diff --git a/CHANGELOG.md b/CHANGELOG.md
index a83391e6..e5df697b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -34,6 +34,7 @@ Breaking changes
 * The _reflections_ API such as `Engine::gen_fn_signatures`, `Module::update_fn_metadata` etc. are put under the `metadata` feature gate.
 * The shebang `#!` is now a reserved symbol.
 * Shebangs at the very beginning of script files are skipped when loading them.
+* [`smartstring`](https://crates.io/crates/smartstring) is used for identifiers.
 
 Enhancements
 ------------
diff --git a/Cargo.toml b/Cargo.toml
index b16375d0..f2f17533 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -17,6 +17,7 @@ categories = ["no-std", "embedded", "wasm", "parser-implementations"]
 
 [dependencies]
 smallvec = { version = "1.6", default-features = false, features = ["union"] }
+smartstring = { version = "0.2.6" }
 ahash = { version = "0.7", default-features = false }
 num-traits = { version = "0.2", default_features = false }
 rhai_codegen = { version = "0.3.4", path = "codegen", features = ["metadata"] }
diff --git a/README.md b/README.md
index 57cc560e..8042448b 100644
--- a/README.md
+++ b/README.md
@@ -35,7 +35,7 @@ Standard features
 * Built-in support for most common [data types](https://rhai.rs/book/language/values-and-types.html) including booleans, [integers](https://rhai.rs/book/language/numbers.html), [floating-point numbers](https://rhai.rs/book/language/numbers.html) (including [`Decimal`](https://crates.io/crates/rust_decimal)), [strings](https://rhai.rs/book/language/strings-chars.html), [Unicode characters](https://rhai.rs/book/language/strings-chars.html), [arrays](https://rhai.rs/book/language/arrays.html) and [object maps](https://rhai.rs/book/language/object-maps.html).
 * Easily [call a script-defined function](https://rhai.rs/book/engine/call-fn.html) from Rust.
 * Relatively little `unsafe` code (yes there are some for performance reasons).
-* Few dependencies (currently only [`smallvec`](https://crates.io/crates/smallvec), [`num-traits`](https://crates.io/crates/num-traits) and [`ahash`](https://crates.io/crates/ahash)).
+* Few dependencies (currently only [`smallvec`](https://crates.io/crates/smallvec), [`num-traits`](https://crates.io/crates/num-traits), [`ahash`](https://crates.io/crates/ahash)) and [`smartstring`](https://crates.io/crates/smartstring)).
 * Re-entrant scripting engine can be made `Send + Sync` (via the `sync` feature).
 * Compile once to [AST](https://rhai.rs/book/engine/compile.html) form for repeated evaluations.
 * Scripts are [optimized](https://rhai.rs/book/engine/optimize.html) (useful for template-based machine-generated scripts).
diff --git a/src/ast.rs b/src/ast.rs
index 07c0870f..02f30e1f 100644
--- a/src/ast.rs
+++ b/src/ast.rs
@@ -16,7 +16,8 @@ use crate::stdlib::{
 };
 use crate::token::Token;
 use crate::{
-    Dynamic, FnNamespace, FnPtr, ImmutableString, Module, Position, Shared, StaticVec, INT,
+    Dynamic, FnNamespace, FnPtr, Identifier, ImmutableString, Module, Position, Shared, StaticVec,
+    INT,
 };
 
 #[cfg(not(feature = "no_float"))]
@@ -50,14 +51,14 @@ pub struct ScriptFnDef {
     #[cfg(not(feature = "no_module"))]
     pub mods: crate::engine::Imports,
     /// Function name.
-    pub name: ImmutableString,
+    pub name: Identifier,
     /// Function access mode.
     pub access: FnAccess,
     /// Names of function parameters.
-    pub params: StaticVec<ImmutableString>,
+    pub params: StaticVec<Identifier>,
     /// Access to external variables.
     #[cfg(not(feature = "no_closure"))]
-    pub externals: BTreeSet<ImmutableString>,
+    pub externals: BTreeSet<Identifier>,
     /// Function doc-comments (if any).
     pub comments: StaticVec<String>,
 }
@@ -144,7 +145,7 @@ impl<'a> Into<ScriptFnMetadata<'a>> for &'a ScriptFnDef {
 #[derive(Debug, Clone)]
 pub struct AST {
     /// Source of the [`AST`].
-    source: Option<ImmutableString>,
+    source: Option<Identifier>,
     /// Global statements.
     body: StmtBlock,
     /// Script-defined functions.
@@ -190,7 +191,7 @@ impl AST {
     pub fn new_with_source(
         statements: impl IntoIterator<Item = Stmt>,
         functions: impl Into<Shared<Module>>,
-        source: impl Into<ImmutableString>,
+        source: impl Into<Identifier>,
     ) -> Self {
         Self {
             source: Some(source.into()),
@@ -210,12 +211,12 @@ impl AST {
     }
     /// Clone the source, if any.
     #[inline(always)]
-    pub(crate) fn clone_source(&self) -> Option<ImmutableString> {
+    pub(crate) fn clone_source(&self) -> Option<Identifier> {
         self.source.clone()
     }
     /// Set the source.
     #[inline(always)]
-    pub fn set_source(&mut self, source: impl Into<ImmutableString>) -> &mut Self {
+    pub fn set_source(&mut self, source: impl Into<Identifier>) -> &mut Self {
         self.source = Some(source.into());
 
         if let Some(module) = Shared::get_mut(&mut self.functions) {
@@ -756,7 +757,7 @@ impl AsRef<Module> for AST {
     }
 }
 
-/// _(INTERNALS)_ An identifier containing an [immutable string][ImmutableString] name and a [position][Position].
+/// _(INTERNALS)_ An identifier containing a name and a [position][Position].
 /// Exported under the `internals` feature only.
 ///
 /// # Volatile Data Structure
@@ -765,7 +766,7 @@ impl AsRef<Module> for AST {
 #[derive(Clone, Eq, PartialEq, Hash)]
 pub struct Ident {
     /// Identifier name.
-    pub name: ImmutableString,
+    pub name: Identifier,
     /// Declaration position.
     pub pos: Position,
 }
@@ -869,11 +870,11 @@ pub enum Stmt {
     /// `do` `{` stmt `}` `while`|`until` expr
     Do(Box<StmtBlock>, Expr, bool, Position),
     /// `for` id `in` expr `{` stmt `}`
-    For(Expr, Box<(String, StmtBlock)>, Position),
+    For(Expr, Box<(Identifier, StmtBlock)>, Position),
     /// \[`export`\] `let` id `=` expr
-    Let(Expr, Ident, bool, Position),
+    Let(Expr, Box<Ident>, bool, Position),
     /// \[`export`\] `const` id `=` expr
-    Const(Expr, Ident, bool, Position),
+    Const(Expr, Box<Ident>, bool, Position),
     /// expr op`=` expr
     Assignment(Box<(Expr, Expr, Option<OpAssignment>)>, Position),
     /// `{` stmt`;` ... `}`
@@ -894,7 +895,7 @@ pub enum Stmt {
     Return(ReturnType, Option<Expr>, Position),
     /// `import` expr `as` var
     #[cfg(not(feature = "no_module"))]
-    Import(Expr, Option<Ident>, Position),
+    Import(Expr, Option<Box<Ident>>, Position),
     /// `export` var `as` var `,` ...
     #[cfg(not(feature = "no_module"))]
     Export(Vec<(Ident, Option<Ident>)>, Position),
@@ -1256,7 +1257,7 @@ pub struct CustomExpr {
     /// List of keywords.
     pub keywords: StaticVec<Expr>,
     /// List of tokens actually parsed.
-    pub tokens: Vec<ImmutableString>,
+    pub tokens: Vec<Identifier>,
     /// Delta number of variables in the scope.
     pub scope_delta: isize,
 }
@@ -1400,11 +1401,11 @@ pub struct FnCallExpr {
     /// List of function call argument expressions.
     pub args: StaticVec<Expr>,
     /// List of function call arguments that are constants.
-    pub constant_args: StaticVec<(Dynamic, Position)>,
+    pub constant_args: smallvec::SmallVec<[(Dynamic, Position); 2]>,
     /// Namespace of the function, if any. Boxed because it occurs rarely.
     pub namespace: Option<NamespaceRef>,
     /// Function name.
-    pub name: ImmutableString,
+    pub name: Identifier,
 }
 
 impl FnCallExpr {
@@ -1554,7 +1555,7 @@ pub enum Expr {
     /// Variable access - (optional index, optional (hash, modules), variable name)
     Variable(Box<(Option<NonZeroUsize>, Option<(u64, NamespaceRef)>, Ident)>),
     /// Property access - ((getter, hash), (setter, hash), prop)
-    Property(Box<((ImmutableString, u64), (ImmutableString, u64), Ident)>),
+    Property(Box<((Identifier, u64), (Identifier, u64), Ident)>),
     /// { [statement][Stmt] ... }
     Stmt(Box<StmtBlock>),
     /// func `(` expr `,` ... `)`
@@ -1609,7 +1610,7 @@ impl Expr {
             Self::Map(x, _) if self.is_constant() => {
                 let mut map = x.1.clone();
                 x.0.iter().for_each(|(k, v)| {
-                    *map.get_mut(&k.name).unwrap() = v.get_constant_value().unwrap()
+                    *map.get_mut(k.name.as_str()).unwrap() = v.get_constant_value().unwrap()
                 });
                 Dynamic(Union::Map(Box::new(map), AccessMode::ReadOnly))
             }
diff --git a/src/dynamic.rs b/src/dynamic.rs
index 7bde7ba2..f32b68b7 100644
--- a/src/dynamic.rs
+++ b/src/dynamic.rs
@@ -1667,6 +1667,18 @@ impl<S: Into<ImmutableString>> From<S> for Dynamic {
         Self(Union::Str(value.into(), AccessMode::ReadWrite))
     }
 }
+impl From<&ImmutableString> for Dynamic {
+    #[inline(always)]
+    fn from(value: &ImmutableString) -> Self {
+        value.clone().into()
+    }
+}
+impl<C: smartstring::SmartStringMode> From<&smartstring::SmartString<C>> for Dynamic {
+    #[inline(always)]
+    fn from(value: &smartstring::SmartString<C>) -> Self {
+        value.to_string().into()
+    }
+}
 #[cfg(not(feature = "no_index"))]
 impl<T: Variant + Clone> From<crate::stdlib::vec::Vec<T>> for Dynamic {
     #[inline(always)]
diff --git a/src/engine.rs b/src/engine.rs
index 442e806f..e8255c63 100644
--- a/src/engine.rs
+++ b/src/engine.rs
@@ -25,8 +25,8 @@ use crate::stdlib::{
 use crate::syntax::CustomSyntax;
 use crate::utils::get_hasher;
 use crate::{
-    Dynamic, EvalAltResult, FnPtr, ImmutableString, Module, Position, RhaiResult, Scope, Shared,
-    StaticVec,
+    Dynamic, EvalAltResult, FnPtr, Identifier, ImmutableString, Module, Position, RhaiResult,
+    Scope, Shared, StaticVec,
 };
 
 #[cfg(not(feature = "no_index"))]
@@ -50,7 +50,7 @@ pub type Precedence = NonZeroU8;
 // the module name will live beyond the AST of the eval script text.
 // The best we can do is a shared reference.
 #[derive(Debug, Clone, Default)]
-pub struct Imports(StaticVec<ImmutableString>, StaticVec<Shared<Module>>);
+pub struct Imports(StaticVec<Identifier>, StaticVec<Shared<Module>>);
 
 impl Imports {
     /// Get the length of this stack of imported [modules][Module].
@@ -79,7 +79,7 @@ impl Imports {
     }
     /// Push an imported [modules][Module] onto the stack.
     #[inline(always)]
-    pub fn push(&mut self, name: impl Into<ImmutableString>, module: impl Into<Shared<Module>>) {
+    pub fn push(&mut self, name: impl Into<Identifier>, module: impl Into<Shared<Module>>) {
         self.0.push(name.into());
         self.1.push(module.into());
     }
@@ -102,18 +102,18 @@ impl Imports {
     /// Get an iterator to this stack of imported [modules][Module] in reverse order.
     #[allow(dead_code)]
     #[inline(always)]
-    pub(crate) fn iter_raw(&self) -> impl Iterator<Item = (&ImmutableString, &Shared<Module>)> {
+    pub(crate) fn iter_raw(&self) -> impl Iterator<Item = (&Identifier, &Shared<Module>)> {
         self.0.iter().rev().zip(self.1.iter().rev())
     }
     /// Get an iterator to this stack of imported [modules][Module] in forward order.
     #[allow(dead_code)]
     #[inline(always)]
-    pub(crate) fn scan_raw(&self) -> impl Iterator<Item = (&ImmutableString, &Shared<Module>)> {
+    pub(crate) fn scan_raw(&self) -> impl Iterator<Item = (&Identifier, &Shared<Module>)> {
         self.0.iter().zip(self.1.iter())
     }
     /// Get a consuming iterator to this stack of imported [modules][Module] in reverse order.
     #[inline(always)]
-    pub fn into_iter(self) -> impl Iterator<Item = (ImmutableString, Shared<Module>)> {
+    pub fn into_iter(self) -> impl Iterator<Item = (Identifier, Shared<Module>)> {
         self.0.into_iter().rev().zip(self.1.into_iter().rev())
     }
     /// Does the specified function hash key exist in this stack of imported [modules][Module]?
@@ -124,7 +124,7 @@ impl Imports {
     }
     /// Get specified function via its hash key.
     #[inline(always)]
-    pub fn get_fn(&self, hash: u64) -> Option<(&CallableFunction, Option<&ImmutableString>)> {
+    pub fn get_fn(&self, hash: u64) -> Option<(&CallableFunction, Option<&Identifier>)> {
         self.1
             .iter()
             .rev()
@@ -491,7 +491,7 @@ pub struct FnResolutionCacheEntry {
     /// Function.
     pub func: CallableFunction,
     /// Optional source.
-    pub source: Option<ImmutableString>,
+    pub source: Option<Identifier>,
 }
 
 /// A function resolution cache.
@@ -506,7 +506,7 @@ pub type FnResolutionCache = BTreeMap<u64, Option<FnResolutionCacheEntry>>;
 #[derive(Debug, Clone, Default)]
 pub struct State {
     /// Source of the current context.
-    pub source: Option<ImmutableString>,
+    pub source: Option<Identifier>,
     /// Normally, access to variables are parsed with a relative offset into the scope to avoid a lookup.
     /// In some situation, e.g. after running an `eval` statement, subsequent offsets become mis-aligned.
     /// When that happens, this flag is turned on to force a scope lookup by name.
@@ -703,7 +703,7 @@ pub struct Engine {
     /// A collection of all modules loaded into the global namespace of the Engine.
     pub(crate) global_modules: StaticVec<Shared<Module>>,
     /// A collection of all sub-modules directly loaded into the Engine.
-    pub(crate) global_sub_modules: BTreeMap<ImmutableString, Shared<Module>>,
+    pub(crate) global_sub_modules: BTreeMap<Identifier, Shared<Module>>,
 
     /// A module resolution service.
     #[cfg(not(feature = "no_module"))]
@@ -717,7 +717,7 @@ pub struct Engine {
     /// A map containing custom keywords and precedence to recognize.
     pub(crate) custom_keywords: BTreeMap<String, Option<Precedence>>,
     /// Custom syntax.
-    pub(crate) custom_syntax: BTreeMap<ImmutableString, CustomSyntax>,
+    pub(crate) custom_syntax: BTreeMap<Identifier, CustomSyntax>,
     /// Callback closure for resolving variable access.
     pub(crate) resolve_var: Option<OnVarCallback>,
 
@@ -1183,7 +1183,7 @@ impl Engine {
                     // {xxx:map}.id op= ???
                     Expr::Property(x) if target_val.is::<Map>() && new_val.is_some() => {
                         let Ident { name, pos, .. } = &x.2;
-                        let index = name.clone().into();
+                        let index = name.into();
                         let val = self.get_indexed_mut(
                             mods, state, lib, target_val, index, *pos, true, is_ref, false, level,
                         )?;
@@ -1196,7 +1196,7 @@ impl Engine {
                     // {xxx:map}.id
                     Expr::Property(x) if target_val.is::<Map>() => {
                         let Ident { name, pos, .. } = &x.2;
-                        let index = name.clone().into();
+                        let index = name.into();
                         let val = self.get_indexed_mut(
                             mods, state, lib, target_val, index, *pos, false, is_ref, false, level,
                         )?;
@@ -1231,7 +1231,7 @@ impl Engine {
                         let mut val = match &x.lhs {
                             Expr::Property(p) => {
                                 let Ident { name, pos, .. } = &p.2;
-                                let index = name.clone().into();
+                                let index = name.into();
                                 self.get_indexed_mut(
                                     mods, state, lib, target_val, index, *pos, false, is_ref, true,
                                     level,
@@ -1696,7 +1696,7 @@ impl Engine {
             Expr::Map(x, _) => {
                 let mut map = x.1.clone();
                 for (Ident { name: key, .. }, expr) in &x.0 {
-                    *map.get_mut(key).unwrap() = self
+                    *map.get_mut(key.as_str()).unwrap() = self
                         .eval_expr(scope, mods, state, lib, this_ptr, expr, level)?
                         .flatten();
                 }
@@ -2209,7 +2209,7 @@ impl Engine {
                 if let Some(func) = func {
                     // Add the loop variable
                     let var_name: Cow<'_, str> = if state.is_global() {
-                        name.clone().into()
+                        name.to_string().into()
                     } else {
                         unsafe_cast_var_name_to_lifetime(name).into()
                     };
@@ -2300,7 +2300,7 @@ impl Engine {
                                 err_map.insert("message".into(), err.to_string().into());
 
                                 if let Some(ref source) = state.source {
-                                    err_map.insert("source".into(), source.clone().into());
+                                    err_map.insert("source".into(), source.into());
                                 }
 
                                 if err_pos.is_none() {
@@ -2382,8 +2382,8 @@ impl Engine {
             }
 
             // Let/const statement
-            Stmt::Let(expr, Ident { name, .. }, export, _)
-            | Stmt::Const(expr, Ident { name, .. }, export, _) => {
+            Stmt::Let(expr, x, export, _) | Stmt::Const(expr, x, export, _) => {
+                let name = &x.name;
                 let entry_type = match stmt {
                     Stmt::Let(_, _, _, _) => AccessMode::ReadWrite,
                     Stmt::Const(_, _, _, _) => AccessMode::ReadOnly,
diff --git a/src/engine_api.rs b/src/engine_api.rs
index 8703d3d0..cd37ac6c 100644
--- a/src/engine_api.rs
+++ b/src/engine_api.rs
@@ -12,7 +12,7 @@ use crate::stdlib::{
     string::String,
 };
 use crate::{
-    scope::Scope, Dynamic, Engine, EvalAltResult, FnAccess, FnNamespace, ImmutableString, Module,
+    scope::Scope, Dynamic, Engine, EvalAltResult, FnAccess, FnNamespace, Identifier, Module,
     NativeCallContext, ParseError, Position, RhaiResult, Shared, AST,
 };
 
@@ -53,7 +53,7 @@ impl Engine {
     #[inline]
     pub fn register_fn<N, A, F>(&mut self, name: N, func: F) -> &mut Self
     where
-        N: AsRef<str> + Into<ImmutableString>,
+        N: AsRef<str> + Into<Identifier>,
         F: RegisterNativeFunction<A, ()>,
     {
         let param_types = F::param_types();
@@ -113,7 +113,7 @@ impl Engine {
     #[inline]
     pub fn register_result_fn<N, A, F, R>(&mut self, name: N, func: F) -> &mut Self
     where
-        N: AsRef<str> + Into<ImmutableString>,
+        N: AsRef<str> + Into<Identifier>,
         F: RegisterNativeFunction<A, Result<R, Box<EvalAltResult>>>,
     {
         let param_types = F::param_types();
@@ -170,7 +170,7 @@ impl Engine {
             + 'static,
     ) -> &mut Self
     where
-        N: AsRef<str> + Into<ImmutableString>,
+        N: AsRef<str> + Into<Identifier>,
         T: Variant + Clone,
     {
         self.global_namespace.set_raw_fn(
@@ -901,12 +901,12 @@ impl Engine {
     #[cfg(not(feature = "no_module"))]
     pub fn register_static_module(
         &mut self,
-        name: impl AsRef<str> + Into<ImmutableString>,
+        name: impl AsRef<str> + Into<Identifier>,
         module: Shared<Module>,
     ) -> &mut Self {
         fn register_static_module_raw(
-            root: &mut crate::stdlib::collections::BTreeMap<crate::ImmutableString, Shared<Module>>,
-            name: impl AsRef<str> + Into<ImmutableString>,
+            root: &mut crate::stdlib::collections::BTreeMap<Identifier, Shared<Module>>,
+            name: impl AsRef<str> + Into<Identifier>,
             module: Shared<Module>,
         ) {
             let separator = crate::token::Token::DoubleColon.syntax();
@@ -952,7 +952,7 @@ impl Engine {
     #[deprecated(since = "0.19.9", note = "use `register_static_module` instead")]
     pub fn register_module(
         &mut self,
-        name: impl AsRef<str> + Into<ImmutableString>,
+        name: impl AsRef<str> + Into<Identifier>,
         module: impl Into<Shared<Module>>,
     ) -> &mut Self {
         self.register_static_module(name, module.into())
@@ -1045,14 +1045,14 @@ impl Engine {
         fn collect_imports(
             ast: &AST,
             resolver: &StaticModuleResolver,
-            imports: &mut BTreeSet<ImmutableString>,
+            imports: &mut BTreeSet<Identifier>,
         ) {
             ast.walk(&mut |path| match path.last().unwrap() {
                 // Collect all `import` statements with a string constant path
                 ASTNode::Stmt(Stmt::Import(Expr::StringConstant(s, _), _, _))
-                    if !resolver.contains_path(s) && !imports.contains(s) =>
+                    if !resolver.contains_path(s) && !imports.contains(s.as_str()) =>
                 {
-                    imports.insert(s.clone());
+                    imports.insert(s.clone().into());
                     true
                 }
                 _ => true,
diff --git a/src/fn_native.rs b/src/fn_native.rs
index e893550b..4535cf8f 100644
--- a/src/fn_native.rs
+++ b/src/fn_native.rs
@@ -13,8 +13,8 @@ use crate::stdlib::{
 };
 use crate::token::is_valid_identifier;
 use crate::{
-    calc_fn_hash, Dynamic, Engine, EvalAltResult, EvalContext, ImmutableString, Module, Position,
-    RhaiResult, StaticVec,
+    calc_fn_hash, Dynamic, Engine, EvalAltResult, EvalContext, Identifier, ImmutableString, Module,
+    Position, RhaiResult, StaticVec,
 };
 
 /// Trait that maps to `Send + Sync` only under the `sync` feature.
@@ -144,9 +144,7 @@ impl<'a> NativeCallContext<'a> {
     #[cfg(not(feature = "no_module"))]
     #[allow(dead_code)]
     #[inline(always)]
-    pub(crate) fn iter_imports_raw(
-        &self,
-    ) -> impl Iterator<Item = (&ImmutableString, &Shared<Module>)> {
+    pub(crate) fn iter_imports_raw(&self) -> impl Iterator<Item = (&Identifier, &Shared<Module>)> {
         self.mods.iter().flat_map(|&m| m.iter_raw())
     }
     /// _(INTERNALS)_ The current set of modules imported via `import` statements.
diff --git a/src/lib.rs b/src/lib.rs
index 87996279..04173c37 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -292,6 +292,8 @@ type StaticVec<T> = smallvec::SmallVec<[T; 4]>;
 #[cfg(feature = "internals")]
 pub type StaticVec<T> = smallvec::SmallVec<[T; 4]>;
 
+pub type Identifier = smartstring::SmartString<smartstring::Compact>;
+
 // Compiler guards against mutually-exclusive feature flags
 
 #[cfg(feature = "no_float")]
diff --git a/src/module/mod.rs b/src/module/mod.rs
index 472e4319..2e7924be 100644
--- a/src/module/mod.rs
+++ b/src/module/mod.rs
@@ -18,8 +18,8 @@ use crate::stdlib::{
 use crate::token::Token;
 use crate::utils::StringInterner;
 use crate::{
-    calc_fn_hash, calc_fn_params_hash, combine_hashes, Dynamic, EvalAltResult, ImmutableString,
-    NativeCallContext, Position, Shared, StaticVec,
+    calc_fn_hash, calc_fn_params_hash, combine_hashes, Dynamic, EvalAltResult, Identifier,
+    ImmutableString, NativeCallContext, Position, Shared, StaticVec,
 };
 
 #[cfg(not(feature = "no_index"))]
@@ -55,14 +55,14 @@ pub struct FuncInfo {
     /// Function access mode.
     pub access: FnAccess,
     /// Function name.
-    pub name: ImmutableString,
+    pub name: Identifier,
     /// Number of parameters.
     pub params: usize,
     /// Parameter types (if applicable).
     pub param_types: StaticVec<TypeId>,
     /// Parameter names (if available).
     #[cfg(feature = "metadata")]
-    pub param_names: StaticVec<ImmutableString>,
+    pub param_names: StaticVec<Identifier>,
 }
 
 impl FuncInfo {
@@ -128,11 +128,11 @@ fn calc_native_fn_hash<'a>(
 #[derive(Clone)]
 pub struct Module {
     /// ID identifying the module.
-    id: Option<ImmutableString>,
+    id: Option<Identifier>,
     /// Sub-modules.
-    modules: BTreeMap<ImmutableString, Shared<Module>>,
+    modules: BTreeMap<Identifier, Shared<Module>>,
     /// [`Module`] variables.
-    variables: BTreeMap<ImmutableString, Dynamic>,
+    variables: BTreeMap<Identifier, Dynamic>,
     /// Flattened collection of all [`Module`] variables, including those in sub-modules.
     all_variables: BTreeMap<u64, Dynamic>,
     /// External Rust functions.
@@ -288,7 +288,7 @@ impl Module {
         self.id_raw().map(|s| s.as_str())
     }
 
-    /// Get the ID of the [`Module`] as an [`ImmutableString`], if any.
+    /// Get the ID of the [`Module`] as an [`Identifier`], if any.
     ///
     /// # Example
     ///
@@ -300,7 +300,7 @@ impl Module {
     /// assert_eq!(module.id_raw().map(|s| s.as_str()), Some("hello"));
     /// ```
     #[inline(always)]
-    pub fn id_raw(&self) -> Option<&ImmutableString> {
+    pub fn id_raw(&self) -> Option<&Identifier> {
         self.id.as_ref()
     }
 
@@ -316,7 +316,7 @@ impl Module {
     /// assert_eq!(module.id(), Some("hello"));
     /// ```
     #[inline(always)]
-    pub fn set_id<S: Into<ImmutableString>>(&mut self, id: Option<S>) {
+    pub fn set_id<S: Into<Identifier>>(&mut self, id: Option<S>) {
         self.id = id.map(|s| s.into());
     }
 
@@ -440,7 +440,7 @@ impl Module {
     #[inline(always)]
     pub fn set_var(
         &mut self,
-        name: impl Into<ImmutableString>,
+        name: impl Into<Identifier>,
         value: impl Variant + Clone,
     ) -> &mut Self {
         self.variables.insert(name.into(), Dynamic::from(value));
@@ -514,7 +514,7 @@ impl Module {
     /// Thus the [`Module`] is automatically set to be non-indexed.
     #[cfg(not(feature = "no_module"))]
     #[inline(always)]
-    pub(crate) fn sub_modules_mut(&mut self) -> &mut BTreeMap<ImmutableString, Shared<Module>> {
+    pub(crate) fn sub_modules_mut(&mut self) -> &mut BTreeMap<Identifier, Shared<Module>> {
         // We must assume that the user has changed the sub-modules
         // (otherwise why take a mutable reference?)
         self.all_functions.clear();
@@ -577,7 +577,7 @@ impl Module {
     #[inline(always)]
     pub fn set_sub_module(
         &mut self,
-        name: impl Into<ImmutableString>,
+        name: impl Into<Identifier>,
         sub_module: impl Into<Shared<Module>>,
     ) -> &mut Self {
         self.modules.insert(name.into(), sub_module.into());
@@ -672,7 +672,7 @@ impl Module {
     #[inline]
     pub fn set_fn(
         &mut self,
-        name: impl AsRef<str> + Into<ImmutableString>,
+        name: impl AsRef<str> + Into<Identifier>,
         namespace: FnNamespace,
         access: FnAccess,
         _arg_names: Option<&[&str]>,
@@ -793,7 +793,7 @@ impl Module {
         func: F,
     ) -> u64
     where
-        N: AsRef<str> + Into<ImmutableString>,
+        N: AsRef<str> + Into<Identifier>,
         T: Variant + Clone,
         F: Fn(NativeCallContext, &mut FnCallArgs) -> Result<T, Box<EvalAltResult>>
             + SendSync
@@ -838,7 +838,7 @@ impl Module {
     #[inline(always)]
     pub fn set_native_fn<ARGS, N, T, F>(&mut self, name: N, func: F) -> u64
     where
-        N: AsRef<str> + Into<ImmutableString>,
+        N: AsRef<str> + Into<Identifier>,
         T: Variant + Clone,
         F: RegisterNativeFunction<ARGS, Result<T, Box<EvalAltResult>>>,
     {
diff --git a/src/optimize.rs b/src/optimize.rs
index 61edaac9..e4b62c26 100644
--- a/src/optimize.rs
+++ b/src/optimize.rs
@@ -1,6 +1,6 @@
 //! Module implementing the [`AST`] optimizer.
 
-use crate::ast::{Expr, Ident, Stmt, StmtBlock};
+use crate::ast::{Expr, Stmt, StmtBlock};
 use crate::dynamic::AccessMode;
 use crate::engine::{KEYWORD_DEBUG, KEYWORD_EVAL, KEYWORD_PRINT, KEYWORD_TYPE_OF};
 use crate::fn_builtin::get_builtin_binary_op_fn;
@@ -214,17 +214,17 @@ fn optimize_stmt_block(
         statements.iter_mut().for_each(|stmt| {
             match stmt {
                 // Add constant literals into the state
-                Stmt::Const(value_expr, Ident { name, .. }, _, _) => {
+                Stmt::Const(value_expr, x, _, _) => {
                     optimize_expr(value_expr, state);
 
                     if value_expr.is_constant() {
-                        state.push_var(name, AccessMode::ReadOnly, value_expr.clone());
+                        state.push_var(&x.name, AccessMode::ReadOnly, value_expr.clone());
                     }
                 }
                 // Add variables into the state
-                Stmt::Let(value_expr, Ident { name, pos, .. }, _, _) => {
+                Stmt::Let(value_expr, x, _, _) => {
                     optimize_expr(value_expr, state);
-                    state.push_var(name, AccessMode::ReadWrite, Expr::Unit(*pos));
+                    state.push_var(&x.name, AccessMode::ReadWrite, Expr::Unit(x.pos));
                 }
                 // Optimize the statement
                 _ => optimize_stmt(stmt, state, preserve_result),
@@ -649,7 +649,7 @@ fn optimize_expr(expr: &mut Expr, state: &mut State) {
                 // Map literal where everything is pure - promote the indexed item.
                 // All other items can be thrown away.
                 state.set_dirty();
-                *expr = mem::take(&mut m.0).into_iter().find(|(x, _)| x.name == *s)
+                *expr = mem::take(&mut m.0).into_iter().find(|(x, _)| x.name.as_str() == s.as_str())
                             .map(|(_, mut expr)| { expr.set_position(*pos); expr })
                             .unwrap_or_else(|| Expr::Unit(*pos));
             }
diff --git a/src/packages/fn_basic.rs b/src/packages/fn_basic.rs
index 65ae4469..634f5253 100644
--- a/src/packages/fn_basic.rs
+++ b/src/packages/fn_basic.rs
@@ -1,5 +1,5 @@
 use crate::plugin::*;
-use crate::{def_package, FnPtr, ImmutableString, NativeCallContext};
+use crate::{def_package, FnPtr, Identifier, ImmutableString, NativeCallContext};
 
 def_package!(crate:BasicFnPackage:"Basic Fn functions.", lib, {
     combine_with_exported_module!(lib, "FnPtr", fn_ptr_functions);
@@ -38,18 +38,21 @@ fn collect_fn_metadata(ctx: NativeCallContext) -> crate::Array {
 
     // Create a metadata record for a function.
     fn make_metadata(
-        dict: &BTreeSet<ImmutableString>,
-        namespace: Option<ImmutableString>,
+        dict: &BTreeSet<Identifier>,
+        namespace: Option<Identifier>,
         f: &ScriptFnDef,
     ) -> Map {
         let mut map = Map::new();
 
         if let Some(ns) = namespace {
-            map.insert(dict.get("namespace").unwrap().clone(), ns.into());
+            map.insert(dict.get("namespace").unwrap().clone().into(), ns.into());
         }
-        map.insert(dict.get("name").unwrap().clone(), f.name.clone().into());
         map.insert(
-            dict.get("access").unwrap().clone(),
+            dict.get("name").unwrap().clone().into(),
+            f.name.clone().into(),
+        );
+        map.insert(
+            dict.get("access").unwrap().clone().into(),
             match f.access {
                 FnAccess::Public => dict.get("public").unwrap().clone(),
                 FnAccess::Private => dict.get("private").unwrap().clone(),
@@ -57,11 +60,11 @@ fn collect_fn_metadata(ctx: NativeCallContext) -> crate::Array {
             .into(),
         );
         map.insert(
-            dict.get("is_anonymous").unwrap().clone(),
+            dict.get("is_anonymous").unwrap().clone().into(),
             f.name.starts_with(crate::engine::FN_ANONYMOUS).into(),
         );
         map.insert(
-            dict.get("params").unwrap().clone(),
+            dict.get("params").unwrap().clone().into(),
             f.params
                 .iter()
                 .cloned()
@@ -74,7 +77,7 @@ fn collect_fn_metadata(ctx: NativeCallContext) -> crate::Array {
     }
 
     // Intern strings
-    let dict: BTreeSet<ImmutableString> = [
+    let dict: BTreeSet<Identifier> = [
         "namespace",
         "name",
         "access",
@@ -98,8 +101,8 @@ fn collect_fn_metadata(ctx: NativeCallContext) -> crate::Array {
         // Recursively scan modules for script-defined functions.
         fn scan_module(
             list: &mut Array,
-            dict: &BTreeSet<ImmutableString>,
-            namespace: ImmutableString,
+            dict: &BTreeSet<Identifier>,
+            namespace: Identifier,
             module: &Module,
         ) {
             module.iter_script_fn().for_each(|(_, _, _, _, f)| {
diff --git a/src/parser.rs b/src/parser.rs
index a662aaee..daa02cfb 100644
--- a/src/parser.rs
+++ b/src/parser.rs
@@ -24,8 +24,8 @@ use crate::syntax::{CustomSyntax, MARKER_BLOCK, MARKER_EXPR, MARKER_IDENT};
 use crate::token::{is_keyword_function, is_valid_identifier, Token, TokenStream};
 use crate::utils::{get_hasher, StringInterner};
 use crate::{
-    calc_fn_hash, Dynamic, Engine, ImmutableString, LexError, ParseError, ParseErrorType, Position,
-    Scope, Shared, StaticVec, AST,
+    calc_fn_hash, Dynamic, Engine, Identifier, ImmutableString, LexError, ParseError,
+    ParseErrorType, Position, Scope, Shared, StaticVec, AST,
 };
 
 #[cfg(not(feature = "no_float"))]
@@ -46,12 +46,12 @@ struct ParseState<'e> {
     /// Interned strings.
     interned_strings: StringInterner,
     /// Encapsulates a local stack with variable names to simulate an actual runtime scope.
-    stack: Vec<(ImmutableString, AccessMode)>,
+    stack: Vec<(Identifier, AccessMode)>,
     /// Size of the local variables stack upon entry of the current block scope.
     entry_stack_len: usize,
     /// Tracks a list of external variables (variables that are not explicitly declared in the scope).
     #[cfg(not(feature = "no_closure"))]
-    external_vars: BTreeMap<ImmutableString, Position>,
+    external_vars: BTreeMap<Identifier, Position>,
     /// An indicator that disables variable capturing into externals one single time
     /// up until the nearest consumed Identifier token.
     /// If set to false the next call to `access_var` will not capture the variable.
@@ -60,7 +60,7 @@ struct ParseState<'e> {
     allow_capture: bool,
     /// Encapsulates a local stack with imported [module][crate::Module] names.
     #[cfg(not(feature = "no_module"))]
-    modules: StaticVec<ImmutableString>,
+    modules: StaticVec<Identifier>,
     /// Maximum levels of expression nesting.
     #[cfg(not(feature = "unchecked"))]
     max_expr_depth: Option<NonZeroUsize>,
@@ -166,10 +166,7 @@ impl<'e> ParseState<'e> {
 
     /// Get an interned string, creating one if it is not yet interned.
     #[inline(always)]
-    pub fn get_interned_string(
-        &mut self,
-        text: impl AsRef<str> + Into<ImmutableString>,
-    ) -> ImmutableString {
+    pub fn get_interned_string(&mut self, text: impl AsRef<str>) -> Identifier {
         self.interned_strings.get(text)
     }
 }
@@ -310,7 +307,7 @@ fn parse_fn_call(
     input: &mut TokenStream,
     state: &mut ParseState,
     lib: &mut FunctionsLib,
-    id: ImmutableString,
+    id: Identifier,
     capture: bool,
     mut namespace: Option<NamespaceRef>,
     settings: ParseSettings,
@@ -668,6 +665,8 @@ fn parse_array_literal(
         };
     }
 
+    arr.shrink_to_fit();
+
     Ok(Expr::Array(Box::new(arr), settings.pos))
 }
 
@@ -707,7 +706,7 @@ fn parse_map_literal(
 
         let (name, pos) = match input.next().unwrap() {
             (Token::Identifier(s), pos) | (Token::StringConstant(s), pos) => {
-                if map.iter().any(|(p, _)| p.name == &s) {
+                if map.iter().any(|(p, _)| p.name == s) {
                     return Err(PERR::DuplicatedProperty(s).into_err(pos));
                 }
                 (s, pos)
@@ -757,7 +756,7 @@ fn parse_map_literal(
 
         let expr = parse_expr(input, state, lib, settings.level_up())?;
         let name = state.get_interned_string(name);
-        template.insert(name.clone(), Default::default());
+        template.insert(name.clone().into(), Default::default());
         map.push((Ident { name, pos }, expr));
 
         match input.peek().unwrap() {
@@ -782,6 +781,8 @@ fn parse_map_literal(
         }
     }
 
+    map.shrink_to_fit();
+
     Ok(Expr::Map(Box::new((map, template)), settings.pos))
 }
 
@@ -935,7 +936,7 @@ fn parse_primary(
             Token::IntegerConstant(x) => Expr::IntegerConstant(x, settings.pos),
             Token::CharConstant(c) => Expr::CharConstant(c, settings.pos),
             Token::StringConstant(s) => {
-                Expr::StringConstant(state.get_interned_string(s), settings.pos)
+                Expr::StringConstant(state.get_interned_string(s).into(), settings.pos)
             }
             Token::True => Expr::BoolConstant(true, settings.pos),
             Token::False => Expr::BoolConstant(false, settings.pos),
@@ -1830,7 +1831,7 @@ fn parse_custom_syntax(
             MARKER_IDENT => match input.next().unwrap() {
                 (Token::Identifier(s), pos) => {
                     let name = state.get_interned_string(s);
-                    segments.push(name.clone());
+                    segments.push(name.clone().into());
                     tokens.push(state.get_interned_string(MARKER_IDENT));
                     let var_name_def = Ident { name, pos };
                     keywords.push(Expr::Variable(Box::new((None, None, var_name_def))));
@@ -1843,14 +1844,14 @@ fn parse_custom_syntax(
             MARKER_EXPR => {
                 keywords.push(parse_expr(input, state, lib, settings)?);
                 let keyword = state.get_interned_string(MARKER_EXPR);
-                segments.push(keyword.clone());
+                segments.push(keyword.clone().into());
                 tokens.push(keyword);
             }
             MARKER_BLOCK => match parse_block(input, state, lib, settings)? {
                 block @ Stmt::Block(_, _) => {
                     keywords.push(Expr::Stmt(Box::new(block.into())));
                     let keyword = state.get_interned_string(MARKER_BLOCK);
-                    segments.push(keyword.clone());
+                    segments.push(keyword.clone().into());
                     tokens.push(keyword);
                 }
                 stmt => unreachable!("expecting Stmt::Block, but gets {:?}", stmt),
@@ -1859,7 +1860,7 @@ fn parse_custom_syntax(
                 (Token::LexError(err), pos) => return Err(err.into_err(pos)),
                 (t, _) if t.syntax().as_ref() == s => {
                     segments.push(required_token.clone());
-                    tokens.push(required_token.clone());
+                    tokens.push(required_token.clone().into());
                 }
                 (_, pos) => {
                     return Err(PERR::MissingToken(
@@ -1901,7 +1902,7 @@ fn parse_expr(
 
         match token {
             Token::Custom(key) | Token::Reserved(key) | Token::Identifier(key) => {
-                match state.engine.custom_syntax.get_key_value(key) {
+                match state.engine.custom_syntax.get_key_value(key.as_str()) {
                     Some((key, syntax)) => {
                         input.next().unwrap();
                         return parse_custom_syntax(
@@ -2119,16 +2120,20 @@ fn parse_for(
     ensure_not_statement_expr(input, "a boolean")?;
     let expr = parse_expr(input, state, lib, settings.level_up())?;
 
-    let loop_var = state.get_interned_string(name.clone());
+    let loop_var = state.get_interned_string(name);
     let prev_stack_len = state.stack.len();
-    state.stack.push((loop_var, AccessMode::ReadWrite));
+    state.stack.push((loop_var.clone(), AccessMode::ReadWrite));
 
     settings.is_breakable = true;
     let body = parse_block(input, state, lib, settings.level_up())?;
 
     state.stack.truncate(prev_stack_len);
 
-    Ok(Stmt::For(expr, Box::new((name, body.into())), settings.pos))
+    Ok(Stmt::For(
+        expr,
+        Box::new((loop_var, body.into())),
+        settings.pos,
+    ))
 }
 
 /// Parse a variable definition statement.
@@ -2174,9 +2179,9 @@ fn parse_let(
 
     match var_type {
         // let name = expr
-        AccessMode::ReadWrite => Ok(Stmt::Let(expr, var_def, export, settings.pos)),
+        AccessMode::ReadWrite => Ok(Stmt::Let(expr, var_def.into(), export, settings.pos)),
         // const name = { expr:constant }
-        AccessMode::ReadOnly => Ok(Stmt::Const(expr, var_def, export, settings.pos)),
+        AccessMode::ReadOnly => Ok(Stmt::Const(expr, var_def.into(), export, settings.pos)),
     }
 }
 
@@ -2217,10 +2222,13 @@ fn parse_import(
 
     Ok(Stmt::Import(
         expr,
-        Some(Ident {
-            name,
-            pos: name_pos,
-        }),
+        Some(
+            Ident {
+                name,
+                pos: name_pos,
+            }
+            .into(),
+        ),
         settings.pos,
     ))
 }
@@ -2511,7 +2519,7 @@ fn parse_stmt(
 
                     if lib.contains_key(&hash) {
                         return Err(PERR::FnDuplicatedDefinition(
-                            func.name.into_owned(),
+                            func.name.to_string(),
                             func.params.len(),
                         )
                         .into_err(pos));
@@ -2904,7 +2912,7 @@ fn parse_anon_fn(
         comments: Default::default(),
     };
 
-    let expr = Expr::FnPointer(fn_name, settings.pos);
+    let expr = Expr::FnPointer(fn_name.into(), settings.pos);
 
     #[cfg(not(feature = "no_closure"))]
     let expr = make_curry_from_externals(state, expr, externals, settings.pos);
diff --git a/src/scope.rs b/src/scope.rs
index ac19bf2c..7d4b114b 100644
--- a/src/scope.rs
+++ b/src/scope.rs
@@ -2,7 +2,7 @@
 
 use crate::dynamic::{AccessMode, Variant};
 use crate::stdlib::{borrow::Cow, boxed::Box, iter, vec::Vec};
-use crate::{Dynamic, ImmutableString, StaticVec};
+use crate::{Dynamic, Identifier, StaticVec};
 
 /// Keep a number of entries inline (since [`Dynamic`] is usually small enough).
 const SCOPE_SIZE: usize = 16;
@@ -54,7 +54,7 @@ pub struct Scope<'a> {
     /// Current value of the entry.
     values: smallvec::SmallVec<[Dynamic; SCOPE_SIZE]>,
     /// (Name, aliases) of the entry.
-    names: Vec<(Cow<'a, str>, Option<Box<StaticVec<ImmutableString>>>)>,
+    names: Vec<(Cow<'a, str>, Option<Box<StaticVec<Identifier>>>)>,
 }
 
 impl Default for Scope<'_> {
@@ -414,7 +414,7 @@ impl<'a> Scope<'a> {
     pub(crate) fn add_entry_alias(
         &mut self,
         index: usize,
-        alias: impl Into<ImmutableString> + PartialEq<ImmutableString>,
+        alias: impl Into<Identifier> + PartialEq<Identifier>,
     ) -> &mut Self {
         let entry = self.names.get_mut(index).expect("invalid index in Scope");
         if entry.1.is_none() {
@@ -449,7 +449,7 @@ impl<'a> Scope<'a> {
     #[allow(dead_code)]
     pub(crate) fn into_iter(
         self,
-    ) -> impl Iterator<Item = (Cow<'a, str>, Dynamic, Vec<ImmutableString>)> {
+    ) -> impl Iterator<Item = (Cow<'a, str>, Dynamic, Vec<Identifier>)> {
         self.names
             .into_iter()
             .zip(self.values.into_iter())
diff --git a/src/syntax.rs b/src/syntax.rs
index 2e39554b..3eea6244 100644
--- a/src/syntax.rs
+++ b/src/syntax.rs
@@ -6,7 +6,8 @@ use crate::fn_native::SendSync;
 use crate::stdlib::{boxed::Box, format, string::ToString};
 use crate::token::{is_valid_identifier, Token};
 use crate::{
-    Engine, ImmutableString, LexError, ParseError, Position, RhaiResult, Shared, StaticVec,
+    Engine, Identifier, ImmutableString, LexError, ParseError, Position, RhaiResult, Shared,
+    StaticVec,
 };
 
 pub const MARKER_EXPR: &str = "$expr$";
@@ -103,7 +104,7 @@ impl Engine {
     ///
     /// If `new_vars` is negative, then it is expected that only the top `new_vars` variables in the
     /// current [`Scope`][crate::Scope] will be _popped_.  Do not randomly remove variables.
-    pub fn register_custom_syntax<S: AsRef<str> + Into<ImmutableString>>(
+    pub fn register_custom_syntax<S: AsRef<str> + Into<Identifier>>(
         &mut self,
         keywords: &[S],
         new_vars: isize,
@@ -221,7 +222,7 @@ impl Engine {
     /// Otherwise, custom keywords won't be recognized.
     pub fn register_custom_syntax_raw(
         &mut self,
-        key: impl Into<ImmutableString>,
+        key: impl Into<Identifier>,
         parse: impl Fn(&[ImmutableString], &str) -> Result<Option<ImmutableString>, ParseError>
             + SendSync
             + 'static,
diff --git a/src/utils.rs b/src/utils.rs
index 60fa7e0f..380a1f8b 100644
--- a/src/utils.rs
+++ b/src/utils.rs
@@ -15,7 +15,7 @@ use crate::stdlib::{
     str::FromStr,
     string::{String, ToString},
 };
-use crate::Shared;
+use crate::{Identifier, Shared};
 
 /// A hasher that only takes one single [`u64`] and returns it as a hash key.
 ///
@@ -590,6 +590,20 @@ impl PartialOrd<ImmutableString> for String {
     }
 }
 
+impl From<ImmutableString> for Identifier {
+    #[inline(always)]
+    fn from(value: ImmutableString) -> Self {
+        value.into_owned().into()
+    }
+}
+
+impl From<Identifier> for ImmutableString {
+    #[inline(always)]
+    fn from(value: Identifier) -> Self {
+        value.to_string().into()
+    }
+}
+
 impl ImmutableString {
     /// Consume the [`ImmutableString`] and convert it into a [`String`].
     /// If there are other references to the same string, a cloned copy is returned.
@@ -608,16 +622,12 @@ impl ImmutableString {
 
 /// A collection of interned strings.
 #[derive(Debug, Clone, Default, Hash)]
-pub struct StringInterner(BTreeSet<ImmutableString>);
+pub struct StringInterner(BTreeSet<Identifier>);
 
 impl StringInterner {
     /// Get an interned string, creating one if it is not yet interned.
     #[inline(always)]
-    pub fn get(&mut self, text: impl AsRef<str> + Into<ImmutableString>) -> ImmutableString {
-        self.0.get(text.as_ref()).cloned().unwrap_or_else(|| {
-            let s = text.into();
-            self.0.insert(s.clone());
-            s
-        })
+    pub fn get(&mut self, text: impl AsRef<str>) -> Identifier {
+        text.as_ref().into()
     }
 }