Show discriminant before overflow in diagnostic.

src/test/ui/enum/enum-discrim-autosizing.rs

@@ -4,8 +4,10 @@
 // so force the repr.
 #[cfg_attr(not(target_pointer_width = "32"), repr(i32))]
 enum Eu64 {
-    Au64 = 0,
-    Bu64 = 0x8000_0000_0000_0000 //~ERROR already exists
+    Au64 = 0, //~NOTE first use of `0`
+    Bu64 = 0x8000_0000_0000_0000
+    //~^ ERROR discriminant value `0` already exists
+    //~| NOTE enum already has `0` (overflowed from `9223372036854775808`)
 }
 
 fn main() {}

src/test/ui/enum/enum-discrim-autosizing.stderr

@@ -4,7 +4,7 @@ error[E0081]: discriminant value `0` already exists
 LL |     Au64 = 0,
    |            - first use of `0`
 LL |     Bu64 = 0x8000_0000_0000_0000
-   |            ^^^^^^^^^^^^^^^^^^^^^ enum already has `0`
+   |            ^^^^^^^^^^^^^^^^^^^^^ enum already has `0` (overflowed from `9223372036854775808`)
 
 error: aborting due to previous error
 

src/test/ui/error-codes/E0081.rs

@@ -1,9 +1,20 @@
 enum Enum {
     P = 3,
+    //~^ NOTE first use of `3`
     X = 3,
     //~^ ERROR discriminant value `3` already exists
+    //~| NOTE enum already has `3`
     Y = 5
 }
 
+#[repr(u8)]
+enum EnumOverflowRepr {
+    P = 257,
+    //~^ NOTE first use of `1` (overflowed from `257`)
+    X = 513,
+    //~^ ERROR discriminant value `1` already exists
+    //~| NOTE enum already has `1` (overflowed from `513`)
+}
+
 fn main() {
 }

src/test/ui/error-codes/E0081.stderr

@@ -1,11 +1,21 @@
 error[E0081]: discriminant value `3` already exists
-  --> $DIR/E0081.rs:3:9
+  --> $DIR/E0081.rs:4:9
    |
 LL |     P = 3,
    |         - first use of `3`
+LL |
 LL |     X = 3,
    |         ^ enum already has `3`
 
-error: aborting due to previous error
+error[E0081]: discriminant value `1` already exists
+  --> $DIR/E0081.rs:14:9
+   |
+LL |     P = 257,
+   |         --- first use of `1` (overflowed from `257`)
+LL |
+LL |     X = 513,
+   |         ^^^ enum already has `1` (overflowed from `513`)
+
+error: aborting due to 2 previous errors
 
 For more information about this error, try `rustc --explain E0081`.