Further improve error message for E0081

Multiple duplicate assignments of the same discriminant are now reported
in the samme error. We now point out the incrementation start point for
discriminants that are not explicitly assigned that are also duplicates.
Removed old test related to E0081 that is now covered by error-codes/E0081.rs.
Also refactored parts of the `check_enum` function.

2 files changed, 54 insertions, 15 deletions

diff --git a/src/test/ui/error-codes/E0081.rs b/src/test/ui/error-codes/E0081.rs
index 5aa6a786339..5e970a89307 100644
--- a/src/test/ui/error-codes/E0081.rs
+++ b/src/test/ui/error-codes/E0081.rs
@@ -1,9 +1,9 @@
 enum Enum {
     //~^ ERROR discriminant value `3` assigned more than once
     P = 3,
-    //~^ NOTE first assignment of `3`
+    //~^ NOTE `3` assigned here
     X = 3,
-    //~^ NOTE second assignment of `3`
+    //~^ NOTE `3` assigned here
     Y = 5
 }
 
@@ -11,20 +11,38 @@ enum Enum {
 enum EnumOverflowRepr {
     //~^ ERROR discriminant value `1` assigned more than once
     P = 257,
-    //~^ NOTE first assignment of `1` (overflowed from `257`)
+    //~^ NOTE `1` (overflowed from `257`) assigned here
     X = 513,
-    //~^ NOTE second assignment of `1` (overflowed from `513`)
+    //~^ NOTE `1` (overflowed from `513`) assigned here
 }
 
 #[repr(i8)]
 enum NegDisEnum {
     //~^ ERROR discriminant value `-1` assigned more than once
     First = -1,
-    //~^ NOTE first assignment of `-1`
+    //~^ NOTE `-1` assigned here
     Second = -2,
-    //~^ NOTE assigned discriminant for `Last` was incremented from this discriminant
+    //~^ NOTE discriminant for `Last` incremented from this startpoint (`Second` + 1 variant later => `Last` = -1)
     Last,
-    //~^ NOTE second assignment of `-1`
+    //~^ NOTE `-1` assigned here
+}
+
+#[repr(i32)]
+enum MultipleDuplicates {
+    //~^ ERROR discriminant value `0` assigned more than once
+    V0,
+    //~^ NOTE `0` assigned here
+    V1 = 0,
+    //~^ NOTE `0` assigned here
+    V2,
+    V3,
+    V4 = 0,
+    //~^ NOTE `0` assigned here
+    V5 = -2,
+    //~^ NOTE discriminant for `V7` incremented from this startpoint (`V5` + 2 variant later => `V7` = 0)
+    V6,
+    V7,
+    //~^ NOTE `0` assigned here
 }
 
 fn main() {
diff --git a/src/test/ui/error-codes/E0081.stderr b/src/test/ui/error-codes/E0081.stderr
index ff6113646bb..d27861aca5f 100644
--- a/src/test/ui/error-codes/E0081.stderr
+++ b/src/test/ui/error-codes/E0081.stderr
@@ -5,10 +5,10 @@ LL | enum Enum {
    | ^^^^^^^^^
 LL |
 LL |     P = 3,
-   |         - first assignment of `3`
+   |         - `3` assigned here
 LL |
 LL |     X = 3,
-   |         - second assignment of `3`
+   |         - `3` assigned here
 
 error[E0081]: discriminant value `1` assigned more than once
   --> $DIR/E0081.rs:11:1
@@ -17,10 +17,10 @@ LL | enum EnumOverflowRepr {
    | ^^^^^^^^^^^^^^^^^^^^^
 LL |
 LL |     P = 257,
-   |         --- first assignment of `1` (overflowed from `257`)
+   |         --- `1` (overflowed from `257`) assigned here
 LL |
 LL |     X = 513,
-   |         --- second assignment of `1` (overflowed from `513`)
+   |         --- `1` (overflowed from `513`) assigned here
 
 error[E0081]: discriminant value `-1` assigned more than once
   --> $DIR/E0081.rs:20:1
@@ -29,14 +29,35 @@ LL | enum NegDisEnum {
    | ^^^^^^^^^^^^^^^
 LL |
 LL |     First = -1,
-   |             -- first assignment of `-1`
+   |             -- `-1` assigned here
 LL |
 LL |     Second = -2,
-   |     ----------- assigned discriminant for `Last` was incremented from this discriminant
+   |     ----------- discriminant for `Last` incremented from this startpoint (`Second` + 1 variant later => `Last` = -1)
 LL |
 LL |     Last,
-   |     ---- second assignment of `-1`
+   |     ---- `-1` assigned here
 
-error: aborting due to 3 previous errors
+error[E0081]: discriminant value `0` assigned more than once
+  --> $DIR/E0081.rs:31:1
+   |
+LL | enum MultipleDuplicates {
+   | ^^^^^^^^^^^^^^^^^^^^^^^
+LL |
+LL |     V0,
+   |     -- `0` assigned here
+LL |
+LL |     V1 = 0,
+   |          - `0` assigned here
+...
+LL |     V4 = 0,
+   |          - `0` assigned here
+LL |
+LL |     V5 = -2,
+   |     ------- discriminant for `V7` incremented from this startpoint (`V5` + 2 variant later => `V7` = 0)
+...
+LL |     V7,
+   |     -- `0` assigned here
+
+error: aborting due to 4 previous errors
 
 For more information about this error, try `rustc --explain E0081`.