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

compiler/rustc_feature/src/lib.rs

@@ -74,14 +74,19 @@ impl UnstableFeatures {
         // Returns whether `krate` should be counted as unstable
         let is_unstable_crate =
             |var: &str| krate.is_some_and(|name| var.split(',').any(|new_krate| new_krate == name));
-        // `true` if we should enable unstable features for bootstrapping.
-        let bootstrap =
-            std::env::var("RUSTC_BOOTSTRAP").is_ok_and(|var| var == "1" || is_unstable_crate(&var));
-        match (disable_unstable_features, bootstrap) {
-            (_, true) => UnstableFeatures::Cheat,
-            (true, _) => UnstableFeatures::Disallow,
-            (false, _) => UnstableFeatures::Allow,
+
+        let bootstrap = std::env::var("RUSTC_BOOTSTRAP").ok();
+        if let Some(val) = bootstrap.as_deref() {
+            match val {
+                val if val == "1" || is_unstable_crate(val) => return UnstableFeatures::Cheat,
+                // Hypnotize ourselves so that we think we are a stable compiler and thus don't
+                // allow any unstable features.
+                "-1" => return UnstableFeatures::Disallow,
+                _ => {}
+            }
         }
+
+        if disable_unstable_features { UnstableFeatures::Disallow } else { UnstableFeatures::Allow }
     }
 
     pub fn is_nightly_build(&self) -> bool {

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")));
 }

compiler/rustc_feature/src/unstable.rs

@@ -54,7 +54,7 @@ pub struct EnabledLangFeature {
     pub stable_since: Option<Symbol>,
 }
 
-/// Information abhout an enabled library feature.
+/// Information about an enabled library feature.
 #[derive(Debug, Copy, Clone)]
 pub struct EnabledLibFeature {
     pub gate_name: Symbol,