diff --git a/src/any.rs b/src/any.rs
index 0b6d81d2..0647d219 100644
--- a/src/any.rs
+++ b/src/any.rs
@@ -1,5 +1,6 @@
 //! Helper module which defines the `Any` trait to to allow dynamic value handling.
 
+use crate::fn_native::SendSync;
 use crate::module::Module;
 use crate::parser::{ImmutableString, INT};
 use crate::r#unsafe::{unsafe_cast_box, unsafe_try_cast};
@@ -54,31 +55,6 @@ pub trait Variant: Any {
     fn _closed(&self) -> _Private;
 }
 
-#[cfg(not(feature = "sync"))]
-impl<T: Any + Clone> Variant for T {
-    fn as_any(&self) -> &dyn Any {
-        self as &dyn Any
-    }
-    fn as_mut_any(&mut self) -> &mut dyn Any {
-        self as &mut dyn Any
-    }
-    fn as_box_any(self: Box<Self>) -> Box<dyn Any> {
-        self as Box<dyn Any>
-    }
-    fn type_name(&self) -> &'static str {
-        type_name::<T>()
-    }
-    fn into_dynamic(self) -> Dynamic {
-        Dynamic::from(self)
-    }
-    fn clone_into_dynamic(&self) -> Dynamic {
-        Dynamic::from(self.clone())
-    }
-    fn _closed(&self) -> _Private {
-        _Private
-    }
-}
-
 /// Trait to represent any type.
 ///
 /// `From<_>` is implemented for `i64` (`i32` if `only_i32`), `f64` (if not `no_float`),
@@ -108,8 +84,7 @@ pub trait Variant: Any + Send + Sync {
     fn _closed(&self) -> _Private;
 }
 
-#[cfg(feature = "sync")]
-impl<T: Any + Clone + Send + Sync> Variant for T {
+impl<T: Any + Clone + SendSync> Variant for T {
     fn as_any(&self) -> &dyn Any {
         self as &dyn Any
     }
diff --git a/src/fn_register.rs b/src/fn_register.rs
index 1384f666..86e7ada9 100644
--- a/src/fn_register.rs
+++ b/src/fn_register.rs
@@ -4,7 +4,7 @@
 
 use crate::any::{Dynamic, Variant};
 use crate::engine::Engine;
-use crate::fn_native::{CallableFunction, FnAny, FnCallArgs};
+use crate::fn_native::{CallableFunction, FnAny, FnCallArgs, SendSync};
 use crate::parser::FnAccess;
 use crate::result::EvalAltResult;
 use crate::utils::ImmutableString;
@@ -101,10 +101,10 @@ pub fn by_ref<T: Variant + Clone>(data: &mut Dynamic) -> &mut T {
 #[inline(always)]
 pub fn by_value<T: Variant + Clone>(data: &mut Dynamic) -> T {
     if TypeId::of::<T>() == TypeId::of::<&str>() {
-        // &str parameters are mapped to the underlying ImmutableString
-        let r = data.as_str().unwrap();
-        let x = unsafe { mem::transmute::<_, &T>(&r) };
-        x.clone()
+        // If T is &str, data must be ImmutableString, so map directly to it
+        let ref_str = data.as_str().unwrap();
+        let ref_T = unsafe { mem::transmute::<_, &T>(&ref_str) };
+        ref_T.clone()
     } else {
         // We consume the argument and then replace it with () - the argument is not supposed to be used again.
         // This way, we avoid having to clone the argument again, because it is already a clone when passed here.
@@ -178,13 +178,7 @@ macro_rules! def_register {
     //                                                         ^ dereferencing function
         impl<
             $($par: Variant + Clone,)*
-
-            #[cfg(feature = "sync")]
-            FN: Fn($($param),*) -> RET + Send + Sync + 'static,
-
-            #[cfg(not(feature = "sync"))]
-            FN: Fn($($param),*) -> RET + 'static,
-
+            FN: Fn($($param),*) -> RET + SendSync + 'static,
             RET: Variant + Clone
         > RegisterFn<FN, ($($mark,)*), RET> for Engine
         {
@@ -198,11 +192,7 @@ macro_rules! def_register {
 
         impl<
             $($par: Variant + Clone,)*
-
-            #[cfg(feature = "sync")]
-            FN: Fn($($param),*) -> Result<Dynamic, Box<EvalAltResult>> + Send + Sync + 'static,
-            #[cfg(not(feature = "sync"))]
-            FN: Fn($($param),*) -> Result<Dynamic, Box<EvalAltResult>> + 'static,
+            FN: Fn($($param),*) -> Result<Dynamic, Box<EvalAltResult>> + SendSync + 'static,
         > RegisterResultFn<FN, ($($mark,)*)> for Engine
         {
             fn register_result_fn(&mut self, name: &str, f: FN) {
diff --git a/src/module.rs b/src/module.rs
index 2a722e3b..1deddee4 100644
--- a/src/module.rs
+++ b/src/module.rs
@@ -321,8 +321,7 @@ impl Module {
     pub fn set_fn_0<T: Variant + Clone>(
         &mut self,
         name: impl Into<String>,
-        #[cfg(not(feature = "sync"))] func: impl Fn() -> FuncReturn<T> + 'static,
-        #[cfg(feature = "sync")] func: impl Fn() -> FuncReturn<T> + Send + Sync + 'static,
+        func: impl Fn() -> FuncReturn<T> + SendSync + 'static,
     ) -> u64 {
         let f = move |_: &mut FnCallArgs| func().map(Dynamic::from);
         let args = [];
@@ -350,8 +349,7 @@ impl Module {
     pub fn set_fn_1<A: Variant + Clone, T: Variant + Clone>(
         &mut self,
         name: impl Into<String>,
-        #[cfg(not(feature = "sync"))] func: impl Fn(A) -> FuncReturn<T> + 'static,
-        #[cfg(feature = "sync")] func: impl Fn(A) -> FuncReturn<T> + Send + Sync + 'static,
+        func: impl Fn(A) -> FuncReturn<T> + SendSync + 'static,
     ) -> u64 {
         let f =
             move |args: &mut FnCallArgs| func(mem::take(args[0]).cast::<A>()).map(Dynamic::from);
@@ -380,8 +378,7 @@ impl Module {
     pub fn set_fn_1_mut<A: Variant + Clone, T: Variant + Clone>(
         &mut self,
         name: impl Into<String>,
-        #[cfg(not(feature = "sync"))] func: impl Fn(&mut A) -> FuncReturn<T> + 'static,
-        #[cfg(feature = "sync")] func: impl Fn(&mut A) -> FuncReturn<T> + Send + Sync + 'static,
+        func: impl Fn(&mut A) -> FuncReturn<T> + SendSync + 'static,
     ) -> u64 {
         let f = move |args: &mut FnCallArgs| {
             func(args[0].downcast_mut::<A>().unwrap()).map(Dynamic::from)
@@ -412,8 +409,7 @@ impl Module {
     pub fn set_getter_fn<A: Variant + Clone, T: Variant + Clone>(
         &mut self,
         name: impl Into<String>,
-        #[cfg(not(feature = "sync"))] func: impl Fn(&mut A) -> FuncReturn<T> + 'static,
-        #[cfg(feature = "sync")] func: impl Fn(&mut A) -> FuncReturn<T> + Send + Sync + 'static,
+        func: impl Fn(&mut A) -> FuncReturn<T> + SendSync + 'static,
     ) -> u64 {
         self.set_fn_1_mut(make_getter(&name.into()), func)
     }
@@ -436,8 +432,7 @@ impl Module {
     pub fn set_fn_2<A: Variant + Clone, B: Variant + Clone, T: Variant + Clone>(
         &mut self,
         name: impl Into<String>,
-        #[cfg(not(feature = "sync"))] func: impl Fn(A, B) -> FuncReturn<T> + 'static,
-        #[cfg(feature = "sync")] func: impl Fn(A, B) -> FuncReturn<T> + Send + Sync + 'static,
+        func: impl Fn(A, B) -> FuncReturn<T> + SendSync + 'static,
     ) -> u64 {
         let f = move |args: &mut FnCallArgs| {
             let a = mem::take(args[0]).cast::<A>();
@@ -473,8 +468,7 @@ impl Module {
     pub fn set_fn_2_mut<A: Variant + Clone, B: Variant + Clone, T: Variant + Clone>(
         &mut self,
         name: impl Into<String>,
-        #[cfg(not(feature = "sync"))] func: impl Fn(&mut A, B) -> FuncReturn<T> + 'static,
-        #[cfg(feature = "sync")] func: impl Fn(&mut A, B) -> FuncReturn<T> + Send + Sync + 'static,
+        func: impl Fn(&mut A, B) -> FuncReturn<T> + SendSync + 'static,
     ) -> u64 {
         let f = move |args: &mut FnCallArgs| {
             let b = mem::take(args[1]).cast::<B>();
@@ -512,8 +506,7 @@ impl Module {
     pub fn set_setter_fn<A: Variant + Clone, B: Variant + Clone>(
         &mut self,
         name: impl Into<String>,
-        #[cfg(not(feature = "sync"))] func: impl Fn(&mut A, B) -> FuncReturn<()> + 'static,
-        #[cfg(feature = "sync")] func: impl Fn(&mut A, B) -> FuncReturn<()> + Send + Sync + 'static,
+        func: impl Fn(&mut A, B) -> FuncReturn<()> + SendSync + 'static,
     ) -> u64 {
         self.set_fn_2_mut(make_setter(&name.into()), func)
     }
@@ -538,8 +531,7 @@ impl Module {
     #[cfg(not(feature = "no_index"))]
     pub fn set_indexer_get_fn<A: Variant + Clone, B: Variant + Clone, T: Variant + Clone>(
         &mut self,
-        #[cfg(not(feature = "sync"))] func: impl Fn(&mut A, B) -> FuncReturn<T> + 'static,
-        #[cfg(feature = "sync")] func: impl Fn(&mut A, B) -> FuncReturn<T> + Send + Sync + 'static,
+        func: impl Fn(&mut A, B) -> FuncReturn<T> + SendSync + 'static,
     ) -> u64 {
         self.set_fn_2_mut(FUNC_INDEXER_GET, func)
     }
@@ -567,8 +559,7 @@ impl Module {
     >(
         &mut self,
         name: impl Into<String>,
-        #[cfg(not(feature = "sync"))] func: impl Fn(A, B, C) -> FuncReturn<T> + 'static,
-        #[cfg(feature = "sync")] func: impl Fn(A, B, C) -> FuncReturn<T> + Send + Sync + 'static,
+        func: impl Fn(A, B, C) -> FuncReturn<T> + SendSync + 'static,
     ) -> u64 {
         let f = move |args: &mut FnCallArgs| {
             let a = mem::take(args[0]).cast::<A>();
@@ -610,8 +601,7 @@ impl Module {
     >(
         &mut self,
         name: impl Into<String>,
-        #[cfg(not(feature = "sync"))] func: impl Fn(&mut A, B, C) -> FuncReturn<T> + 'static,
-        #[cfg(feature = "sync")] func: impl Fn(&mut A, B, C) -> FuncReturn<T> + Send + Sync + 'static,
+        func: impl Fn(&mut A, B, C) -> FuncReturn<T> + SendSync + 'static,
     ) -> u64 {
         let f = move |args: &mut FnCallArgs| {
             let b = mem::take(args[1]).cast::<B>();
@@ -648,8 +638,7 @@ impl Module {
     /// ```
     pub fn set_indexer_set_fn<A: Variant + Clone, B: Variant + Clone>(
         &mut self,
-        #[cfg(not(feature = "sync"))] func: impl Fn(&mut A, B, A) -> FuncReturn<()> + 'static,
-        #[cfg(feature = "sync")] func: impl Fn(&mut A, B, A) -> FuncReturn<()> + Send + Sync + 'static,
+        func: impl Fn(&mut A, B, A) -> FuncReturn<()> + SendSync + 'static,
     ) -> u64 {
         let f = move |args: &mut FnCallArgs| {
             let b = mem::take(args[1]).cast::<B>();
@@ -691,8 +680,7 @@ impl Module {
     >(
         &mut self,
         name: impl Into<String>,
-        #[cfg(not(feature = "sync"))] func: impl Fn(A, B, C, D) -> FuncReturn<T> + 'static,
-        #[cfg(feature = "sync")] func: impl Fn(A, B, C, D) -> FuncReturn<T> + Send + Sync + 'static,
+        func: impl Fn(A, B, C, D) -> FuncReturn<T> + SendSync + 'static,
     ) -> u64 {
         let f = move |args: &mut FnCallArgs| {
             let a = mem::take(args[0]).cast::<A>();
@@ -741,8 +729,7 @@ impl Module {
     >(
         &mut self,
         name: impl Into<String>,
-        #[cfg(not(feature = "sync"))] func: impl Fn(&mut A, B, C, D) -> FuncReturn<T> + 'static,
-        #[cfg(feature = "sync")] func: impl Fn(&mut A, B, C, D) -> FuncReturn<T> + Send + Sync + 'static,
+        func: impl Fn(&mut A, B, C, D) -> FuncReturn<T> + SendSync + 'static,
     ) -> u64 {
         let f = move |args: &mut FnCallArgs| {
             let b = mem::take(args[1]).cast::<B>();