diff --git a/src/api/events.rs b/src/api/events.rs
index 134d9486..0f0061d4 100644
--- a/src/api/events.rs
+++ b/src/api/events.rs
@@ -5,6 +5,19 @@ use crate::{Dynamic, Engine, EvalContext, Position, RhaiResultOf};
 #[cfg(feature = "no_std")]
 use std::prelude::v1::*;
 
+/// Information on a variable definition.
+#[non_exhaustive]
+pub struct VarDefInfo<'a> {
+    /// Name of the variable to be defined.
+    pub name: &'a str,
+    /// `true` if the statement is `const`, otherwise it is `let`.
+    pub is_const: bool,
+    /// The current nesting level, with zero being the global level.
+    pub nesting_level: usize,
+    /// Will the variable _shadow_ an existing variable?
+    pub will_shadow: bool,
+}
+
 impl Engine {
     /// Provide a callback that will be invoked before each variable access.
     ///
@@ -65,18 +78,19 @@ impl Engine {
     }
     /// Provide a callback that will be invoked before the definition of each variable .
     ///
+    /// # WARNING - Unstable API
+    ///
+    /// This API is volatile and may change in the future.
+    ///
     /// # Callback Function Signature
     ///
     /// The callback function signature takes the following form:
     ///
-    /// > `Fn(name: &str, is_const: bool, block_level: usize, will_shadow: bool, context: &EvalContext) -> Result<bool, Box<EvalAltResult>>`
+    /// > `Fn(is_runtime: bool, info: VarInfo, context: &EvalContext) -> Result<bool, Box<EvalAltResult>>`
     ///
     /// where:
-    /// * `name`: name of the variable to be defined.
     /// * `is_runtime`: `true` if the variable definition event happens during runtime, `false` if during compilation.
-    /// * `is_const`: `true` if the statement is `const`, otherwise it is `let`.
-    /// * `block_level`: the current nesting level of statement blocks, with zero being the global level.
-    /// * `will_shadow`: will the variable _shadow_ an existing variable?
+    /// * `info`: information on the variable.
     /// * `context`: the current [evaluation context][`EvalContext`].
     ///
     /// ## Return value
@@ -97,9 +111,9 @@ impl Engine {
     /// let mut engine = Engine::new();
     ///
     /// // Register a variable definition filter.
-    /// engine.on_def_var(|name, _, is_const, _, _, _| {
+    /// engine.on_def_var(|_, info, _| {
     ///     // Disallow defining MYSTIC_NUMBER as a constant
-    ///     if name == "MYSTIC_NUMBER" && is_const {
+    ///     if info.name == "MYSTIC_NUMBER" && info.is_const {
     ///         Ok(false)
     ///     } else {
     ///         Ok(true)
@@ -115,12 +129,11 @@ impl Engine {
     /// # Ok(())
     /// # }
     /// ```
+    #[deprecated = "This API is volatile and may change in the future."]
     #[inline(always)]
     pub fn on_def_var(
         &mut self,
-        callback: impl Fn(&str, bool, bool, usize, bool, &EvalContext) -> RhaiResultOf<bool>
-            + SendSync
-            + 'static,
+        callback: impl Fn(bool, VarDefInfo, &EvalContext) -> RhaiResultOf<bool> + SendSync + 'static,
     ) -> &mut Self {
         self.def_var_filter = Some(Box::new(callback));
         self
@@ -322,8 +335,13 @@ impl Engine {
         self.debug = Some(Box::new(callback));
         self
     }
-    /// _(debugging)_ Register callbacks for debugging.
+    /// _(debugging)_ Register a callback for debugging.
     /// Exported under the `debugging` feature only.
+    ///
+    /// # WARNING - Unstable API
+    ///
+    /// This API is volatile and may change in the future.
+    #[deprecated = "This API is volatile and may change in the future."]
     #[cfg(feature = "debugging")]
     #[inline(always)]
     pub fn register_debugger(
diff --git a/src/eval/stmt.rs b/src/eval/stmt.rs
index 3e04d3e8..294705ac 100644
--- a/src/eval/stmt.rs
+++ b/src/eval/stmt.rs
@@ -1,6 +1,7 @@
 //! Module defining functions for evaluating a statement.
 
 use super::{EvalContext, EvalState, GlobalRuntimeState, Target};
+use crate::api::events::VarDefInfo;
 use crate::ast::{
     BinaryExpr, Expr, Ident, OpAssignment, Stmt, SwitchCases, TryCatchBlock, AST_OPTION_FLAGS::*,
 };
@@ -818,9 +819,15 @@ impl Engine {
                 let export = options.contains(AST_OPTION_EXPORTED);
 
                 let result = if let Some(ref filter) = self.def_var_filter {
-                    let shadowing = scope.contains(var_name);
-                    let scope_level = state.scope_level;
+                    let will_shadow = scope.contains(var_name);
+                    let nesting_level = state.scope_level;
                     let is_const = entry_type == AccessMode::ReadOnly;
+                    let info = VarDefInfo {
+                        name: var_name,
+                        is_const,
+                        nesting_level,
+                        will_shadow,
+                    };
                     let context = EvalContext {
                         engine: self,
                         scope,
@@ -828,10 +835,10 @@ impl Engine {
                         state,
                         lib,
                         this_ptr,
-                        level: level,
+                        level,
                     };
 
-                    match filter(var_name, true, is_const, scope_level, shadowing, &context) {
+                    match filter(true, info, &context) {
                         Ok(true) => None,
                         Ok(false) => {
                             Some(Err(
diff --git a/src/func/native.rs b/src/func/native.rs
index 9d97eb96..66e793a3 100644
--- a/src/func/native.rs
+++ b/src/func/native.rs
@@ -1,6 +1,7 @@
 //! Module defining interfaces to native-Rust functions.
 
 use super::call::FnCallArgs;
+use crate::api::events::VarDefInfo;
 use crate::ast::FnCallHashes;
 use crate::eval::{EvalState, GlobalRuntimeState};
 use crate::plugin::PluginFunction;
@@ -447,9 +448,8 @@ pub type OnVarCallback =
 
 /// Callback function for variable definition.
 #[cfg(not(feature = "sync"))]
-pub type OnDefVarCallback =
-    dyn Fn(&str, bool, bool, usize, bool, &EvalContext) -> RhaiResultOf<bool>;
+pub type OnDefVarCallback = dyn Fn(bool, VarDefInfo, &EvalContext) -> RhaiResultOf<bool>;
 /// Callback function for variable definition.
 #[cfg(feature = "sync")]
 pub type OnDefVarCallback =
-    dyn Fn(&str, bool, bool, usize, bool, &EvalContext) -> RhaiResultOf<bool> + Send + Sync;
+    dyn Fn(bool, VarDefInfo, &EvalContext) -> RhaiResultOf<bool> + Send + Sync;
diff --git a/src/lib.rs b/src/lib.rs
index 72417e22..07612641 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -147,6 +147,7 @@ type ExclusiveRange = std::ops::Range<INT>;
 type InclusiveRange = std::ops::RangeInclusive<INT>;
 
 pub use api::custom_syntax::Expression;
+pub use api::events::VarDefInfo;
 pub use ast::{FnAccess, AST};
 pub use engine::{Engine, OP_CONTAINS, OP_EQUALS};
 pub use eval::EvalContext;
diff --git a/src/parser.rs b/src/parser.rs
index 2bcabfe0..74f97c2a 100644
--- a/src/parser.rs
+++ b/src/parser.rs
@@ -1,6 +1,7 @@
 //! Main module defining the lexer and parser.
 
 use crate::api::custom_syntax::{markers::*, CustomSyntax};
+use crate::api::events::VarDefInfo;
 use crate::api::options::LanguageOptions;
 use crate::ast::{
     BinaryExpr, ConditionalStmtBlock, CustomExpr, Expr, FnCallExpr, FnCallHashes, Ident,
@@ -109,7 +110,7 @@ impl<'e> ParseState<'e> {
     #[inline]
     #[must_use]
     pub fn access_var(&mut self, name: &str, pos: Position) -> Option<NonZeroUsize> {
-        let mut barrier = false;
+        let mut hit_barrier = false;
         let _pos = pos;
 
         let index = self
@@ -119,7 +120,7 @@ impl<'e> ParseState<'e> {
             .find(|&(.., (n, ..))| {
                 if n == SCOPE_SEARCH_BARRIER_MARKER {
                     // Do not go beyond the barrier
-                    barrier = true;
+                    hit_barrier = true;
                     false
                 } else {
                     n == name
@@ -139,7 +140,7 @@ impl<'e> ParseState<'e> {
             self.allow_capture = true
         }
 
-        if barrier {
+        if hit_barrier {
             None
         } else {
             index
@@ -2566,16 +2567,13 @@ fn parse_for(
         let name = state.get_identifier("", name);
         let pos = counter_pos.expect("`Some`");
         state.stack.push(name.clone(), ());
-        Ident {
-            name: name.clone(),
-            pos,
-        }
+        Ident { name, pos }
     });
 
     let loop_var = state.get_identifier("", name);
     state.stack.push(loop_var.clone(), ());
     let loop_var = Ident {
-        name: loop_var.clone(),
+        name: loop_var,
         pos: name_pos,
     };
 
@@ -2617,9 +2615,15 @@ fn parse_let(
     }
 
     if let Some(ref filter) = state.engine.def_var_filter {
-        let shadowing = state.stack.iter().any(|(v, ..)| v == name.as_ref());
+        let will_shadow = state.stack.iter().any(|(v, ..)| v == name.as_ref());
         let level = settings.level;
         let is_const = var_type == AccessMode::ReadOnly;
+        let info = VarDefInfo {
+            name: &name,
+            is_const,
+            nesting_level: level,
+            will_shadow,
+        };
         let context = EvalContext {
             engine: state.engine,
             scope: &mut state.stack,
@@ -2630,7 +2634,7 @@ fn parse_let(
             level,
         };
 
-        match filter(&name, false, is_const, level, shadowing, &context) {
+        match filter(false, info, &context) {
             Ok(true) => (),
             Ok(false) => return Err(PERR::ForbiddenVariable(name.to_string()).into_err(pos)),
             Err(err) => match *err {
diff --git a/tests/var_scope.rs b/tests/var_scope.rs
index 4b07ab71..84678747 100644
--- a/tests/var_scope.rs
+++ b/tests/var_scope.rs
@@ -128,7 +128,7 @@ fn test_var_def_filter() -> Result<(), Box<EvalAltResult>> {
     let ast = engine.compile("let x = 42;")?;
     engine.run_ast(&ast)?;
 
-    engine.on_def_var(|name, _, _, scope_level, _, _| match (name, scope_level) {
+    engine.on_def_var(|_, info, _| match (info.name, info.nesting_level) {
         ("x", 0 | 1) => Ok(false),
         _ => Ok(true),
     });