lowering casts in constants now creates multiple uses

This can trigger more errors than before. Not sure what is the best
fix here.

Author: nikomatsakis

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

@@ -14,9 +14,10 @@ const X: u32 = 5;
 const Y: u32 = 6;
 const FOO: u32 = [X - Y, Y - X][(X < Y) as usize];
 //~^ WARN this constant cannot be used
+//~| ERROR
 
 fn main() {
     println!("{}", FOO);
-    //~^ ERROR erroneous constant used
-    //~| E0080
+    //~^ ERROR
+    //~| ERROR
 }

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

@@ -13,20 +13,28 @@ LL | #![warn(const_err)]
    |         ^^^^^^^^^
 
 error[E0080]: referenced constant has errors
-  --> $DIR/conditional_array_execution.rs:19:20
+  --> $DIR/conditional_array_execution.rs:20:20
    |
 LL | const FOO: u32 = [X - Y, Y - X][(X < Y) as usize];
    |                   ----- attempt to subtract with overflow
 ...
 LL |     println!("{}", FOO);
    |                    ^^^
 
-error[E0080]: erroneous constant used
-  --> $DIR/conditional_array_execution.rs:19:20
+error[E0080]: could not evaluate constant
+  --> $DIR/conditional_array_execution.rs:20:20
    |
 LL |     println!("{}", FOO);
    |                    ^^^ referenced constant has errors
 
-error: aborting due to 2 previous errors
+error[E0080]: constant evaluation error
+  --> $DIR/conditional_array_execution.rs:15:1
+   |
+LL | const FOO: u32 = [X - Y, Y - X][(X < Y) as usize];
+   | ^^^^^^^^^^^^^^^^^^-----^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |                   |
+   |                   attempt to subtract with overflow
+
+error: aborting due to 3 previous errors
 
 For more information about this error, try `rustc --explain E0080`.

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

@@ -34,6 +34,12 @@ warning: attempt to divide by zero
 LL |     println!("{}", 1/(false as u32));
    |                    ^^^^^^^^^^^^^^^^
 
+warning: this expression will panic at runtime
+  --> $DIR/promoted_errors.rs:24:20
+   |
+LL |     println!("{}", 1/(false as u32));
+   |                    ^^^^^^^^^^^^^^^^ attempt to divide by zero
+
 warning: attempt to divide by zero
   --> $DIR/promoted_errors.rs:26:14
    |