Migration on ops.rs for unstable const functions

compiler/rustc_const_eval/src/errors.rs

@@ -103,3 +103,11 @@ pub(crate) struct UnallowedFnPointerCall {
     pub span: Span,
     pub kind: ConstContext,
 }
+
+#[derive(SessionDiagnostic)]
+#[error(const_eval::unstable_const_fn)]
+pub(crate) struct UnstableConstFn {
+    #[primary_span]
+    pub span: Span,
+    pub def_id: String,
+}

compiler/rustc_const_eval/src/transform/check_consts/ops.rs

@@ -25,7 +25,7 @@ use rustc_trait_selection::traits::SelectionContext;
 use super::ConstCx;
 use crate::errors::{
     MutDerefErr, NonConstOpErr, PanicNonStrErr, RawPtrToIntErr, StaticAccessErr,
-    TransientMutBorrowErr, TransientMutBorrowErrRaw, UnallowedFnPointerCall,
+    TransientMutBorrowErr, TransientMutBorrowErrRaw, UnallowedFnPointerCall, UnstableConstFn,
 };
 use crate::util::{call_kind, CallDesugaringKind, CallKind};
 
@@ -351,10 +351,8 @@ impl<'tcx> NonConstOp<'tcx> for FnCallUnstable {
     ) -> DiagnosticBuilder<'tcx, ErrorGuaranteed> {
         let FnCallUnstable(def_id, feature) = *self;
 
-        let mut err = ccx.tcx.sess.struct_span_err(
-            span,
-            &format!("`{}` is not yet stable as a const fn", ccx.tcx.def_path_str(def_id)),
-        );
+        let mut err =
+            ccx.tcx.sess.create_err(UnstableConstFn { span, def_id: ccx.tcx.def_path_str(def_id) });
 
         if ccx.is_const_stable_const_fn() {
             err.help("const-stable functions can only call other const-stable functions");

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

@@ -32,4 +32,6 @@ const_eval_transient_mut_borrow_raw = raw mutable references are not allowed in
 
 const_evaL_max_num_nodes_exceeded = maximum number of nodes exceeded in constant {$s}
 
-const_eval_unallowed_fn_pointer_call = function pointer calls are not allowed in {$const_kind}s
+const_eval_unallowed_fn_pointer_call = function pointer calls are not allowed in {$kind}s
+
+const_eval_unstable_const_fn = `{$def_id}` is not yet stable as a const fn