the const evaluator might run before check_const

So we cannot assume that the function call was marked NOT_CONST by check_const.

src/librustc/middle/const_eval.rs

@@ -1031,24 +1031,23 @@ pub fn eval_const_expr_partial<'tcx>(tcx: &ty::ctxt<'tcx>,
               UncheckedExprNoHint // we cannot reason about UncheckedExprHint here
           };
           let (
-            decl,
-            unsafety,
-            abi,
-            block,
+              decl,
+              unsafety,
+              abi,
+              block,
+              constness,
           ) = match try!(eval_const_expr_partial(tcx, callee, sub_ty_hint, fn_args)) {
               Function(did) => if did.is_local() {
                   match tcx.map.find(did.index.as_u32()) {
                       Some(ast_map::NodeItem(it)) => match it.node {
                           hir::ItemFn(
                               ref decl,
                               unsafety,
-                              _, // no need to check for constness... either check_const
-                                 // already forbids this or we const eval over whatever
-                                 // we want
+                              constness,
                               abi,
                               _, // ducktype generics? types are funky in const_eval
                               ref block,
-                          ) => (decl, unsafety, abi, block),
+                          ) => (decl, unsafety, abi, block, constness),
                           _ => signal!(e, NonConstPath),
                       },
                       _ => signal!(e, NonConstPath),
@@ -1058,6 +1057,15 @@ pub fn eval_const_expr_partial<'tcx>(tcx: &ty::ctxt<'tcx>,
               },
               _ => signal!(e, NonConstPath),
           };
+          if let ExprTypeChecked = ty_hint {
+              // no need to check for constness... either check_const
+              // already forbids this or we const eval over whatever
+              // we want
+          } else {
+              // we don't know much about the function, so we force it to be a const fn
+              // so compilation will fail later in case the const fn's body is not const
+              assert_eq!(constness, hir::Constness::Const)
+          }
           assert_eq!(decl.inputs.len(), args.len());
           assert_eq!(unsafety, hir::Unsafety::Normal);
           assert_eq!(abi, abi::Abi::Rust);