add test for unused ill-formed constant

tests/compile-fail/erroneous_const.rs

@@ -0,0 +1,19 @@
+//! Make sure we detect erroneous constants post-monomorphization even when they are unused.
+//! (https://github.com/rust-lang/miri/issues/1382)
+#![feature(const_panic)]
+#![feature(never_type)]
+#![warn(const_err)]
+
+struct PrintName<T>(T);
+impl<T> PrintName<T> {
+    const VOID: ! = panic!(); //~WARN any use of this value will cause an error
+}
+
+fn no_codegen<T>() {
+    if false {
+        let _ = PrintName::<T>::VOID; //~ERROR referenced constant has errors
+    }
+}
+fn main() {
+    no_codegen::<i32>();
+}