diff --git a/src/any.rs b/src/any.rs
index 6a5d9050..3b42acf8 100644
--- a/src/any.rs
+++ b/src/any.rs
@@ -550,8 +550,15 @@ impl Dynamic {
     /// Convert the `Dynamic` into `String` and return it.
     /// Returns the name of the actual type if the cast fails.
     pub fn take_string(self) -> Result<String, &'static str> {
+        self.take_immutable_string()
+            .map(ImmutableString::into_owned)
+    }
+
+    /// Convert the `Dynamic` into `ImmutableString` and return it.
+    /// Returns the name of the actual type if the cast fails.
+    pub(crate) fn take_immutable_string(self) -> Result<ImmutableString, &'static str> {
         match self.0 {
-            Union::Str(s) => Ok(s.into_owned()),
+            Union::Str(s) => Ok(s),
             _ => Err(self.type_name()),
         }
     }
@@ -615,7 +622,7 @@ impl<T: Variant + Clone> From<HashMap<String, T>> for Dynamic {
         Self(Union::Map(Box::new(
             value
                 .into_iter()
-                .map(|(k, v)| (k, Dynamic::from(v)))
+                .map(|(k, v)| (k.into(), Dynamic::from(v)))
                 .collect(),
         )))
     }
diff --git a/src/engine.rs b/src/engine.rs
index f1260824..4a473b42 100644
--- a/src/engine.rs
+++ b/src/engine.rs
@@ -38,7 +38,7 @@ pub type Array = Vec<Dynamic>;
 ///
 /// Not available under the `no_object` feature.
 #[cfg(not(feature = "no_object"))]
-pub type Map = HashMap<String, Dynamic>;
+pub type Map = HashMap<ImmutableString, Dynamic>;
 
 #[cfg(not(feature = "unchecked"))]
 #[cfg(debug_assertions)]
@@ -1389,7 +1389,7 @@ impl Engine {
                 // val_map[idx]
                 Ok(if create {
                     let index = idx
-                        .take_string()
+                        .take_immutable_string()
                         .map_err(|_| EvalAltResult::ErrorStringIndexExpr(idx_pos))?;
 
                     map.entry(index).or_insert(Default::default()).into()
@@ -1398,7 +1398,7 @@ impl Engine {
                         .downcast_ref::<String>()
                         .ok_or_else(|| EvalAltResult::ErrorStringIndexExpr(idx_pos))?;
 
-                    map.get_mut(index)
+                    map.get_mut(index.as_str())
                         .map(Target::from)
                         .unwrap_or_else(|| Target::from(()))
                 })
@@ -1498,7 +1498,9 @@ impl Engine {
             Dynamic(Union::Map(rhs_value)) => match lhs_value {
                 // Only allows String or char
                 Dynamic(Union::Str(s)) => Ok(rhs_value.contains_key(s.as_str()).into()),
-                Dynamic(Union::Char(c)) => Ok(rhs_value.contains_key(&c.to_string()).into()),
+                Dynamic(Union::Char(c)) => {
+                    Ok(rhs_value.contains_key(c.to_string().as_str()).into())
+                }
                 _ => Err(Box::new(EvalAltResult::ErrorInExpr(lhs.position()))),
             },
             Dynamic(Union::Str(rhs_value)) => match lhs_value {
diff --git a/src/optimize.rs b/src/optimize.rs
index 7895fc68..6acdcc2b 100644
--- a/src/optimize.rs
+++ b/src/optimize.rs
@@ -408,7 +408,7 @@ fn optimize_expr(expr: Expr, state: &mut State) -> Expr {
                 // All other items can be thrown away.
                 state.set_dirty();
                 let pos = m.1;
-                m.0.into_iter().find(|((name, _), _)| name == prop)
+                m.0.into_iter().find(|((name, _), _)| name.as_str() == prop)
                     .map(|(_, expr)| expr.set_position(pos))
                     .unwrap_or_else(|| Expr::Unit(pos))
             }
@@ -434,7 +434,7 @@ fn optimize_expr(expr: Expr, state: &mut State) -> Expr {
                 // All other items can be thrown away.
                 state.set_dirty();
                 let pos = m.1;
-                m.0.into_iter().find(|((name, _), _)| name == s.0.as_ref())
+                m.0.into_iter().find(|((name, _), _)| *name == s.0)
                     .map(|(_, expr)| expr.set_position(pos))
                     .unwrap_or_else(|| Expr::Unit(pos))
             }
@@ -472,7 +472,7 @@ fn optimize_expr(expr: Expr, state: &mut State) -> Expr {
             // "xxx" in #{...}
             (Expr::StringConstant(a), Expr::Map(b)) => {
                 state.set_dirty();
-                if b.0.iter().find(|((name, _), _)| name == a.0.as_ref()).is_some() {
+                if b.0.iter().find(|((name, _), _)| *name == a.0).is_some() {
                     Expr::True(a.1)
                 } else {
                     Expr::False(a.1)
@@ -483,7 +483,7 @@ fn optimize_expr(expr: Expr, state: &mut State) -> Expr {
                 state.set_dirty();
                 let ch = a.0.to_string();
 
-                if b.0.iter().find(|((name, _), _)| name == &ch).is_some() {
+                if b.0.iter().find(|((name, _), _)| name.as_str() == ch.as_str()).is_some() {
                     Expr::True(a.1)
                 } else {
                     Expr::False(a.1)
diff --git a/src/packages/map_basic.rs b/src/packages/map_basic.rs
index 9be0fb42..1488e655 100644
--- a/src/packages/map_basic.rs
+++ b/src/packages/map_basic.rs
@@ -6,10 +6,10 @@ use crate::engine::Map;
 use crate::module::FuncReturn;
 use crate::parser::{ImmutableString, INT};
 
-use crate::stdlib::{string::ToString, vec::Vec};
+use crate::stdlib::vec::Vec;
 
 fn map_get_keys(map: &mut Map) -> FuncReturn<Vec<Dynamic>> {
-    Ok(map.iter().map(|(k, _)| k.to_string().into()).collect())
+    Ok(map.iter().map(|(k, _)| k.clone().into()).collect())
 }
 fn map_get_values(map: &mut Map) -> FuncReturn<Vec<Dynamic>> {
     Ok(map.iter().map(|(_, v)| v.clone()).collect())
diff --git a/src/parser.rs b/src/parser.rs
index 5d0bd902..1f856791 100644
--- a/src/parser.rs
+++ b/src/parser.rs
@@ -464,7 +464,7 @@ pub enum Expr {
     /// [ expr, ... ]
     Array(Box<(StaticVec<Expr>, Position)>),
     /// #{ name:expr, ... }
-    Map(Box<(StaticVec<((String, Position), Expr)>, Position)>),
+    Map(Box<(StaticVec<((ImmutableString, Position), Expr)>, Position)>),
     /// lhs in rhs
     In(Box<(Expr, Expr, Position)>),
     /// lhs && rhs
@@ -1196,7 +1196,7 @@ fn parse_map_literal(
                 }
 
                 let expr = parse_expr(input, state, settings.level_up())?;
-                map.push(((name, pos), expr));
+                map.push(((Into::<ImmutableString>::into(name), pos), expr));
             }
         }