diff --git a/src/libsyntax/feature_gate.rs b/src/libsyntax/feature_gate.rs
index f8eb4508b1606d0ba2b9c8c1137b2e33e236a860..2b4f03bd4f6ef6904b82d798cc87ca0aa7ecb3b9 100644
--- a/src/libsyntax/feature_gate.rs
+++ b/src/libsyntax/feature_gate.rs
@@ -36,6 +36,7 @@
 use parse::token::InternedString;
 
 use std::ascii::AsciiExt;
+use std::env;
 
 macro_rules! setter {
     ($field: ident) => {{
@@ -1296,6 +1297,23 @@ pub enum UnstableFeatures {
     Cheat
 }
 
+impl UnstableFeatures {
+    pub fn from_environment() -> UnstableFeatures {
+        // Whether this is a feature-staged build, i.e. on the beta or stable channel
+        let disable_unstable_features = option_env!("CFG_DISABLE_UNSTABLE_FEATURES").is_some();
+        // The secret key needed to get through the rustc build itself by
+        // subverting the unstable features lints
+        let bootstrap_secret_key = option_env!("CFG_BOOTSTRAP_KEY");
+        // The matching key to the above, only known by the build system
+        let bootstrap_provided_key = env::var("RUSTC_BOOTSTRAP_KEY").ok();
+        match (disable_unstable_features, bootstrap_secret_key, bootstrap_provided_key) {
+            (_, Some(ref s), Some(ref p)) if s == p => UnstableFeatures::Cheat,
+            (true, _, _) => UnstableFeatures::Disallow,
+            (false, _, _) => UnstableFeatures::Allow
+        }
+    }
+}
+
 fn maybe_stage_features(span_handler: &Handler, krate: &ast::Crate,
                         unstable: UnstableFeatures) {
     let allow_features = match unstable {
diff --git a/src/libsyntax/parse/mod.rs b/src/libsyntax/parse/mod.rs
index 5aa0efdec11a2565a75323770dbf1817502a6dd0..1e286c143de9b4b5ad97718343f6f1ea2c9ed053 100644
--- a/src/libsyntax/parse/mod.rs
+++ b/src/libsyntax/parse/mod.rs
@@ -14,6 +14,7 @@
 use codemap::CodeMap;
 use syntax_pos::{self, Span, FileMap};
 use errors::{Handler, ColorConfig, DiagnosticBuilder};
+use feature_gate::UnstableFeatures;
 use parse::parser::Parser;
 use parse::token::InternedString;
 use ptr::P;
@@ -42,6 +43,7 @@
 /// Info about a parsing session.
 pub struct ParseSess {
     pub span_diagnostic: Handler, // better be the same as the one in the reader!
+    pub unstable_features: UnstableFeatures,
     /// Used to determine and report recursive mod inclusions
     included_mod_stack: RefCell<Vec<PathBuf>>,
     code_map: Rc<CodeMap>,
@@ -60,6 +62,7 @@ pub fn new() -> ParseSess {
     pub fn with_span_handler(handler: Handler, code_map: Rc<CodeMap>) -> ParseSess {
         ParseSess {
             span_diagnostic: handler,
+            unstable_features: UnstableFeatures::from_environment(),
             included_mod_stack: RefCell::new(vec![]),
             code_map: code_map
         }