Avoid warnings in standard build.

src/syntax.rs

@@ -14,11 +14,14 @@ use crate::utils::StaticVec;
 use crate::stdlib::{
     boxed::Box,
     fmt, format,
-    rc::Rc,
     string::{String, ToString},
-    sync::Arc,
 };
 
+#[cfg(not(feature = "sync"))]
+use crate::stdlib::rc::Rc;
+#[cfg(feature = "sync")]
+use crate::stdlib::sync::Arc;
+
 /// A general expression evaluation trait object.
 #[cfg(not(feature = "sync"))]
 pub type FnCustomSyntaxEval = dyn Fn(