diff --git a/CHANGELOG.md b/CHANGELOG.md
index 28e36210..d1cf25db 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -15,11 +15,12 @@ Script-breaking changes
 
 * _Strict Variables Mode_ no longer returns an error when an undeclared variable matches a variable/constant in the provided external `Scope`.
 
-Changes to unstable API's
--------------------------
+Potentially breaking API changes
+--------------------------------
 
 * The `Engine::on_var` and `Engine::on_parse_token` API's are now marked unstable/volatile.
 * The closures passed to `Engine::on_var`, `Engine::on_def_var` and `Engine::register_debugger` take `EvalContext` instead of `&EvalContext` or `&mut EvalContext`.
+* The following enum's are marked `non_exhaustive`: `AccessMode`, `FnAccess`, `FnNamespace`, `FnMetadata`, `OptimizationLevel`
 
 New API
 -------
diff --git a/src/ast/flags.rs b/src/ast/flags.rs
index 6d2506ba..34a3bbaa 100644
--- a/src/ast/flags.rs
+++ b/src/ast/flags.rs
@@ -6,6 +6,7 @@ use std::prelude::v1::*;
 
 /// A type representing the access mode of a function.
 #[derive(Debug, Clone, Copy, Eq, PartialEq, Ord, PartialOrd, Hash)]
+#[non_exhaustive]
 pub enum FnAccess {
     /// Private function.
     Private,
diff --git a/src/module/mod.rs b/src/module/mod.rs
index ea9b7ebe..5ad8e48e 100644
--- a/src/module/mod.rs
+++ b/src/module/mod.rs
@@ -23,6 +23,7 @@ use std::{
 
 /// A type representing the namespace of a function.
 #[derive(Debug, Clone, Copy, Eq, PartialEq, Ord, PartialOrd, Hash)]
+#[non_exhaustive]
 pub enum FnNamespace {
     /// Module namespace only.
     ///
@@ -34,6 +35,7 @@ pub enum FnNamespace {
 
 /// A type containing all metadata for a registered function.
 #[derive(Debug, Clone, Eq, PartialEq, Hash)]
+#[non_exhaustive]
 pub struct FnMetadata {
     /// Function namespace.
     pub namespace: FnNamespace,
diff --git a/src/optimizer.rs b/src/optimizer.rs
index f18b295b..69731719 100644
--- a/src/optimizer.rs
+++ b/src/optimizer.rs
@@ -23,6 +23,7 @@ use std::{
 
 /// Level of optimization performed.
 #[derive(Debug, Eq, PartialEq, Hash, Clone, Copy)]
+#[non_exhaustive]
 pub enum OptimizationLevel {
     /// No optimization performed.
     None,
diff --git a/src/types/dynamic.rs b/src/types/dynamic.rs
index 792d008b..0b5e891c 100644
--- a/src/types/dynamic.rs
+++ b/src/types/dynamic.rs
@@ -127,6 +127,7 @@ impl dyn Variant {
 /// _(internals)_ Modes of access.
 /// Exported under the `internals` feature only.
 #[derive(Debug, Eq, PartialEq, Hash, Copy, Clone)]
+#[non_exhaustive]
 pub enum AccessMode {
     /// Mutable.
     ReadWrite,