add test for calling non-const fn

src/test/ui/consts/miri_unleashed/non_const_fn.rs

@@ -0,0 +1,12 @@
+// compile-flags: -Zunleash-the-miri-inside-of-you
+#![allow(const_err)]
+
+// A test demonstrating that we prevent calling non-const fn during CTFE.
+
+fn foo() {}
+
+const C: () = foo(); //~ WARN: skipping const checks
+
+fn main() {
+    println!("{:?}", C); //~ ERROR: evaluation of constant expression failed
+}

src/test/ui/consts/miri_unleashed/non_const_fn.stderr

@@ -0,0 +1,15 @@
+warning: skipping const checks
+  --> $DIR/non_const_fn.rs:8:15
+   |
+LL | const C: () = foo();
+   |               ^^^^^
+
+error[E0080]: evaluation of constant expression failed
+  --> $DIR/non_const_fn.rs:11:22
+   |
+LL |     println!("{:?}", C);
+   |                      ^ referenced constant has errors
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0080`.