Remove dead code and extend test coverage and diagnostics around it

We lost the following comment during refactorings:

The current code for niche-filling relies on variant indices instead of actual discriminants, so enums with explicit discriminants (RFC 2363) would misbehave.

1 files changed, 29 insertions, 9 deletions

diff --git a/compiler/rustc_hir_analysis/src/check/check.rs b/compiler/rustc_hir_analysis/src/check/check.rs
index 03c026cd6c8..bb1de5bcfc3 100644
--- a/compiler/rustc_hir_analysis/src/check/check.rs
+++ b/compiler/rustc_hir_analysis/src/check/check.rs
@@ -1642,20 +1642,40 @@ fn check_enum(tcx: TyCtxt<'_>, def_id: LocalDefId) {
 
     if def.repr().int.is_none() {
         let is_unit = |var: &ty::VariantDef| matches!(var.ctor_kind(), Some(CtorKind::Const));
-        let has_disr = |var: &ty::VariantDef| matches!(var.discr, ty::VariantDiscr::Explicit(_));
+        let get_disr = |var: &ty::VariantDef| match var.discr {
+            ty::VariantDiscr::Explicit(disr) => Some(disr),
+            ty::VariantDiscr::Relative(_) => None,
+        };
 
-        let has_non_units = def.variants().iter().any(|var| !is_unit(var));
-        let disr_units = def.variants().iter().any(|var| is_unit(var) && has_disr(var));
-        let disr_non_unit = def.variants().iter().any(|var| !is_unit(var) && has_disr(var));
+        let non_unit = def.variants().iter().find(|var| !is_unit(var));
+        let disr_unit =
+            def.variants().iter().filter(|var| is_unit(var)).find_map(|var| get_disr(var));
+        let disr_non_unit =
+            def.variants().iter().filter(|var| !is_unit(var)).find_map(|var| get_disr(var));
 
-        if disr_non_unit || (disr_units && has_non_units) {
-            struct_span_code_err!(
+        if disr_non_unit.is_some() || (disr_unit.is_some() && non_unit.is_some()) {
+            let mut err = struct_span_code_err!(
                 tcx.dcx(),
                 tcx.def_span(def_id),
                 E0732,
-                "`#[repr(inttype)]` must be specified"
-            )
-            .emit();
+                "`#[repr(inttype)]` must be specified for enums with explicit discriminants and non-unit variants"
+            );
+            if let Some(disr_non_unit) = disr_non_unit {
+                err.span_label(
+                    tcx.def_span(disr_non_unit),
+                    "explicit discriminant on non-unit variant specified here",
+                );
+            } else {
+                err.span_label(
+                    tcx.def_span(disr_unit.unwrap()),
+                    "explicit discriminant specified here",
+                );
+                err.span_label(
+                    tcx.def_span(non_unit.unwrap().def_id),
+                    "non-unit discriminant declared here",
+                );
+            }
+            err.emit();
         }
     }