Refine data structures.

src/engine.rs

@@ -112,7 +112,7 @@ pub struct Engine {
     /// A map containing custom keywords and precedence to recognize.
     pub(crate) custom_keywords: BTreeMap<Identifier, Option<Precedence>>,
     /// Custom syntax.
-    pub(crate) custom_syntax: BTreeMap<Identifier, Box<CustomSyntax>>,
+    pub(crate) custom_syntax: BTreeMap<Identifier, CustomSyntax>,
     /// Callback closure for filtering variable definition.
     pub(crate) def_var_filter: Option<Box<OnDefVarCallback>>,
     /// Callback closure for resolving variable access.
@@ -155,7 +155,14 @@ impl fmt::Debug for Engine {
 
         f.field("disabled_symbols", &self.disabled_symbols)
             .field("custom_keywords", &self.custom_keywords)
-            .field("custom_syntax", &(!self.custom_syntax.is_empty()))
+            .field(
+                "custom_syntax",
+                &self
+                    .custom_syntax
+                    .keys()
+                    .map(|s| s.as_str())
+                    .collect::<String>(),
+            )
             .field("def_var_filter", &self.def_var_filter.is_some())
             .field("resolve_var", &self.resolve_var.is_some())
             .field("token_mapper", &self.token_mapper.is_some());