Avoid span_delayed_bug on one path in AdtDef::eval_explicit_discr.

Author: nnethercote

compiler/rustc_middle/src/ty/adt.rs

@@ -494,11 +494,15 @@ impl<'tcx> AdtDef<'tcx> {
                 }
             }
             Err(err) => {
-                let msg = match err {
-                    ErrorHandled::Reported(..) => "enum discriminant evaluation failed",
-                    ErrorHandled::TooGeneric(..) => "enum discriminant depends on generics",
+                match err {
+                    ErrorHandled::Reported(..) => {} // error guaranteed by `Reported`
+                    ErrorHandled::TooGeneric(..) => {
+                        tcx.dcx().span_delayed_bug(
+                            tcx.def_span(expr_did),
+                            "enum discriminant depends on generics",
+                        );
+                    }
                 };
-                tcx.dcx().span_delayed_bug(tcx.def_span(expr_did), msg);
                 None
             }
         }