Add regression test for const prop

Author: oli-obk

src/test/ui/const-eval/no_lint_for_statically_known_error.rs

@@ -0,0 +1,28 @@
+// Copyright 2018 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// must-compile-successfully
+
+// if `X` were used instead of `x`, `X - 10` would result in a lint.
+// This file should never produce a lint, no matter how the const
+// propagator is improved.
+
+#![deny(warnings)]
+
+const X: u32 = 5;
+
+fn main() {
+    let x = X;
+    if x > 10 {
+        println!("{}", x - 10);
+    } else {
+        println!("{}", 10 - x);
+    }
+}