
     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
+    // `RUSTC_BOOTSTRAP=0` is not recognized.
     assert!(!is_bootstrap("0", None));
+
+    // `RUSTC_BOOTSTRAP=-1` is force-stable, no unstable features allowed.
+    let is_force_stable = |krate| {
+        std::env::set_var("RUSTC_BOOTSTRAP", "-1");
+        matches!(UnstableFeatures::from_environment(krate), UnstableFeatures::Disallow)
+    };
+    assert!(is_force_stable(None));
+    // Does not support specifying any crate.
+    assert!(is_force_stable(Some("x")));
+    assert!(is_force_stable(Some("x,y,z")));
 }