Add feature-gate to calling const fn

src/librustc/middle/check_const.rs

@@ -199,8 +199,23 @@ fn add_qualif(&mut self, qualif: ConstQualif) {
     }
 
     /// Returns true if the call is to a const fn or method.
-    fn handle_const_fn_call(&mut self, def_id: ast::DefId, ret_ty: Ty<'tcx>) -> bool {
+    fn handle_const_fn_call(&mut self,
+                            expr: &ast::Expr,
+                            def_id: ast::DefId,
+                            ret_ty: Ty<'tcx>)
+                            -> bool {
         if let Some(fn_like) = const_eval::lookup_const_fn_by_id(self.tcx, def_id) {
+            if self.mode != Mode::Var && !self.tcx.sess.features.borrow().const_fn {
+                self.tcx.sess.span_err(
+                    expr.span,
+                    &format!("const fns are an unstable feature"));
+                fileline_help!(
+                    self.tcx.sess,
+                    expr.span,
+                    "in Nightly builds, add `#![feature(const_fn)]` to the crate \
+                     attributes to enable");
+            }
+
             let qualif = self.fn_like(fn_like.kind(),
                                       fn_like.decl(),
                                       fn_like.body(),
@@ -657,7 +672,7 @@ fn check_expr<'a, 'tcx>(v: &mut CheckCrateVisitor<'a, 'tcx>,
                 }
                 Some(def::DefMethod(did, def::FromImpl(_))) |
                 Some(def::DefFn(did, _)) => {
-                    v.handle_const_fn_call(did, node_ty)
+                    v.handle_const_fn_call(e, did, node_ty)
                 }
                 _ => false
             };
@@ -677,7 +692,7 @@ fn check_expr<'a, 'tcx>(v: &mut CheckCrateVisitor<'a, 'tcx>,
                 _ => None
             };
             let is_const = match method_did {
-                Some(did) => v.handle_const_fn_call(did, node_ty),
+                Some(did) => v.handle_const_fn_call(e, did, node_ty),
                 None => false
             };
             if !is_const {

src/libsyntax/feature_gate.rs

@@ -332,7 +332,8 @@ pub struct Features {
     /// spans of #![feature] attrs for stable language features. for error reporting
     pub declared_stable_lang_features: Vec<Span>,
     /// #![feature] attrs for non-language (library) features
-    pub declared_lib_features: Vec<(InternedString, Span)>
+    pub declared_lib_features: Vec<(InternedString, Span)>,
+    pub const_fn: bool,
 }
 
 impl Features {
@@ -352,7 +353,8 @@ pub fn new() -> Features {
             unmarked_api: false,
             negate_unsigned: false,
             declared_stable_lang_features: Vec::new(),
-            declared_lib_features: Vec::new()
+            declared_lib_features: Vec::new(),
+            const_fn: false,
         }
     }
 }
@@ -802,7 +804,8 @@ fn check_crate_inner<F>(cm: &CodeMap, span_handler: &SpanHandler,
         unmarked_api: cx.has_feature("unmarked_api"),
         negate_unsigned: cx.has_feature("negate_unsigned"),
         declared_stable_lang_features: accepted_features,
-        declared_lib_features: unknown_features
+        declared_lib_features: unknown_features,
+        const_fn: cx.has_feature("const_fn"),
     }
 }
 

src/test/compile-fail/const-fn-stability.rs

@@ -25,4 +25,9 @@ impl Foo for u32 {
     const fn foo() -> u32 { 0 } //~ ERROR const fn is unstable
 }
 
-fn main() { }
+static FOO: usize = foo();
+const BAR: usize = foo();
+
+fn main() {
+    let x: [usize; foo()] = [];
+}