Change Option<usize> to usize for variable resolver index.

src/api.rs

@@ -1721,12 +1721,7 @@ impl Engine {
     #[inline(always)]
     pub fn on_var(
         &mut self,
-        callback: impl Fn(
-                &str,
-                Option<usize>,
-                &Scope,
-                &EvalContext,
-            ) -> Result<Option<Dynamic>, Box<EvalAltResult>>
+        callback: impl Fn(&str, usize, &Scope, &EvalContext) -> Result<Option<Dynamic>, Box<EvalAltResult>>
             + SendSync
             + 'static,
     ) -> &mut Self {

src/engine.rs

@@ -745,7 +745,7 @@ impl Engine {
                 this_ptr,
                 level: 0,
             };
-            if let Some(result) = resolve_var(name, index.map(|v| v.get()), scope, &context)
+            if let Some(result) = resolve_var(name, index.map_or(0, |v| v.get()), scope, &context)
                 .map_err(|err| err.fill_position(*pos))?
             {
                 return Ok((result.into(), name, ScopeEntryType::Constant, *pos));

src/fn_native.rs

@@ -224,13 +224,13 @@ pub type Callback<T, R> = Box<dyn Fn(&T) -> R + Send + Sync + 'static>;
 /// A standard callback function.
 #[cfg(not(feature = "sync"))]
 pub type OnVarCallback = Box<
-    dyn Fn(&str, Option<usize>, &Scope, &EvalContext) -> Result<Option<Dynamic>, Box<EvalAltResult>>
+    dyn Fn(&str, usize, &Scope, &EvalContext) -> Result<Option<Dynamic>, Box<EvalAltResult>>
         + 'static,
 >;
 /// A standard callback function.
 #[cfg(feature = "sync")]
 pub type OnVarCallback = Box<
-    dyn Fn(&str, Option<usize>, &Scope, &EvalContext) -> Result<Option<Dynamic>, Box<EvalAltResult>>
+    dyn Fn(&str, usize, &Scope, &EvalContext) -> Result<Option<Dynamic>, Box<EvalAltResult>>
         + Send
         + Sync
         + 'static,