librustc_typeck: use diag item instead of string compare

src/librustc_span/symbol.rs

@@ -1043,6 +1043,7 @@
         stop_after_dataflow,
         str,
         str_alloc,
+        string_type,
         stringify,
         struct_field_attributes,
         struct_inherit,

src/librustc_typeck/check/op.rs

@@ -562,7 +562,11 @@ fn check_str_addition(
                    on the left and may require reallocation. This \
                    requires ownership of the string on the left";
 
-        let is_std_string = |ty| &format!("{:?}", ty) == "std::string::String";
+        let string_type = self.tcx.get_diagnostic_item(sym::string_type);
+        let is_std_string = |ty: Ty<'tcx>| match &ty.ty_adt_def() {
+            Some(ty_def) => Some(ty_def.did) == string_type,
+            None => false,
+        };
 
         match (&lhs_ty.kind, &rhs_ty.kind) {
             (&Ref(_, l_ty, _), &Ref(_, r_ty, _)) // &str or &String + &str, &String or &&str