add test for unused erroneous const in CTFE

Author: RalfJung

src/test/ui/consts/const-eval/erroneous-const.rs

@@ -0,0 +1,20 @@
+//! Make sure we error on erroneous consts even if they are unused.
+#![warn(const_err, unconditional_panic)]
+
+struct PrintName<T>(T);
+impl<T> PrintName<T> {
+    const VOID: () = [()][2]; //~WARN any use of this value will cause an error
+    //~^ WARN this operation will panic at runtime
+}
+
+const fn no_codegen<T>() {
+    if false { //~ERROR evaluation of constant value failed
+        let _ = PrintName::<T>::VOID;
+    }
+}
+
+pub static FOO: () = no_codegen::<i32>(); //~ERROR could not evaluate static initializer
+
+fn main() {
+    FOO
+}

src/test/ui/consts/const-eval/erroneous-const.stderr

@@ -0,0 +1,43 @@
+warning: this operation will panic at runtime
+  --> $DIR/erroneous-const.rs:6:22
+   |
+LL |     const VOID: () = [()][2];
+   |                      ^^^^^^^ index out of bounds: the len is 1 but the index is 2
+   |
+note: the lint level is defined here
+  --> $DIR/erroneous-const.rs:2:20
+   |
+LL | #![warn(const_err, unconditional_panic)]
+   |                    ^^^^^^^^^^^^^^^^^^^
+
+warning: any use of this value will cause an error
+  --> $DIR/erroneous-const.rs:6:22
+   |
+LL |     const VOID: () = [()][2];
+   |     -----------------^^^^^^^-
+   |                      |
+   |                      index out of bounds: the len is 1 but the index is 2
+   |
+note: the lint level is defined here
+  --> $DIR/erroneous-const.rs:2:9
+   |
+LL | #![warn(const_err, unconditional_panic)]
+   |         ^^^^^^^^^
+
+error[E0080]: evaluation of constant value failed
+  --> $DIR/erroneous-const.rs:11:5
+   |
+LL | /     if false {
+LL | |         let _ = PrintName::<T>::VOID;
+LL | |     }
+   | |_____^ referenced constant has errors
+
+error[E0080]: could not evaluate static initializer
+  --> $DIR/erroneous-const.rs:16:22
+   |
+LL | pub static FOO: () = no_codegen::<i32>();
+   |                      ^^^^^^^^^^^^^^^^^^^ referenced constant has errors
+
+error: aborting due to 2 previous errors; 2 warnings emitted
+
+For more information about this error, try `rustc --explain E0080`.