make is_nightly_build a method on UnstableFeatures

src/librustc/session/config.rs

@@ -1583,10 +1583,7 @@ pub mod nightly_options {
     }
 
     pub fn is_nightly_build() -> bool {
-        match get_unstable_features_setting() {
-            UnstableFeatures::Allow | UnstableFeatures::Cheat => true,
-            _ => false,
-        }
+        UnstableFeatures::from_environment().is_nightly_build()
     }
 
     pub fn check_nightly_options(matches: &getopts::Matches, flags: &[RustcOptGroup]) {

src/libsyntax/feature_gate.rs

@@ -1312,6 +1312,13 @@ impl UnstableFeatures {
             (false, _, _) => UnstableFeatures::Allow
         }
     }
+
+    pub fn is_nightly_build(&self) -> bool {
+        match *self {
+            UnstableFeatures::Allow | UnstableFeatures::Cheat => true,
+            _ => false,
+        }
+    }
 }
 
 fn maybe_stage_features(span_handler: &Handler, krate: &ast::Crate,