Show const_err lint in addition to the hard error

Author: oli-obk

src/test/ui/consts/assoc_const_generic_impl.rs

@@ -1,12 +1,12 @@
-#![allow(const_err)]
+#![warn(const_err)]
 
 trait ZeroSized: Sized {
     const I_AM_ZERO_SIZED: ();
     fn requires_zero_size(self);
 }
 
 impl<T: Sized> ZeroSized for T {
-    const I_AM_ZERO_SIZED: ()  = [()][std::mem::size_of::<Self>()];
+    const I_AM_ZERO_SIZED: ()  = [()][std::mem::size_of::<Self>()]; //~ WARN any use of this value
     fn requires_zero_size(self) {
         let () = Self::I_AM_ZERO_SIZED; //~ ERROR erroneous constant encountered
         println!("requires_zero_size called");

src/test/ui/consts/assoc_const_generic_impl.stderr

@@ -1,3 +1,17 @@
+warning: any use of this value will cause an error
+  --> $DIR/assoc_const_generic_impl.rs:9:34
+   |
+LL |     const I_AM_ZERO_SIZED: ()  = [()][std::mem::size_of::<Self>()];
+   |     -----------------------------^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^-
+   |                                  |
+   |                                  index out of bounds: the len is 1 but the index is 4
+   |
+note: lint level defined here
+  --> $DIR/assoc_const_generic_impl.rs:1:9
+   |
+LL | #![warn(const_err)]
+   |         ^^^^^^^^^
+
 error: erroneous constant encountered
   --> $DIR/assoc_const_generic_impl.rs:11:18
    |