Fix plural form of variant in error message not formatting correctly

due to ordering, added/improved comments and removed redundant test
already caught by `E0081.rs`

compiler/rustc_typeck/src/check/check.rs

@@ -1499,18 +1499,19 @@ fn check_enum<'tcx>(tcx: TyCtxt<'tcx>, vs: &'tcx [hir::Variant<'tcx>], def_id: L
     check_transparent(tcx, sp, def);
 }
 
-/// Part of enum check, errors if two or more discriminants are equal
+/// Part of enum check. Given the discriminants of an enum, errors if two or more discriminants are equal
 fn detect_discriminant_duplicate<'tcx>(
     tcx: TyCtxt<'tcx>,
     mut discrs: Vec<(VariantIdx, Discr<'tcx>)>,
     vs: &'tcx [hir::Variant<'tcx>],
     self_span: Span,
 ) {
-    // Helper closure to reduce duplicate code. This gets called everytime we detect a duplicate
+    // Helper closure to reduce duplicate code. This gets called everytime we detect a duplicate.
+    // Here `idx` refers to the order of which the discriminant appears, and its index in `vs`
     let report = |dis: Discr<'tcx>,
                   idx: usize,
                   err: &mut DiagnosticBuilder<'_, ErrorGuaranteed>| {
-        let var = &vs[idx];
+        let var = &vs[idx]; // HIR for the duplicate discriminant
         let (span, display_discr) = match var.disr_expr {
             Some(ref expr) => {
                 // In the case the discriminant is both a duplicate and overflowed, let the user know
@@ -1533,8 +1534,8 @@ fn detect_discriminant_duplicate<'tcx>(
                     vs[..idx].iter().rev().enumerate().find(|v| v.1.disr_expr.is_some())
                 {
                     let ve_ident = var.ident;
-                    let sp = if n > 1 { "variants" } else { "variant" };
                     let n = n + 1;
+                    let sp = if n > 1 { "variants" } else { "variant" };
 
                     err.span_label(
                         *span,
@@ -1549,7 +1550,7 @@ fn detect_discriminant_duplicate<'tcx>(
         err.span_label(span, format!("{display_discr} assigned here"));
     };
 
-    // Here we are loop through the discriminants, comparing each discriminant to another.
+    // Here we loop through the discriminants, comparing each discriminant to another.
     // When a duplicate is detected, we instatiate an error and point to both
     // initial and duplicate value. The duplicate discriminant is then discarded by swapping
     // it with the last element and decrementing the `vec.len` (which is why we have to evaluate

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

@@ -27,9 +27,9 @@ enum NegDisEnum {
     //~^ NOTE `-1` assigned here
 }
 
-#[repr(i32)]
 enum MultipleDuplicates {
     //~^ ERROR discriminant value `0` assigned more than once
+    //~^^ ERROR discriminant value `-2` assigned more than once
     V0,
     //~^ NOTE `0` assigned here
     V1 = 0,
@@ -39,10 +39,15 @@ enum MultipleDuplicates {
     V4 = 0,
     //~^ NOTE `0` assigned here
     V5 = -2,
-    //~^ NOTE discriminant for `V7` incremented from this startpoint (`V5` + 2 variant later => `V7` = 0)
+    //~^ NOTE discriminant for `V7` incremented from this startpoint (`V5` + 2 variants later => `V7` = 0)
+    //~^^ NOTE `-2` assigned here
     V6,
     V7,
     //~^ NOTE `0` assigned here
+    V8 = -3,
+    //~^ NOTE discriminant for `V9` incremented from this startpoint (`V8` + 1 variant later => `V9` = -2)
+    V9,
+    //~^ NOTE `-2` assigned here
 }
 
 fn main() {

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

@@ -38,11 +38,11 @@ LL |     Last,
    |     ---- `-1` assigned here
 
 error[E0081]: discriminant value `0` assigned more than once
-  --> $DIR/E0081.rs:31:1
+  --> $DIR/E0081.rs:30:1
    |
 LL | enum MultipleDuplicates {
    | ^^^^^^^^^^^^^^^^^^^^^^^
-LL |
+...
 LL |     V0,
    |     -- `0` assigned here
 LL |
@@ -53,11 +53,26 @@ LL |     V4 = 0,
    |          - `0` assigned here
 LL |
 LL |     V5 = -2,
-   |     ------- discriminant for `V7` incremented from this startpoint (`V5` + 2 variant later => `V7` = 0)
+   |     ------- discriminant for `V7` incremented from this startpoint (`V5` + 2 variants later => `V7` = 0)
 ...
 LL |     V7,
    |     -- `0` assigned here
 
-error: aborting due to 4 previous errors
+error[E0081]: discriminant value `-2` assigned more than once
+  --> $DIR/E0081.rs:30:1
+   |
+LL | enum MultipleDuplicates {
+   | ^^^^^^^^^^^^^^^^^^^^^^^
+...
+LL |     V5 = -2,
+   |          -- `-2` assigned here
+...
+LL |     V8 = -3,
+   |     ------- discriminant for `V9` incremented from this startpoint (`V8` + 1 variant later => `V9` = -2)
+LL |
+LL |     V9,
+   |     -- `-2` assigned here
+
+error: aborting due to 5 previous errors
 
 For more information about this error, try `rustc --explain E0081`.

src/test/ui/tag-variant-disr-dup.rs

@@ -1,12 +0,0 @@
-// Black and White have the same discriminator value ...
-
-enum Color {
-    //~^ ERROR discriminant value `0` assigned more than once
-    Red = 0xff0000,
-    Green = 0x00ff00,
-    Blue = 0x0000ff,
-    Black = 0x000000,
-    White = 0x000000,
-}
-
-fn main() { }

src/test/ui/tag-variant-disr-dup.stderr

@@ -1,14 +0,0 @@
-error[E0081]: discriminant value `0` assigned more than once
-  --> $DIR/tag-variant-disr-dup.rs:3:1
-   |
-LL | enum Color {
-   | ^^^^^^^^^^
-...
-LL |     Black = 0x000000,
-   |             -------- `0` assigned here
-LL |     White = 0x000000,
-   |             -------- `0` assigned here
-
-error: aborting due to previous error
-
-For more information about this error, try `rustc --explain E0081`.