const_eval machine: use original instance for replaced MIR bodies

compiler/rustc_const_eval/src/const_eval/machine.rs

@@ -279,7 +279,17 @@ impl<'mir, 'tcx> interpret::Machine<'mir, 'tcx> for CompileTimeInterpreter<'mir,
 
             if let Some(new_instance) = ecx.hook_special_const_fn(instance, args)? {
                 // We call another const fn instead.
-                return Self::find_mir_or_eval_fn(ecx, new_instance, _abi, args, _ret, _unwind);
+                // However, we return the *original* instance to make backtraces work out
+                // (and we hope this does not confuse the FnAbi checks too much).
+                return Ok(Self::find_mir_or_eval_fn(
+                    ecx,
+                    new_instance,
+                    _abi,
+                    args,
+                    _ret,
+                    _unwind,
+                )?
+                .map(|(body, _instance)| (body, instance)));
             }
         }
         // This is a const fn. Call it.