Improve error message for E0081

compiler/rustc_typeck/src/check/check.rs

@@ -1376,6 +1376,8 @@ fn check_enum<'tcx>(
     }
 
     let mut disr_vals: Vec<Discr<'tcx>> = Vec::with_capacity(vs.len());
+    // This tracks the previous variant span (in the loop) incase we need it for diagnostics
+    let mut prev_variant_span: Span = DUMMY_SP;
     for ((_, discr), v) in iter::zip(def.discriminants(tcx), vs) {
         // Check for duplicate discriminant values
         if let Some(i) = disr_vals.iter().position(|&x| x.val == discr.val) {
@@ -1390,42 +1392,59 @@ fn check_enum<'tcx>(
                 Some(ref expr) => tcx.hir().span(expr.hir_id),
                 None => v.span,
             };
-            let display_discr = display_discriminant_value(tcx, v, discr.val);
-            let display_discr_i = display_discriminant_value(tcx, variant_i, disr_vals[i].val);
-            struct_span_err!(
+            let display_discr = format_discriminant_overflow(tcx, v, discr);
+            let display_discr_i = format_discriminant_overflow(tcx, variant_i, disr_vals[i]);
+            let no_disr = v.disr_expr.is_none();
+            let mut err = struct_span_err!(
                 tcx.sess,
-                span,
+                sp,
                 E0081,
-                "discriminant value `{}` already exists",
-                discr.val,
-            )
-            .span_label(i_span, format!("first use of {display_discr_i}"))
-            .span_label(span, format!("enum already has {display_discr}"))
-            .emit();
+                "discriminant value `{}` assigned more than once",
+                discr,
+            );
+
+            err.span_label(i_span, format!("first assignment of {display_discr_i}"));
+            err.span_label(span, format!("second assignment of {display_discr}"));
+
+            if no_disr {
+                err.span_label(
+                    prev_variant_span,
+                    format!(
+                        "assigned discriminant for `{}` was incremented from this discriminant",
+                        v.ident
+                    ),
+                );
+            }
+            err.emit();
         }
+
         disr_vals.push(discr);
+        prev_variant_span = v.span;
     }
 
     check_representable(tcx, sp, def_id);
     check_transparent(tcx, sp, def);
 }
 
-/// Format an enum discriminant value for use in a diagnostic message.
-fn display_discriminant_value<'tcx>(
+/// In the case that a discriminant is both a duplicate and an overflowing literal,
+/// we insert both the assigned discriminant and the literal it overflowed from into the formatted
+/// output. Otherwise we format the discriminant normally.
+fn format_discriminant_overflow<'tcx>(
     tcx: TyCtxt<'tcx>,
     variant: &hir::Variant<'_>,
-    evaluated: u128,
+    dis: Discr<'tcx>,
 ) -> String {
     if let Some(expr) = &variant.disr_expr {
         let body = &tcx.hir().body(expr.body).value;
         if let hir::ExprKind::Lit(lit) = &body.kind
             && let rustc_ast::LitKind::Int(lit_value, _int_kind) = &lit.node
-            && evaluated != *lit_value
+            && dis.val != *lit_value
         {
-                    return format!("`{evaluated}` (overflowed from `{lit_value}`)");
+                    return format!("`{dis}` (overflowed from `{lit_value}`)");
         }
     }
-    format!("`{}`", evaluated)
+
+    format!("`{dis}`")
 }
 
 pub(super) fn check_type_params_are_used<'tcx>(

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

@@ -4,10 +4,11 @@
 // so force the repr.
 #[cfg_attr(not(target_pointer_width = "32"), repr(i32))]
 enum Eu64 {
-    Au64 = 0, //~NOTE first use of `0`
+    //~^ ERROR discriminant value `0` assigned more than once
+    Au64 = 0,
+    //~^NOTE first assignment of `0`
     Bu64 = 0x8000_0000_0000_0000
-    //~^ ERROR discriminant value `0` already exists
-    //~| NOTE enum already has `0` (overflowed from `9223372036854775808`)
+    //~^NOTE second assignment of `0` (overflowed from `9223372036854775808`)
 }
 
 fn main() {}

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

@@ -1,10 +1,16 @@
-error[E0081]: discriminant value `0` already exists
-  --> $DIR/enum-discrim-autosizing.rs:8:12
+error[E0081]: discriminant value `0` assigned more than once
+  --> $DIR/enum-discrim-autosizing.rs:6:1
    |
-LL |     Au64 = 0,
-   |            - first use of `0`
-LL |     Bu64 = 0x8000_0000_0000_0000
-   |            ^^^^^^^^^^^^^^^^^^^^^ enum already has `0` (overflowed from `9223372036854775808`)
+LL | / enum Eu64 {
+LL | |
+LL | |     Au64 = 0,
+   | |            - first assignment of `0`
+LL | |
+LL | |     Bu64 = 0x8000_0000_0000_0000
+   | |            --------------------- second assignment of `0` (overflowed from `9223372036854775808`)
+LL | |
+LL | | }
+   | |_^
 
 error: aborting due to previous error
 

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

@@ -1,19 +1,30 @@
 enum Enum {
+    //~^ ERROR discriminant value `3` assigned more than once
     P = 3,
-    //~^ NOTE first use of `3`
+    //~^ NOTE first assignment of `3`
     X = 3,
-    //~^ ERROR discriminant value `3` already exists
-    //~| NOTE enum already has `3`
+    //~^ NOTE second assignment of `3`
     Y = 5
 }
 
 #[repr(u8)]
 enum EnumOverflowRepr {
+    //~^ ERROR discriminant value `1` assigned more than once
     P = 257,
-    //~^ NOTE first use of `1` (overflowed from `257`)
+    //~^ NOTE first assignment of `1` (overflowed from `257`)
     X = 513,
-    //~^ ERROR discriminant value `1` already exists
-    //~| NOTE enum already has `1` (overflowed from `513`)
+    //~^ NOTE second assignment of `1` (overflowed from `513`)
+}
+
+#[repr(i8)]
+enum NegDisEnum {
+    //~^ ERROR discriminant value `-1` assigned more than once
+    First = -1,
+    //~^ NOTE first assignment of `-1`
+    Second = -2,
+    //~^ NOTE assigned discriminant for `Last` was incremented from this discriminant
+    Last,
+    //~^ NOTE second assignment of `-1`
 }
 
 fn main() {

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

@@ -1,21 +1,49 @@
-error[E0081]: discriminant value `3` already exists
-  --> $DIR/E0081.rs:4:9
+error[E0081]: discriminant value `3` assigned more than once
+  --> $DIR/E0081.rs:1:1
    |
-LL |     P = 3,
-   |         - first use of `3`
-LL |
-LL |     X = 3,
-   |         ^ enum already has `3`
+LL | / enum Enum {
+LL | |
+LL | |     P = 3,
+   | |         - first assignment of `3`
+LL | |
+LL | |     X = 3,
+   | |         - second assignment of `3`
+LL | |
+LL | |     Y = 5
+LL | | }
+   | |_^
 
-error[E0081]: discriminant value `1` already exists
-  --> $DIR/E0081.rs:14:9
+error[E0081]: discriminant value `1` assigned more than once
+  --> $DIR/E0081.rs:11:1
    |
-LL |     P = 257,
-   |         --- first use of `1` (overflowed from `257`)
-LL |
-LL |     X = 513,
-   |         ^^^ enum already has `1` (overflowed from `513`)
+LL | / enum EnumOverflowRepr {
+LL | |
+LL | |     P = 257,
+   | |         --- first assignment of `1` (overflowed from `257`)
+LL | |
+LL | |     X = 513,
+   | |         --- second assignment of `1` (overflowed from `513`)
+LL | |
+LL | | }
+   | |_^
 
-error: aborting due to 2 previous errors
+error[E0081]: discriminant value `-1` assigned more than once
+  --> $DIR/E0081.rs:20:1
+   |
+LL | / enum NegDisEnum {
+LL | |
+LL | |     First = -1,
+   | |             -- first assignment of `-1`
+LL | |
+LL | |     Second = -2,
+   | |     ----------- assigned discriminant for `Last` was incremented from this discriminant
+LL | |
+LL | |     Last,
+   | |     ---- second assignment of `-1`
+LL | |
+LL | | }
+   | |_^
+
+error: aborting due to 3 previous errors
 
 For more information about this error, try `rustc --explain E0081`.

src/test/ui/issues/issue-15524.rs

@@ -1,15 +1,15 @@
 const N: isize = 1;
 
 enum Foo {
+    //~^ ERROR discriminant value `1` assigned more than once
+    //~| ERROR discriminant value `1` assigned more than once
+    //~| ERROR discriminant value `1` assigned more than once
     A = 1,
     B = 1,
-    //~^ ERROR discriminant value `1` already exists
     C = 0,
     D,
-    //~^ ERROR discriminant value `1` already exists
 
     E = N,
-    //~^ ERROR discriminant value `1` already exists
 
 }
 

src/test/ui/issues/issue-15524.stderr

@@ -1,28 +1,53 @@
-error[E0081]: discriminant value `1` already exists
-  --> $DIR/issue-15524.rs:5:9
+error[E0081]: discriminant value `1` assigned more than once
+  --> $DIR/issue-15524.rs:3:1
    |
-LL |     A = 1,
-   |         - first use of `1`
-LL |     B = 1,
-   |         ^ enum already has `1`
+LL | / enum Foo {
+LL | |
+LL | |
+LL | |
+LL | |     A = 1,
+   | |         - first assignment of `1`
+LL | |     B = 1,
+   | |         - second assignment of `1`
+...  |
+LL | |
+LL | | }
+   | |_^
 
-error[E0081]: discriminant value `1` already exists
-  --> $DIR/issue-15524.rs:8:5
+error[E0081]: discriminant value `1` assigned more than once
+  --> $DIR/issue-15524.rs:3:1
    |
-LL |     A = 1,
-   |         - first use of `1`
-...
-LL |     D,
-   |     ^ enum already has `1`
+LL | / enum Foo {
+LL | |
+LL | |
+LL | |
+LL | |     A = 1,
+   | |         - first assignment of `1`
+LL | |     B = 1,
+LL | |     C = 0,
+   | |     ----- assigned discriminant for `D` was incremented from this discriminant
+LL | |     D,
+   | |     - second assignment of `1`
+...  |
+LL | |
+LL | | }
+   | |_^
 
-error[E0081]: discriminant value `1` already exists
-  --> $DIR/issue-15524.rs:11:9
+error[E0081]: discriminant value `1` assigned more than once
+  --> $DIR/issue-15524.rs:3:1
    |
-LL |     A = 1,
-   |         - first use of `1`
-...
-LL |     E = N,
-   |         ^ enum already has `1`
+LL | / enum Foo {
+LL | |
+LL | |
+LL | |
+LL | |     A = 1,
+   | |         - first assignment of `1`
+...  |
+LL | |     E = N,
+   | |         - second assignment of `1`
+LL | |
+LL | | }
+   | |_^
 
 error: aborting due to 3 previous errors
 

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

@@ -1,11 +1,12 @@
 // 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, //~ ERROR discriminant value `0` already exists
+    White = 0x000000,
 }
 
 fn main() { }

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

@@ -1,10 +1,17 @@
-error[E0081]: discriminant value `0` already exists
-  --> $DIR/tag-variant-disr-dup.rs:8:13
+error[E0081]: discriminant value `0` assigned more than once
+  --> $DIR/tag-variant-disr-dup.rs:3:1
    |
-LL |     Black = 0x000000,
-   |             -------- first use of `0`
-LL |     White = 0x000000,
-   |             ^^^^^^^^ enum already has `0`
+LL | / enum Color {
+LL | |
+LL | |     Red = 0xff0000,
+LL | |     Green = 0x00ff00,
+LL | |     Blue = 0x0000ff,
+LL | |     Black = 0x000000,
+   | |             -------- first assignment of `0`
+LL | |     White = 0x000000,
+   | |             -------- second assignment of `0`
+LL | | }
+   | |_^
 
 error: aborting due to previous error