Add diagnostic struct for const eval error in rustc_middle

Co-authored-by: Michael Goulet <michael@errs.io>

compiler/rustc_error_messages/locales/en-US/middle.ftl

@@ -15,3 +15,6 @@ middle_previous_use_here =
 middle_limit_invalid =
     `limit` must be a non-negative integer
     .label = {$error_str}
+
+middle_const_eval_non_int =
+    constant evaluation of enum discriminant resulted in non-integer

compiler/rustc_middle/src/error.rs

@@ -48,3 +48,10 @@ pub struct LimitInvalid<'a> {
     pub value_span: Span,
     pub error_str: &'a str,
 }
+
+#[derive(Diagnostic)]
+#[diag(middle::const_eval_non_int)]
+pub struct ConstEvalNonIntError {
+    #[primary_span]
+    pub span: Span,
+}

compiler/rustc_middle/src/ty/adt.rs

@@ -458,11 +458,9 @@ pub fn eval_explicit_discr(self, tcx: TyCtxt<'tcx>, expr_did: DefId) -> Option<D
                     Some(Discr { val: b, ty })
                 } else {
                     info!("invalid enum discriminant: {:#?}", val);
-                    crate::mir::interpret::struct_error(
-                        tcx.at(tcx.def_span(expr_did)),
-                        "constant evaluation of enum discriminant resulted in non-integer",
-                    )
-                    .emit();
+                    tcx.sess.emit_err(crate::error::ConstEvalNonIntError {
+                        span: tcx.def_span(expr_did),
+                    });
                     None
                 }
             }