Fixed the std corner case

Author: LucaCappelletti94

src/ast/dml.rs

@@ -16,7 +16,12 @@
 // under the License.
 
 #[cfg(not(feature = "std"))]
-use alloc::{boxed::Box, format, string::ToString, vec::Vec};
+use alloc::{
+    boxed::Box,
+    format,
+    string::{String, ToString},
+    vec::Vec,
+};
 
 use core::fmt::{self, Display};
 #[cfg(feature = "serde")]