From c721858338b27c6cbf74edbf655c3bbf52feac06 Mon Sep 17 00:00:00 2001
From: Hans Larsen <hans@larsen.online>
Date: Thu, 28 Mar 2024 22:41:16 -0700
Subject: [PATCH 01/22] Add more utility traits and funtions to boa_interop

---
 core/interop/src/into_js_function_impls.rs |  91 +++++++++
 core/interop/src/lib.rs                    | 212 +++++++++++++++++++--
 2 files changed, 283 insertions(+), 20 deletions(-)
 create mode 100644 core/interop/src/into_js_function_impls.rs

diff --git a/core/interop/src/into_js_function_impls.rs b/core/interop/src/into_js_function_impls.rs
new file mode 100644
index 00000000000..90b1e2e72df
--- /dev/null
+++ b/core/interop/src/into_js_function_impls.rs
@@ -0,0 +1,91 @@
+//! Implementations of the `IntoJsFunction` trait for various function signatures.
+
+use crate::{IntoJsFunction, TryFromJsArgument};
+use boa_engine::{Context, JsValue, NativeFunction};
+use std::cell::RefCell;
+
+/// A token to represent the context argument in the function signature.
+/// This should not be used directly and has no external meaning.
+#[derive(Debug, Copy, Clone)]
+pub struct ContextArgToken();
+
+macro_rules! impl_into_js_function {
+    ($($id: ident: $t: ident),*) => {
+        impl<$($t,)* R, T> IntoJsFunction<($($t,)*), R> for T
+        where
+            $($t: TryFromJsArgument + 'static,)*
+            R: Into<JsValue>,
+            T: FnMut($($t,)*) -> R + 'static,
+        {
+            #[allow(unused_variables)]
+            fn into_js_function(self, _context: &mut Context) -> NativeFunction {
+                let s = RefCell::new(self);
+                unsafe {
+                    NativeFunction::from_closure(move |this, args, ctx| {
+                        let rest = args;
+                        $(
+                            let ($id, rest) = $t::try_from_js_argument(this, rest, ctx)?;
+                        )*
+                        let r = s.borrow_mut()( $($id),* );
+                        Ok(r.into())
+                    })
+                }
+            }
+        }
+
+        impl<$($t,)* R, T> IntoJsFunction<($($t,)* ContextArgToken), R> for T
+        where
+            $($t: TryFromJsArgument + 'static,)*
+            R: Into<JsValue>,
+            T: FnMut($($t,)* &mut Context) -> R + 'static,
+        {
+            #[allow(unused_variables)]
+            fn into_js_function(self, _context: &mut Context) -> NativeFunction {
+                let s = RefCell::new(self);
+                unsafe {
+                    NativeFunction::from_closure(move |this, args, ctx| {
+                        let rest = args;
+                        $(
+                            let ($id, rest) = $t::try_from_js_argument(this, rest, ctx)?;
+                        )*
+                        let r = s.borrow_mut()( $($id,)* ctx);
+                        Ok(r.into())
+                    })
+                }
+            }
+        }
+    };
+}
+
+impl<R, T> IntoJsFunction<(), R> for T
+where
+    R: Into<JsValue>,
+    T: FnMut() -> R + 'static,
+{
+    fn into_js_function(self, _context: &mut Context) -> NativeFunction {
+        let s = RefCell::new(self);
+        unsafe {
+            NativeFunction::from_closure(move |_this, _args, _ctx| {
+                let r = s.borrow_mut()();
+                Ok(r.into())
+            })
+        }
+    }
+}
+
+// Currently implemented up to 12 arguments. The empty argument list
+// is implemented separately above.
+// Consider that JsRest and JsThis are part of this list, but Context
+// is not, as it is a special specialization of the template.
+impl_into_js_function!(a: A);
+impl_into_js_function!(a: A, b: B);
+impl_into_js_function!(a: A, b: B, c: C);
+impl_into_js_function!(a: A, b: B, c: C, d: D);
+impl_into_js_function!(a: A, b: B, c: C, d: D, e: E);
+impl_into_js_function!(a: A, b: B, c: C, d: D, e: E, f: F);
+impl_into_js_function!(a: A, b: B, c: C, d: D, e: E, f: F, g: G);
+impl_into_js_function!(a: A, b: B, c: C, d: D, e: E, f: F, g: G, h: H);
+impl_into_js_function!(a: A, b: B, c: C, d: D, e: E, f: F, g: G, h: H, i: I);
+impl_into_js_function!(a: A, b: B, c: C, d: D, e: E, f: F, g: G, h: H, i: I, j: J);
+impl_into_js_function!(a: A, b: B, c: C, d: D, e: E, f: F, g: G, h: H, i: I, j: J, k: K);
+impl_into_js_function!(a: A, b: B, c: C, d: D, e: E, f: F, g: G, h: H, i: I, j: J, k: K, l: L);
diff --git a/core/interop/src/lib.rs b/core/interop/src/lib.rs
index f048bf2f655..bcff0c6e9b9 100644
--- a/core/interop/src/lib.rs
+++ b/core/interop/src/lib.rs
@@ -3,7 +3,8 @@
 use std::cell::RefCell;
 
 use boa_engine::module::SyntheticModuleInitializer;
-use boa_engine::{Context, JsString, JsValue, Module, NativeFunction};
+use boa_engine::value::TryFromJs;
+use boa_engine::{Context, JsResult, JsString, JsValue, Module, NativeFunction};
 
 pub mod loaders;
 
@@ -39,6 +40,41 @@ impl<T: IntoIterator<Item = (JsString, NativeFunction)> + Clone> IntoJsModule fo
 /// This trait does not require the implementing type to be `Copy`, which
 /// can lead to undefined behaviour if it contains Garbage Collected objects.
 ///
+/// This trait is implemented for functions with various signatures.
+///
+/// For example:
+/// ```
+/// # use boa_engine::{Context, JsValue, NativeFunction};
+/// # use boa_interop::IntoJsFunctionUnsafe;
+/// # let mut context = Context::default();
+/// let f = |a: i32, b: i32| a + b;
+/// let f = unsafe { f.into_js_function(&mut context) };
+/// let result = f.call(&JsValue::undefined(), &[JsValue::from(1), JsValue::from(2)], &mut context).unwrap();
+/// assert_eq!(result, JsValue::new(3));
+/// ```
+///
+/// Since the `IntoJsFunction` trait is implemented for `FnMut`, you can
+/// also use closures directly:
+/// ```
+/// # use boa_engine::{Context, JsValue, NativeFunction};
+/// # use boa_interop::IntoJsFunctionUnsafe;
+/// # use std::cell::RefCell;
+/// # use std::rc::Rc;
+/// # let mut context = Context::default();
+/// let mut x = Rc::new(RefCell::new(0));
+/// // Because NativeFunction takes ownership of the closure,
+/// // the compiler cannot be certain it won't outlive `x`, so
+/// // we need to create a `Rc<RefCell>` and share it.
+/// let f = unsafe {
+///     let x = x.clone();
+///     move |a: i32| *x.borrow_mut() += a
+/// };
+/// let f = f.into_js_function(&mut context);
+/// f.call(&JsValue::undefined(), &[JsValue::from(1)], &mut context).unwrap();
+/// f.call(&JsValue::undefined(), &[JsValue::from(4)], &mut context).unwrap();
+/// assert_eq!(*x.borrow(), 5);
+/// ```
+///
 /// # Safety
 /// For this trait to be implemented safely, the implementing type must not contain any
 /// garbage collected objects (from [`boa_gc`]).
@@ -63,13 +99,114 @@ unsafe impl<T: FnMut() + 'static> IntoJsFunctionUnsafe for T {
     }
 }
 
+/// Create a Rust value from a JS argument. This trait is used to
+/// convert arguments from JS to Rust types. It allows support
+/// for optional arguments or rest arguments.
+pub trait TryFromJsArgument: Sized {
+    /// Try to convert a JS argument into a Rust value, returning the
+    /// value and the rest of the arguments to be parsed.
+    ///
+    /// # Errors
+    /// Any parsing errors that may occur during the conversion.
+    fn try_from_js_argument<'a>(
+        this: &'a JsValue,
+        rest: &'a [JsValue],
+        context: &mut Context,
+    ) -> JsResult<(Self, &'a [JsValue])>;
+}
+
+impl<T: TryFromJs> TryFromJsArgument for T {
+    fn try_from_js_argument<'a>(
+        _: &'a JsValue,
+        rest: &'a [JsValue],
+        context: &mut Context,
+    ) -> JsResult<(Self, &'a [JsValue])> {
+        match rest.split_first() {
+            Some((first, rest)) => Ok((first.try_js_into(context)?, rest)),
+            None => T::try_from_js(&JsValue::undefined(), context).map(|v| (v, rest)),
+        }
+    }
+}
+
+/// An argument that when used in a JS function will empty the list
+/// of JS arguments as JsValues. This can be used for having the
+/// rest of the arguments in a function.
+#[derive(Debug, Clone)]
+pub struct JsRest(pub Vec<JsValue>);
+
+#[allow(unused)]
+impl JsRest {
+    /// Consumes the `JsRest` and returns the inner list of `JsValue`.
+    fn into_inner(self) -> Vec<JsValue> {
+        self.0
+    }
+
+    /// Returns an iterator over the inner list of `JsValue`.
+    fn iter(&self) -> impl Iterator<Item = &JsValue> {
+        self.0.iter()
+    }
+
+    /// Returns a mutable iterator over the inner list of `JsValue`.
+    fn iter_mut(&mut self) -> impl Iterator<Item = &mut JsValue> {
+        self.0.iter_mut()
+    }
+
+    /// Returns the length of the inner list of `JsValue`.
+    fn len(&self) -> usize {
+        self.0.len()
+    }
+
+    /// Returns `true` if the inner list of `JsValue` is empty.
+    fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
+}
+
+impl IntoIterator for JsRest {
+    type Item = JsValue;
+    type IntoIter = std::vec::IntoIter<JsValue>;
+
+    fn into_iter(self) -> Self::IntoIter {
+        self.into_inner().into_iter()
+    }
+}
+
+impl TryFromJsArgument for JsRest {
+    fn try_from_js_argument<'a>(
+        _: &'a JsValue,
+        rest: &'a [JsValue],
+        _context: &mut Context,
+    ) -> JsResult<(Self, &'a [JsValue])> {
+        Ok((JsRest(rest.to_vec()), &[]))
+    }
+}
+
+/// Captures the `this` value in a JS function. Although this can be
+/// specified multiple times as argument, it will always be filled
+/// with clone of the same value.
+#[derive(Debug, Clone)]
+pub struct JsThis<T: TryFromJs>(pub T);
+
+impl<T: TryFromJs> TryFromJsArgument for JsThis<T> {
+    fn try_from_js_argument<'a>(
+        this: &'a JsValue,
+        rest: &'a [JsValue],
+        context: &mut Context,
+    ) -> JsResult<(Self, &'a [JsValue])> {
+        Ok((JsThis(this.try_js_into(context)?), rest))
+    }
+}
+
+// Implement `IntoJsFunction` for functions with a various list of
+// arguments.
+mod into_js_function_impls;
+
 #[test]
 #[allow(clippy::missing_panics_doc)]
 pub fn into_js_module() {
-    use boa_engine::builtins::promise::PromiseState;
     use boa_engine::{js_string, JsValue, Source};
+    use std::cell::RefCell;
     use std::rc::Rc;
-    use std::sync::atomic::{AtomicU32, Ordering};
 
     let loader = Rc::new(loaders::HashMapModuleLoader::new());
     let mut context = Context::builder()
@@ -77,29 +214,56 @@ pub fn into_js_module() {
         .build()
         .unwrap();
 
-    let foo_count = Rc::new(AtomicU32::new(0));
-    let bar_count = Rc::new(AtomicU32::new(0));
+    let foo_count = Rc::new(RefCell::new(0));
+    let bar_count = Rc::new(RefCell::new(0));
+    let dad_count = Rc::new(RefCell::new(0));
+    let result = Rc::new(RefCell::new(JsValue::undefined()));
     let module = unsafe {
         vec![
             (
                 js_string!("foo"),
+                {
+                    let counter = foo_count.clone();
+                    move || {
+                        *counter.borrow_mut() += 1;
+                        let result = *counter.borrow();
+                        result
+                    }
+                }
+                .into_js_function(&mut context),
+            ),
+            (
+                js_string!("bar"),
                 IntoJsFunctionUnsafe::into_js_function(
                     {
-                        let counter = foo_count.clone();
-                        move || {
-                            counter.fetch_add(1, Ordering::Relaxed);
+                        let counter = bar_count.clone();
+                        move |i: i32| {
+                            *counter.borrow_mut() += i;
                         }
                     },
                     &mut context,
                 ),
             ),
             (
-                js_string!("bar"),
+                js_string!("dad"),
+                {
+                    let counter = dad_count.clone();
+                    move |args: JsRest, context: &mut Context| {
+                        *counter.borrow_mut() += args
+                            .into_iter()
+                            .map(|i| i.try_js_into::<i32>(context).unwrap())
+                            .sum::<i32>();
+                    }
+                }
+                .into_js_function(&mut context),
+            ),
+            (
+                js_string!("send"),
                 IntoJsFunctionUnsafe::into_js_function(
                     {
-                        let counter = bar_count.clone();
-                        move || {
-                            counter.fetch_add(1, Ordering::Relaxed);
+                        let result = result.clone();
+                        move |value: JsValue| {
+                            *result.borrow_mut() = value;
                         }
                     },
                     &mut context,
@@ -115,11 +279,15 @@ pub fn into_js_module() {
         r"
             import * as test from 'test';
             let result = test.foo();
-            for (let i = 0; i < 10; i++) {
-                test.bar();
+            test.foo();
+            for (let i = 1; i <= 5; i++) {
+                test.bar(i);
+            }
+            for (let i = 1; i < 5; i++) {
+                test.dad(1, 2, 3);
             }
 
-            result
+            test.send(result);
         ",
     );
     let root_module = Module::parse(source, None, &mut context).unwrap();
@@ -128,11 +296,15 @@ pub fn into_js_module() {
     context.run_jobs();
 
     // Checking if the final promise didn't return an error.
-    let PromiseState::Fulfilled(v) = promise_result.state() else {
-        panic!("module didn't execute successfully!")
+    if promise_result.state().as_fulfilled().is_none() {
+        panic!(
+            "module didn't execute successfully! Promise: {:?}",
+            promise_result.state()
+        );
     };
 
-    assert_eq!(foo_count.load(Ordering::Relaxed), 1);
-    assert_eq!(bar_count.load(Ordering::Relaxed), 10);
-    assert_eq!(v, JsValue::undefined());
+    assert_eq!(*foo_count.borrow(), 2);
+    assert_eq!(*bar_count.borrow(), 15);
+    assert_eq!(*dad_count.borrow(), 24);
+    assert_eq!(result.borrow().clone().try_js_into(&mut context), Ok(1u32));
 }

From a499b886432d7e68fde17a9f48ad5565c7af5da6 Mon Sep 17 00:00:00 2001
From: Hans Larsen <hans@larsen.online>
Date: Thu, 28 Mar 2024 22:44:57 -0700
Subject: [PATCH 02/22] cargo clippy

---
 core/interop/src/lib.rs | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/core/interop/src/lib.rs b/core/interop/src/lib.rs
index bcff0c6e9b9..39e64fac356 100644
--- a/core/interop/src/lib.rs
+++ b/core/interop/src/lib.rs
@@ -129,7 +129,7 @@ impl<T: TryFromJs> TryFromJsArgument for T {
 }
 
 /// An argument that when used in a JS function will empty the list
-/// of JS arguments as JsValues. This can be used for having the
+/// of JS arguments as `JsValue`s. This can be used for having the
 /// rest of the arguments in a function.
 #[derive(Debug, Clone)]
 pub struct JsRest(pub Vec<JsValue>);
@@ -296,12 +296,11 @@ pub fn into_js_module() {
     context.run_jobs();
 
     // Checking if the final promise didn't return an error.
-    if promise_result.state().as_fulfilled().is_none() {
-        panic!(
-            "module didn't execute successfully! Promise: {:?}",
-            promise_result.state()
-        );
-    };
+    assert!(
+        promise_result.state().as_fulfilled().is_some(),
+        "module didn't execute successfully! Promise: {:?}",
+        promise_result.state()
+    );
 
     assert_eq!(*foo_count.borrow(), 2);
     assert_eq!(*bar_count.borrow(), 15);

From bbcf043d778a5595eb1993c816d67803151c3d5a Mon Sep 17 00:00:00 2001
From: Hans Larsen <hans@larsen.online>
Date: Mon, 1 Apr 2024 10:50:42 -0700
Subject: [PATCH 03/22] Readd the safe version for Fn which also implements
 Copy

---
 core/interop/src/into_js_function_impls.rs | 79 +++++++++++++++++++---
 core/interop/src/lib.rs                    | 67 ++++++++++--------
 2 files changed, 108 insertions(+), 38 deletions(-)

diff --git a/core/interop/src/into_js_function_impls.rs b/core/interop/src/into_js_function_impls.rs
index 90b1e2e72df..7fe4bf12b7c 100644
--- a/core/interop/src/into_js_function_impls.rs
+++ b/core/interop/src/into_js_function_impls.rs
@@ -1,9 +1,11 @@
 //! Implementations of the `IntoJsFunction` trait for various function signatures.
 
-use crate::{IntoJsFunction, TryFromJsArgument};
-use boa_engine::{Context, JsValue, NativeFunction};
 use std::cell::RefCell;
 
+use boa_engine::{Context, JsValue, NativeFunction};
+
+use crate::{IntoJsFunction, IntoJsFunctionUnsafe, TryFromJsArgument};
+
 /// A token to represent the context argument in the function signature.
 /// This should not be used directly and has no external meaning.
 #[derive(Debug, Copy, Clone)]
@@ -11,14 +13,14 @@ pub struct ContextArgToken();
 
 macro_rules! impl_into_js_function {
     ($($id: ident: $t: ident),*) => {
-        impl<$($t,)* R, T> IntoJsFunction<($($t,)*), R> for T
+        unsafe impl<$($t,)* R, T> IntoJsFunctionUnsafe<($($t,)*), R> for T
         where
             $($t: TryFromJsArgument + 'static,)*
             R: Into<JsValue>,
             T: FnMut($($t,)*) -> R + 'static,
         {
             #[allow(unused_variables)]
-            fn into_js_function(self, _context: &mut Context) -> NativeFunction {
+            unsafe fn into_js_function_unsafe(self, _context: &mut Context) -> NativeFunction {
                 let s = RefCell::new(self);
                 unsafe {
                     NativeFunction::from_closure(move |this, args, ctx| {
@@ -33,14 +35,14 @@ macro_rules! impl_into_js_function {
             }
         }
 
-        impl<$($t,)* R, T> IntoJsFunction<($($t,)* ContextArgToken), R> for T
+        unsafe impl<$($t,)* R, T> IntoJsFunctionUnsafe<($($t,)* ContextArgToken), R> for T
         where
             $($t: TryFromJsArgument + 'static,)*
             R: Into<JsValue>,
             T: FnMut($($t,)* &mut Context) -> R + 'static,
         {
             #[allow(unused_variables)]
-            fn into_js_function(self, _context: &mut Context) -> NativeFunction {
+            unsafe fn into_js_function_unsafe(self, _context: &mut Context) -> NativeFunction {
                 let s = RefCell::new(self);
                 unsafe {
                     NativeFunction::from_closure(move |this, args, ctx| {
@@ -54,15 +56,60 @@ macro_rules! impl_into_js_function {
                 }
             }
         }
+
+        // Safe versions for `Fn(..) -> ...`.
+        impl<$($t,)* R, T> IntoJsFunction<($($t,)*), R> for T
+        where
+            $($t: TryFromJsArgument + 'static,)*
+            R: Into<JsValue>,
+            T: Fn($($t,)*) -> R + 'static + Copy,
+        {
+            #[allow(unused_variables)]
+            fn into_js_function(self, _context: &mut Context) -> NativeFunction {
+                let s = self;
+                unsafe {
+                    NativeFunction::from_closure(move |this, args, ctx| {
+                        let rest = args;
+                        $(
+                            let ($id, rest) = $t::try_from_js_argument(this, rest, ctx)?;
+                        )*
+                        let r = s( $($id),* );
+                        Ok(r.into())
+                    })
+                }
+            }
+        }
+
+        impl<$($t,)* R, T> IntoJsFunction<($($t,)* ContextArgToken), R> for T
+        where
+            $($t: TryFromJsArgument + 'static,)*
+            R: Into<JsValue>,
+            T: Fn($($t,)* &mut Context) -> R + 'static + Copy,
+        {
+            #[allow(unused_variables)]
+            fn into_js_function(self, _context: &mut Context) -> NativeFunction {
+                let s = self;
+                unsafe {
+                    NativeFunction::from_closure(move |this, args, ctx| {
+                        let rest = args;
+                        $(
+                            let ($id, rest) = $t::try_from_js_argument(this, rest, ctx)?;
+                        )*
+                        let r = s( $($id,)* ctx);
+                        Ok(r.into())
+                    })
+                }
+            }
+        }
     };
 }
 
-impl<R, T> IntoJsFunction<(), R> for T
+unsafe impl<R, T> IntoJsFunctionUnsafe<(), R> for T
 where
     R: Into<JsValue>,
     T: FnMut() -> R + 'static,
 {
-    fn into_js_function(self, _context: &mut Context) -> NativeFunction {
+    unsafe fn into_js_function_unsafe(self, _context: &mut Context) -> NativeFunction {
         let s = RefCell::new(self);
         unsafe {
             NativeFunction::from_closure(move |_this, _args, _ctx| {
@@ -73,6 +120,22 @@ where
     }
 }
 
+impl<R, T> IntoJsFunction<(), R> for T
+where
+    R: Into<JsValue>,
+    T: Fn() -> R + 'static + Copy,
+{
+    fn into_js_function(self, _context: &mut Context) -> NativeFunction {
+        let s = self;
+        unsafe {
+            NativeFunction::from_closure(move |_this, _args, _ctx| {
+                let r = s();
+                Ok(r.into())
+            })
+        }
+    }
+}
+
 // Currently implemented up to 12 arguments. The empty argument list
 // is implemented separately above.
 // Consider that JsRest and JsThis are part of this list, but Context
diff --git a/core/interop/src/lib.rs b/core/interop/src/lib.rs
index 39e64fac356..72a807f4316 100644
--- a/core/interop/src/lib.rs
+++ b/core/interop/src/lib.rs
@@ -1,7 +1,5 @@
 //! Interop utilities between Boa and its host.
 
-use std::cell::RefCell;
-
 use boa_engine::module::SyntheticModuleInitializer;
 use boa_engine::value::TryFromJs;
 use boa_engine::{Context, JsResult, JsString, JsValue, Module, NativeFunction};
@@ -48,12 +46,12 @@ impl<T: IntoIterator<Item = (JsString, NativeFunction)> + Clone> IntoJsModule fo
 /// # use boa_interop::IntoJsFunctionUnsafe;
 /// # let mut context = Context::default();
 /// let f = |a: i32, b: i32| a + b;
-/// let f = unsafe { f.into_js_function(&mut context) };
+/// let f = unsafe { f.into_js_function_unsafe(&mut context) };
 /// let result = f.call(&JsValue::undefined(), &[JsValue::from(1), JsValue::from(2)], &mut context).unwrap();
 /// assert_eq!(result, JsValue::new(3));
 /// ```
 ///
-/// Since the `IntoJsFunction` trait is implemented for `FnMut`, you can
+/// Since the `IntoJsFunctionUnsafe` trait is implemented for `FnMut`, you can
 /// also use closures directly:
 /// ```
 /// # use boa_engine::{Context, JsValue, NativeFunction};
@@ -69,7 +67,7 @@ impl<T: IntoIterator<Item = (JsString, NativeFunction)> + Clone> IntoJsModule fo
 ///     let x = x.clone();
 ///     move |a: i32| *x.borrow_mut() += a
 /// };
-/// let f = f.into_js_function(&mut context);
+/// let f = unsafe { f.into_js_function_unsafe(&mut context) };
 /// f.call(&JsValue::undefined(), &[JsValue::from(1)], &mut context).unwrap();
 /// f.call(&JsValue::undefined(), &[JsValue::from(4)], &mut context).unwrap();
 /// assert_eq!(*x.borrow(), 5);
@@ -78,25 +76,32 @@ impl<T: IntoIterator<Item = (JsString, NativeFunction)> + Clone> IntoJsModule fo
 /// # Safety
 /// For this trait to be implemented safely, the implementing type must not contain any
 /// garbage collected objects (from [`boa_gc`]).
-pub unsafe trait IntoJsFunctionUnsafe {
+pub unsafe trait IntoJsFunctionUnsafe<Args, Ret> {
     /// Converts the type into a JS function.
     ///
     /// # Safety
     /// This function is unsafe to ensure the callee knows the risks of using this trait.
     /// The implementing type must not contain any garbage collected objects.
-    unsafe fn into_js_function(self, context: &mut Context) -> NativeFunction;
+    unsafe fn into_js_function_unsafe(self, context: &mut Context) -> NativeFunction;
 }
 
-unsafe impl<T: FnMut() + 'static> IntoJsFunctionUnsafe for T {
-    unsafe fn into_js_function(self, _context: &mut Context) -> NativeFunction {
-        let cell = RefCell::new(self);
-        unsafe {
-            NativeFunction::from_closure(move |_, _, _| {
-                cell.borrow_mut()();
-                Ok(JsValue::undefined())
-            })
-        }
-    }
+/// The safe equivalent of the [`IntoJsFunctionUnsafe`] trait.
+/// This can only be used on closures that have the `Copy` trait.
+///
+/// Since this function is implemented for `Fn(...)` closures, we can use
+/// it directly when defining a function:
+/// ```
+/// # use boa_engine::{Context, JsValue, NativeFunction};
+/// # use boa_interop::IntoJsFunction;
+/// # let mut context = Context::default();
+/// let f = |a: i32, b: i32| a + b;
+/// let f = f.into_js_function(&mut context);
+/// let result = f.call(&JsValue::undefined(), &[JsValue::from(1), JsValue::from(2)], &mut context).unwrap();
+/// assert_eq!(result, JsValue::new(3));
+/// ```
+pub trait IntoJsFunction<Args, Ret>: Copy {
+    /// Converts the type into a JS function.
+    fn into_js_function(self, context: &mut Context) -> NativeFunction;
 }
 
 /// Create a Rust value from a JS argument. This trait is used to
@@ -230,11 +235,11 @@ pub fn into_js_module() {
                         result
                     }
                 }
-                .into_js_function(&mut context),
+                .into_js_function_unsafe(&mut context),
             ),
             (
                 js_string!("bar"),
-                IntoJsFunctionUnsafe::into_js_function(
+                IntoJsFunctionUnsafe::into_js_function_unsafe(
                     {
                         let counter = bar_count.clone();
                         move |i: i32| {
@@ -246,20 +251,22 @@ pub fn into_js_module() {
             ),
             (
                 js_string!("dad"),
-                {
-                    let counter = dad_count.clone();
-                    move |args: JsRest, context: &mut Context| {
-                        *counter.borrow_mut() += args
-                            .into_iter()
-                            .map(|i| i.try_js_into::<i32>(context).unwrap())
-                            .sum::<i32>();
-                    }
-                }
-                .into_js_function(&mut context),
+                IntoJsFunctionUnsafe::into_js_function_unsafe(
+                    {
+                        let counter = dad_count.clone();
+                        move |args: JsRest, context: &mut Context| {
+                            *counter.borrow_mut() += args
+                                .into_iter()
+                                .map(|i| i.try_js_into::<i32>(context).unwrap())
+                                .sum::<i32>();
+                        }
+                    },
+                    &mut context,
+                ),
             ),
             (
                 js_string!("send"),
-                IntoJsFunctionUnsafe::into_js_function(
+                IntoJsFunctionUnsafe::into_js_function_unsafe(
                     {
                         let result = result.clone();
                         move |value: JsValue| {

From 4b0f47d9ddca1adfd351478f27c7444f734739af Mon Sep 17 00:00:00 2001
From: Hans Larsen <hans@larsen.online>
Date: Wed, 3 Apr 2024 20:26:04 -0700
Subject: [PATCH 04/22] Use a new trait for converting a Rust type to a
 JsResult<JsValue>

---
 core/interop/src/into_js_function_impls.rs   | 34 ++++++------
 core/interop/src/lib.rs                      | 12 +++++
 core/interop/src/try_into_js_result_impls.rs | 55 ++++++++++++++++++++
 3 files changed, 84 insertions(+), 17 deletions(-)
 create mode 100644 core/interop/src/try_into_js_result_impls.rs

diff --git a/core/interop/src/into_js_function_impls.rs b/core/interop/src/into_js_function_impls.rs
index 7fe4bf12b7c..67ab1cc6028 100644
--- a/core/interop/src/into_js_function_impls.rs
+++ b/core/interop/src/into_js_function_impls.rs
@@ -2,21 +2,21 @@
 
 use std::cell::RefCell;
 
-use boa_engine::{Context, JsValue, NativeFunction};
+use boa_engine::{Context, NativeFunction};
 
-use crate::{IntoJsFunction, IntoJsFunctionUnsafe, TryFromJsArgument};
+use crate::{IntoJsFunction, IntoJsFunctionUnsafe, TryFromJsArgument, TryIntoJsResult};
 
 /// A token to represent the context argument in the function signature.
 /// This should not be used directly and has no external meaning.
 #[derive(Debug, Copy, Clone)]
-pub struct ContextArgToken();
+pub struct ContextArgToken;
 
 macro_rules! impl_into_js_function {
     ($($id: ident: $t: ident),*) => {
         unsafe impl<$($t,)* R, T> IntoJsFunctionUnsafe<($($t,)*), R> for T
         where
             $($t: TryFromJsArgument + 'static,)*
-            R: Into<JsValue>,
+            R: TryIntoJsResult,
             T: FnMut($($t,)*) -> R + 'static,
         {
             #[allow(unused_variables)]
@@ -29,7 +29,7 @@ macro_rules! impl_into_js_function {
                             let ($id, rest) = $t::try_from_js_argument(this, rest, ctx)?;
                         )*
                         let r = s.borrow_mut()( $($id),* );
-                        Ok(r.into())
+                        r.try_into_js_result(ctx)
                     })
                 }
             }
@@ -38,7 +38,7 @@ macro_rules! impl_into_js_function {
         unsafe impl<$($t,)* R, T> IntoJsFunctionUnsafe<($($t,)* ContextArgToken), R> for T
         where
             $($t: TryFromJsArgument + 'static,)*
-            R: Into<JsValue>,
+            R: TryIntoJsResult,
             T: FnMut($($t,)* &mut Context) -> R + 'static,
         {
             #[allow(unused_variables)]
@@ -51,7 +51,7 @@ macro_rules! impl_into_js_function {
                             let ($id, rest) = $t::try_from_js_argument(this, rest, ctx)?;
                         )*
                         let r = s.borrow_mut()( $($id,)* ctx);
-                        Ok(r.into())
+                        r.try_into_js_result(ctx)
                     })
                 }
             }
@@ -61,7 +61,7 @@ macro_rules! impl_into_js_function {
         impl<$($t,)* R, T> IntoJsFunction<($($t,)*), R> for T
         where
             $($t: TryFromJsArgument + 'static,)*
-            R: Into<JsValue>,
+            R: TryIntoJsResult,
             T: Fn($($t,)*) -> R + 'static + Copy,
         {
             #[allow(unused_variables)]
@@ -74,7 +74,7 @@ macro_rules! impl_into_js_function {
                             let ($id, rest) = $t::try_from_js_argument(this, rest, ctx)?;
                         )*
                         let r = s( $($id),* );
-                        Ok(r.into())
+                        r.try_into_js_result(ctx)
                     })
                 }
             }
@@ -83,7 +83,7 @@ macro_rules! impl_into_js_function {
         impl<$($t,)* R, T> IntoJsFunction<($($t,)* ContextArgToken), R> for T
         where
             $($t: TryFromJsArgument + 'static,)*
-            R: Into<JsValue>,
+            R: TryIntoJsResult,
             T: Fn($($t,)* &mut Context) -> R + 'static + Copy,
         {
             #[allow(unused_variables)]
@@ -96,7 +96,7 @@ macro_rules! impl_into_js_function {
                             let ($id, rest) = $t::try_from_js_argument(this, rest, ctx)?;
                         )*
                         let r = s( $($id,)* ctx);
-                        Ok(r.into())
+                        r.try_into_js_result(ctx)
                     })
                 }
             }
@@ -106,15 +106,15 @@ macro_rules! impl_into_js_function {
 
 unsafe impl<R, T> IntoJsFunctionUnsafe<(), R> for T
 where
-    R: Into<JsValue>,
+    R: TryIntoJsResult,
     T: FnMut() -> R + 'static,
 {
     unsafe fn into_js_function_unsafe(self, _context: &mut Context) -> NativeFunction {
         let s = RefCell::new(self);
         unsafe {
-            NativeFunction::from_closure(move |_this, _args, _ctx| {
+            NativeFunction::from_closure(move |_this, _args, ctx| {
                 let r = s.borrow_mut()();
-                Ok(r.into())
+                r.try_into_js_result(ctx)
             })
         }
     }
@@ -122,15 +122,15 @@ where
 
 impl<R, T> IntoJsFunction<(), R> for T
 where
-    R: Into<JsValue>,
+    R: TryIntoJsResult,
     T: Fn() -> R + 'static + Copy,
 {
     fn into_js_function(self, _context: &mut Context) -> NativeFunction {
         let s = self;
         unsafe {
-            NativeFunction::from_closure(move |_this, _args, _ctx| {
+            NativeFunction::from_closure(move |_this, _args, ctx| {
                 let r = s();
-                Ok(r.into())
+                r.try_into_js_result(ctx)
             })
         }
     }
diff --git a/core/interop/src/lib.rs b/core/interop/src/lib.rs
index 72a807f4316..34d5d42aa90 100644
--- a/core/interop/src/lib.rs
+++ b/core/interop/src/lib.rs
@@ -104,6 +104,18 @@ pub trait IntoJsFunction<Args, Ret>: Copy {
     fn into_js_function(self, context: &mut Context) -> NativeFunction;
 }
 
+/// Create a [`JsResult`] from a Rust value. This trait is used to
+/// convert Rust types to JS types, including [`JsResult`] of
+/// Rust values and [`JsValue`]s. It is used to convert the
+/// return value of a function in [`IntoJsFunctionUnsafe`] and
+/// [`IntoJsFunction`].
+pub trait TryIntoJsResult {
+    /// Try to convert a Rust value into a `JsResult<JsValue>`.
+    fn try_into_js_result(self, context: &mut Context) -> JsResult<JsValue>;
+}
+
+mod try_into_js_result_impls;
+
 /// Create a Rust value from a JS argument. This trait is used to
 /// convert arguments from JS to Rust types. It allows support
 /// for optional arguments or rest arguments.
diff --git a/core/interop/src/try_into_js_result_impls.rs b/core/interop/src/try_into_js_result_impls.rs
new file mode 100644
index 00000000000..1be5e1e92e8
--- /dev/null
+++ b/core/interop/src/try_into_js_result_impls.rs
@@ -0,0 +1,55 @@
+//! Declare implementations of [`TryIntoJsResult`] trait for various types.
+//! We cannot rely on a generic implementation based on `TryIntoJs` due
+//! to a limitation of the Rust type system which prevents generalization
+//! of traits based on an upstream crate.
+
+use crate::TryIntoJsResult;
+use boa_engine::{Context, JsBigInt, JsResult, JsString, JsSymbol, JsValue};
+
+impl<T: TryIntoJsResult> TryIntoJsResult for Option<T> {
+    fn try_into_js_result(self, context: &mut Context) -> JsResult<JsValue> {
+        match self {
+            Some(value) => value.try_into_js_result(context),
+            None => Ok(JsValue::undefined()),
+        }
+    }
+}
+
+impl<T: TryIntoJsResult> TryIntoJsResult for JsResult<T> {
+    fn try_into_js_result(self, context: &mut Context) -> JsResult<JsValue> {
+        self.and_then(|value| value.try_into_js_result(context))
+    }
+}
+
+macro_rules! impl_try_into_js_result {
+    ($($t: ty),*) => {
+        $(
+            impl TryIntoJsResult for $t {
+                fn try_into_js_result(self, _context: &mut Context) -> JsResult<JsValue> {
+                    Ok(JsValue::from(self))
+                }
+            }
+        )*
+    };
+}
+
+impl_try_into_js_result!(
+    bool,
+    char,
+    i8,
+    i16,
+    i32,
+    i64,
+    u8,
+    u16,
+    u32,
+    u64,
+    usize,
+    f32,
+    f64,
+    JsBigInt,
+    JsString,
+    JsSymbol,
+    JsValue,
+    ()
+);

From 64fff68ac21c90a6b85fbce202ed97344052bbcb Mon Sep 17 00:00:00 2001
From: Hans Larsen <hans@larsen.online>
Date: Wed, 3 Apr 2024 20:59:13 -0700
Subject: [PATCH 05/22] cargo clippy

---
 core/interop/src/lib.rs | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/core/interop/src/lib.rs b/core/interop/src/lib.rs
index 34d5d42aa90..87bb950d5e1 100644
--- a/core/interop/src/lib.rs
+++ b/core/interop/src/lib.rs
@@ -111,6 +111,10 @@ pub trait IntoJsFunction<Args, Ret>: Copy {
 /// [`IntoJsFunction`].
 pub trait TryIntoJsResult {
     /// Try to convert a Rust value into a `JsResult<JsValue>`.
+    ///
+    /// # Errors
+    /// Any parsing errors that may occur during the conversion, or any
+    /// error that happened during the call to a function.
     fn try_into_js_result(self, context: &mut Context) -> JsResult<JsValue>;
 }
 

From 529dd788168f8e5f20569c56989bd259c37e2b17 Mon Sep 17 00:00:00 2001
From: Hans Larsen <hans@larsen.online>
Date: Wed, 3 Apr 2024 21:33:08 -0700
Subject: [PATCH 06/22] Add a test for returning result and Fn()

---
 core/interop/src/lib.rs | 44 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 44 insertions(+)

diff --git a/core/interop/src/lib.rs b/core/interop/src/lib.rs
index 87bb950d5e1..f5b4f648320 100644
--- a/core/interop/src/lib.rs
+++ b/core/interop/src/lib.rs
@@ -330,3 +330,47 @@ pub fn into_js_module() {
     assert_eq!(*dad_count.borrow(), 24);
     assert_eq!(result.borrow().clone().try_js_into(&mut context), Ok(1u32));
 }
+
+#[test]
+fn can_throw_exception() {
+    use boa_engine::{js_string, JsError, JsValue, Source};
+    use std::rc::Rc;
+
+    let loader = Rc::new(loaders::HashMapModuleLoader::new());
+    let mut context = Context::builder()
+        .module_loader(loader.clone())
+        .build()
+        .unwrap();
+
+    let module = vec![(
+        js_string!("doTheThrow"),
+        IntoJsFunction::into_js_function(
+            |message: JsValue| -> JsResult<()> { JsResult::Err(JsError::from_opaque(message)) },
+            &mut context,
+        ),
+    )]
+    .into_js_module(&mut context);
+
+    loader.register(js_string!("test"), module);
+
+    let source = Source::from_bytes(
+        r"
+            import * as test from 'test';
+            try {
+                test.doTheThrow('javascript');
+            } catch(e) {
+                throw 'from ' + e;
+            }
+        ",
+    );
+    let root_module = Module::parse(source, None, &mut context).unwrap();
+
+    let promise_result = root_module.load_link_evaluate(&mut context);
+    context.run_jobs();
+
+    // Checking if the final promise didn't return an error.
+    assert_eq!(
+        promise_result.state().as_rejected(),
+        Some(&JsString::from("from javascript").into())
+    );
+}

From 921b35a988de544b51d8a7e36ffed4e33c054841 Mon Sep 17 00:00:00 2001
From: Hans Larsen <hans@larsen.online>
Date: Sat, 6 Apr 2024 13:08:48 -0700
Subject: [PATCH 07/22] Seal both IntoJsFunction and IntoJsFunctionUnsafe

---
 core/interop/src/into_js_function_impls.rs | 28 +++++++++++++++++++---
 core/interop/src/lib.rs                    | 15 +++++++-----
 2 files changed, 34 insertions(+), 9 deletions(-)

diff --git a/core/interop/src/into_js_function_impls.rs b/core/interop/src/into_js_function_impls.rs
index 67ab1cc6028..0cffe0f6478 100644
--- a/core/interop/src/into_js_function_impls.rs
+++ b/core/interop/src/into_js_function_impls.rs
@@ -4,6 +4,7 @@ use std::cell::RefCell;
 
 use boa_engine::{Context, NativeFunction};
 
+use crate::private::IntoJsFunctionSealed;
 use crate::{IntoJsFunction, IntoJsFunctionUnsafe, TryFromJsArgument, TryIntoJsResult};
 
 /// A token to represent the context argument in the function signature.
@@ -13,7 +14,21 @@ pub struct ContextArgToken;
 
 macro_rules! impl_into_js_function {
     ($($id: ident: $t: ident),*) => {
-        unsafe impl<$($t,)* R, T> IntoJsFunctionUnsafe<($($t,)*), R> for T
+        impl<$($t,)* R, T> IntoJsFunctionSealed<($($t,)*), R> for T
+        where
+            $($t: TryFromJsArgument + 'static,)*
+            R: TryIntoJsResult,
+            T: FnMut($($t,)*) -> R + 'static
+        {}
+
+        impl<$($t,)* R, T> IntoJsFunctionSealed<($($t,)* ContextArgToken), R> for T
+        where
+            $($t: TryFromJsArgument + 'static,)*
+            R: TryIntoJsResult,
+            T: FnMut($($t,)* &mut Context) -> R + 'static
+        {}
+
+        impl<$($t,)* R, T> IntoJsFunctionUnsafe<($($t,)*), R> for T
         where
             $($t: TryFromJsArgument + 'static,)*
             R: TryIntoJsResult,
@@ -35,7 +50,7 @@ macro_rules! impl_into_js_function {
             }
         }
 
-        unsafe impl<$($t,)* R, T> IntoJsFunctionUnsafe<($($t,)* ContextArgToken), R> for T
+        impl<$($t,)* R, T> IntoJsFunctionUnsafe<($($t,)* ContextArgToken), R> for T
         where
             $($t: TryFromJsArgument + 'static,)*
             R: TryIntoJsResult,
@@ -104,7 +119,14 @@ macro_rules! impl_into_js_function {
     };
 }
 
-unsafe impl<R, T> IntoJsFunctionUnsafe<(), R> for T
+impl<R, T> IntoJsFunctionSealed<(), R> for T
+where
+    R: TryIntoJsResult,
+    T: FnMut() -> R + 'static,
+{
+}
+
+impl<R, T> IntoJsFunctionUnsafe<(), R> for T
 where
     R: TryIntoJsResult,
     T: FnMut() -> R + 'static,
diff --git a/core/interop/src/lib.rs b/core/interop/src/lib.rs
index f3e9d39312b..45c2f168d69 100644
--- a/core/interop/src/lib.rs
+++ b/core/interop/src/lib.rs
@@ -6,6 +6,13 @@ use boa_engine::{Context, JsResult, JsString, JsValue, Module, NativeFunction};
 
 pub mod loaders;
 
+/// Internal module only.
+pub(crate) mod private {
+    /// A sealed trait to prevent users from implementing the `IntoJsModuleFunction`
+    /// and `IntoJsFunctionUnsafe` traits to their own types.
+    pub trait IntoJsFunctionSealed<A, R> {}
+}
+
 /// A trait to convert a type into a JS module.
 pub trait IntoJsModule {
     /// Converts the type into a JS module.
@@ -73,11 +80,7 @@ impl<T: IntoIterator<Item = (JsString, NativeFunction)> + Clone> IntoJsModule fo
 /// f.call(&JsValue::undefined(), &[JsValue::from(4)], &mut context).unwrap();
 /// assert_eq!(*x.borrow(), 5);
 /// ```
-///
-/// # Safety
-/// For this trait to be implemented safely, the implementing type must not contain any
-/// garbage collected objects (from [`boa_gc`]).
-pub unsafe trait IntoJsFunctionUnsafe<Args, Ret> {
+pub trait IntoJsFunctionUnsafe<Args, Ret>: private::IntoJsFunctionSealed<Args, Ret> {
     /// Converts the type into a JS function.
     ///
     /// # Safety
@@ -100,7 +103,7 @@ pub unsafe trait IntoJsFunctionUnsafe<Args, Ret> {
 /// let result = f.call(&JsValue::undefined(), &[JsValue::from(1), JsValue::from(2)], &mut context).unwrap();
 /// assert_eq!(result, JsValue::new(3));
 /// ```
-pub trait IntoJsFunction<Args, Ret>: Copy {
+pub trait IntoJsFunction<Args, Ret>: private::IntoJsFunctionSealed<Args, Ret> + Copy {
     /// Converts the type into a JS function.
     fn into_js_function(self, context: &mut Context) -> NativeFunction;
 }

From d8815ac53781a14f2ec282d7250b0c38870f3ac7 Mon Sep 17 00:00:00 2001
From: Hans Larsen <hans@larsen.online>
Date: Sat, 6 Apr 2024 13:13:10 -0700
Subject: [PATCH 08/22] Try Borrowing and return a nice error instead of
 panicking

---
 core/interop/src/into_js_function_impls.rs | 18 +++++++++++-------
 core/interop/src/lib.rs                    |  8 ++++----
 2 files changed, 15 insertions(+), 11 deletions(-)

diff --git a/core/interop/src/into_js_function_impls.rs b/core/interop/src/into_js_function_impls.rs
index 0cffe0f6478..73c3f200f94 100644
--- a/core/interop/src/into_js_function_impls.rs
+++ b/core/interop/src/into_js_function_impls.rs
@@ -2,10 +2,10 @@
 
 use std::cell::RefCell;
 
-use boa_engine::{Context, NativeFunction};
+use boa_engine::{js_string, Context, JsError, NativeFunction};
 
 use crate::private::IntoJsFunctionSealed;
-use crate::{IntoJsFunction, IntoJsFunctionUnsafe, TryFromJsArgument, TryIntoJsResult};
+use crate::{IntoJsFunctionCopied, IntoJsFunctionUnsafe, TryFromJsArgument, TryIntoJsResult};
 
 /// A token to represent the context argument in the function signature.
 /// This should not be used directly and has no external meaning.
@@ -43,8 +43,12 @@ macro_rules! impl_into_js_function {
                         $(
                             let ($id, rest) = $t::try_from_js_argument(this, rest, ctx)?;
                         )*
-                        let r = s.borrow_mut()( $($id),* );
-                        r.try_into_js_result(ctx)
+                        match s.try_borrow_mut() {
+                            Ok(mut r) => r( $($id),* ).try_into_js_result(ctx),
+                            Err(_) => {
+                                Err(JsError::from_opaque(js_string!("recursive calls to this function not supported").into()))
+                            }
+                        }
                     })
                 }
             }
@@ -73,7 +77,7 @@ macro_rules! impl_into_js_function {
         }
 
         // Safe versions for `Fn(..) -> ...`.
-        impl<$($t,)* R, T> IntoJsFunction<($($t,)*), R> for T
+        impl<$($t,)* R, T> IntoJsFunctionCopied<($($t,)*), R> for T
         where
             $($t: TryFromJsArgument + 'static,)*
             R: TryIntoJsResult,
@@ -95,7 +99,7 @@ macro_rules! impl_into_js_function {
             }
         }
 
-        impl<$($t,)* R, T> IntoJsFunction<($($t,)* ContextArgToken), R> for T
+        impl<$($t,)* R, T> IntoJsFunctionCopied<($($t,)* ContextArgToken), R> for T
         where
             $($t: TryFromJsArgument + 'static,)*
             R: TryIntoJsResult,
@@ -142,7 +146,7 @@ where
     }
 }
 
-impl<R, T> IntoJsFunction<(), R> for T
+impl<R, T> IntoJsFunctionCopied<(), R> for T
 where
     R: TryIntoJsResult,
     T: Fn() -> R + 'static + Copy,
diff --git a/core/interop/src/lib.rs b/core/interop/src/lib.rs
index 45c2f168d69..e88200a3f79 100644
--- a/core/interop/src/lib.rs
+++ b/core/interop/src/lib.rs
@@ -96,14 +96,14 @@ pub trait IntoJsFunctionUnsafe<Args, Ret>: private::IntoJsFunctionSealed<Args, R
 /// it directly when defining a function:
 /// ```
 /// # use boa_engine::{Context, JsValue, NativeFunction};
-/// # use boa_interop::IntoJsFunction;
+/// # use boa_interop::IntoJsFunctionCopied;
 /// # let mut context = Context::default();
 /// let f = |a: i32, b: i32| a + b;
 /// let f = f.into_js_function(&mut context);
 /// let result = f.call(&JsValue::undefined(), &[JsValue::from(1), JsValue::from(2)], &mut context).unwrap();
 /// assert_eq!(result, JsValue::new(3));
 /// ```
-pub trait IntoJsFunction<Args, Ret>: private::IntoJsFunctionSealed<Args, Ret> + Copy {
+pub trait IntoJsFunctionCopied<Args, Ret>: private::IntoJsFunctionSealed<Args, Ret> + Copy {
     /// Converts the type into a JS function.
     fn into_js_function(self, context: &mut Context) -> NativeFunction;
 }
@@ -112,7 +112,7 @@ pub trait IntoJsFunction<Args, Ret>: private::IntoJsFunctionSealed<Args, Ret> +
 /// convert Rust types to JS types, including [`JsResult`] of
 /// Rust values and [`JsValue`]s. It is used to convert the
 /// return value of a function in [`IntoJsFunctionUnsafe`] and
-/// [`IntoJsFunction`].
+/// [`IntoJsFunctionCopied`].
 pub trait TryIntoJsResult {
     /// Try to convert a Rust value into a `JsResult<JsValue>`.
     ///
@@ -348,7 +348,7 @@ fn can_throw_exception() {
 
     let module = vec![(
         js_string!("doTheThrow"),
-        IntoJsFunction::into_js_function(
+        IntoJsFunctionCopied::into_js_function(
             |message: JsValue| -> JsResult<()> { JsResult::Err(JsError::from_opaque(message)) },
             &mut context,
         ),

From 78d541f407118207eed874e543d496e6da4e4648 Mon Sep 17 00:00:00 2001
From: Hans Larsen <hans@larsen.online>
Date: Sat, 6 Apr 2024 14:00:16 -0700
Subject: [PATCH 09/22] Address comments

---
 core/interop/src/into_js_function_impls.rs |  8 ++++----
 core/interop/src/lib.rs                    | 16 ++++++++--------
 2 files changed, 12 insertions(+), 12 deletions(-)

diff --git a/core/interop/src/into_js_function_impls.rs b/core/interop/src/into_js_function_impls.rs
index 73c3f200f94..7ec023c43e8 100644
--- a/core/interop/src/into_js_function_impls.rs
+++ b/core/interop/src/into_js_function_impls.rs
@@ -5,7 +5,7 @@ use std::cell::RefCell;
 use boa_engine::{js_string, Context, JsError, NativeFunction};
 
 use crate::private::IntoJsFunctionSealed;
-use crate::{IntoJsFunctionCopied, IntoJsFunctionUnsafe, TryFromJsArgument, TryIntoJsResult};
+use crate::{IntoJsFunctionCopied, TryFromJsArgument, TryIntoJsResult, UnsafeIntoJsFunction};
 
 /// A token to represent the context argument in the function signature.
 /// This should not be used directly and has no external meaning.
@@ -28,7 +28,7 @@ macro_rules! impl_into_js_function {
             T: FnMut($($t,)* &mut Context) -> R + 'static
         {}
 
-        impl<$($t,)* R, T> IntoJsFunctionUnsafe<($($t,)*), R> for T
+        impl<$($t,)* R, T> UnsafeIntoJsFunction<($($t,)*), R> for T
         where
             $($t: TryFromJsArgument + 'static,)*
             R: TryIntoJsResult,
@@ -54,7 +54,7 @@ macro_rules! impl_into_js_function {
             }
         }
 
-        impl<$($t,)* R, T> IntoJsFunctionUnsafe<($($t,)* ContextArgToken), R> for T
+        impl<$($t,)* R, T> UnsafeIntoJsFunction<($($t,)* ContextArgToken), R> for T
         where
             $($t: TryFromJsArgument + 'static,)*
             R: TryIntoJsResult,
@@ -130,7 +130,7 @@ where
 {
 }
 
-impl<R, T> IntoJsFunctionUnsafe<(), R> for T
+impl<R, T> UnsafeIntoJsFunction<(), R> for T
 where
     R: TryIntoJsResult,
     T: FnMut() -> R + 'static,
diff --git a/core/interop/src/lib.rs b/core/interop/src/lib.rs
index e88200a3f79..30c2d18327b 100644
--- a/core/interop/src/lib.rs
+++ b/core/interop/src/lib.rs
@@ -51,7 +51,7 @@ impl<T: IntoIterator<Item = (JsString, NativeFunction)> + Clone> IntoJsModule fo
 /// For example:
 /// ```
 /// # use boa_engine::{Context, JsValue, NativeFunction};
-/// # use boa_interop::IntoJsFunctionUnsafe;
+/// # use boa_interop::UnsafeIntoJsFunction;
 /// # let mut context = Context::default();
 /// let f = |a: i32, b: i32| a + b;
 /// let f = unsafe { f.into_js_function_unsafe(&mut context) };
@@ -63,7 +63,7 @@ impl<T: IntoIterator<Item = (JsString, NativeFunction)> + Clone> IntoJsModule fo
 /// also use closures directly:
 /// ```
 /// # use boa_engine::{Context, JsValue, NativeFunction};
-/// # use boa_interop::IntoJsFunctionUnsafe;
+/// # use boa_interop::UnsafeIntoJsFunction;
 /// # use std::cell::RefCell;
 /// # use std::rc::Rc;
 /// # let mut context = Context::default();
@@ -80,7 +80,7 @@ impl<T: IntoIterator<Item = (JsString, NativeFunction)> + Clone> IntoJsModule fo
 /// f.call(&JsValue::undefined(), &[JsValue::from(4)], &mut context).unwrap();
 /// assert_eq!(*x.borrow(), 5);
 /// ```
-pub trait IntoJsFunctionUnsafe<Args, Ret>: private::IntoJsFunctionSealed<Args, Ret> {
+pub trait UnsafeIntoJsFunction<Args, Ret>: private::IntoJsFunctionSealed<Args, Ret> {
     /// Converts the type into a JS function.
     ///
     /// # Safety
@@ -89,7 +89,7 @@ pub trait IntoJsFunctionUnsafe<Args, Ret>: private::IntoJsFunctionSealed<Args, R
     unsafe fn into_js_function_unsafe(self, context: &mut Context) -> NativeFunction;
 }
 
-/// The safe equivalent of the [`IntoJsFunctionUnsafe`] trait.
+/// The safe equivalent of the [`UnsafeIntoJsFunction`] trait.
 /// This can only be used on closures that have the `Copy` trait.
 ///
 /// Since this function is implemented for `Fn(...)` closures, we can use
@@ -111,7 +111,7 @@ pub trait IntoJsFunctionCopied<Args, Ret>: private::IntoJsFunctionSealed<Args, R
 /// Create a [`JsResult`] from a Rust value. This trait is used to
 /// convert Rust types to JS types, including [`JsResult`] of
 /// Rust values and [`JsValue`]s. It is used to convert the
-/// return value of a function in [`IntoJsFunctionUnsafe`] and
+/// return value of a function in [`UnsafeIntoJsFunction`] and
 /// [`IntoJsFunctionCopied`].
 pub trait TryIntoJsResult {
     /// Try to convert a Rust value into a `JsResult<JsValue>`.
@@ -259,7 +259,7 @@ pub fn into_js_module() {
             ),
             (
                 js_string!("bar"),
-                IntoJsFunctionUnsafe::into_js_function_unsafe(
+                UnsafeIntoJsFunction::into_js_function_unsafe(
                     {
                         let counter = bar_count.clone();
                         move |i: i32| {
@@ -271,7 +271,7 @@ pub fn into_js_module() {
             ),
             (
                 js_string!("dad"),
-                IntoJsFunctionUnsafe::into_js_function_unsafe(
+                UnsafeIntoJsFunction::into_js_function_unsafe(
                     {
                         let counter = dad_count.clone();
                         move |args: JsRest, context: &mut Context| {
@@ -286,7 +286,7 @@ pub fn into_js_module() {
             ),
             (
                 js_string!("send"),
-                IntoJsFunctionUnsafe::into_js_function_unsafe(
+                UnsafeIntoJsFunction::into_js_function_unsafe(
                     {
                         let result = result.clone();
                         move |value: JsValue| {

From 3ff54d506fb2e45ffae2edda442a6e042bb87305 Mon Sep 17 00:00:00 2001
From: Hans Larsen <hans@larsen.online>
Date: Sat, 6 Apr 2024 16:16:00 -0700
Subject: [PATCH 10/22] Rename into_js_function to into_js_function_copied

---
 core/interop/src/into_js_function_impls.rs | 6 +++---
 core/interop/src/lib.rs                    | 6 +++---
 2 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/core/interop/src/into_js_function_impls.rs b/core/interop/src/into_js_function_impls.rs
index 7ec023c43e8..83fade587d4 100644
--- a/core/interop/src/into_js_function_impls.rs
+++ b/core/interop/src/into_js_function_impls.rs
@@ -84,7 +84,7 @@ macro_rules! impl_into_js_function {
             T: Fn($($t,)*) -> R + 'static + Copy,
         {
             #[allow(unused_variables)]
-            fn into_js_function(self, _context: &mut Context) -> NativeFunction {
+            fn into_js_function_copied(self, _context: &mut Context) -> NativeFunction {
                 let s = self;
                 unsafe {
                     NativeFunction::from_closure(move |this, args, ctx| {
@@ -106,7 +106,7 @@ macro_rules! impl_into_js_function {
             T: Fn($($t,)* &mut Context) -> R + 'static + Copy,
         {
             #[allow(unused_variables)]
-            fn into_js_function(self, _context: &mut Context) -> NativeFunction {
+            fn into_js_function_copied(self, _context: &mut Context) -> NativeFunction {
                 let s = self;
                 unsafe {
                     NativeFunction::from_closure(move |this, args, ctx| {
@@ -151,7 +151,7 @@ where
     R: TryIntoJsResult,
     T: Fn() -> R + 'static + Copy,
 {
-    fn into_js_function(self, _context: &mut Context) -> NativeFunction {
+    fn into_js_function_copied(self, _context: &mut Context) -> NativeFunction {
         let s = self;
         unsafe {
             NativeFunction::from_closure(move |_this, _args, ctx| {
diff --git a/core/interop/src/lib.rs b/core/interop/src/lib.rs
index 30c2d18327b..5bc8b72b4bc 100644
--- a/core/interop/src/lib.rs
+++ b/core/interop/src/lib.rs
@@ -99,13 +99,13 @@ pub trait UnsafeIntoJsFunction<Args, Ret>: private::IntoJsFunctionSealed<Args, R
 /// # use boa_interop::IntoJsFunctionCopied;
 /// # let mut context = Context::default();
 /// let f = |a: i32, b: i32| a + b;
-/// let f = f.into_js_function(&mut context);
+/// let f = f.into_js_function_copied(&mut context);
 /// let result = f.call(&JsValue::undefined(), &[JsValue::from(1), JsValue::from(2)], &mut context).unwrap();
 /// assert_eq!(result, JsValue::new(3));
 /// ```
 pub trait IntoJsFunctionCopied<Args, Ret>: private::IntoJsFunctionSealed<Args, Ret> + Copy {
     /// Converts the type into a JS function.
-    fn into_js_function(self, context: &mut Context) -> NativeFunction;
+    fn into_js_function_copied(self, context: &mut Context) -> NativeFunction;
 }
 
 /// Create a [`JsResult`] from a Rust value. This trait is used to
@@ -348,7 +348,7 @@ fn can_throw_exception() {
 
     let module = vec![(
         js_string!("doTheThrow"),
-        IntoJsFunctionCopied::into_js_function(
+        IntoJsFunctionCopied::into_js_function_copied(
             |message: JsValue| -> JsResult<()> { JsResult::Err(JsError::from_opaque(message)) },
             &mut context,
         ),

From 3ea6baf471b227f0133e976b2a67e84b4487aacf Mon Sep 17 00:00:00 2001
From: Hans Larsen <hans@larsen.online>
Date: Sun, 7 Apr 2024 19:43:35 -0700
Subject: [PATCH 11/22] Move TryIntoJsResult to boa_engine

---
 core/engine/src/lib.rs                       | 16 ++++++
 core/engine/src/try_into_js_result_impls.rs  | 33 ++++++++++++
 core/interop/src/into_js_function_impls.rs   |  4 +-
 core/interop/src/lib.rs                      | 18 +------
 core/interop/src/try_into_js_result_impls.rs | 55 --------------------
 5 files changed, 52 insertions(+), 74 deletions(-)
 create mode 100644 core/engine/src/try_into_js_result_impls.rs
 delete mode 100644 core/interop/src/try_into_js_result_impls.rs

diff --git a/core/engine/src/lib.rs b/core/engine/src/lib.rs
index c017bfb4c60..713906e9bd3 100644
--- a/core/engine/src/lib.rs
+++ b/core/engine/src/lib.rs
@@ -141,6 +141,22 @@ pub use prelude::*;
 /// The result of a Javascript expression is represented like this so it can succeed (`Ok`) or fail (`Err`)
 pub type JsResult<T> = StdResult<T, JsError>;
 
+/// Create a [`JsResult`] from a Rust value. This trait is used to
+/// convert Rust types to JS types, including [`JsResult`] of
+/// Rust values and [`JsValue`]s.
+///
+/// This trait is implemented for any that can be converted into a [`JsValue`].
+pub trait TryIntoJsResult {
+    /// Try to convert a Rust value into a `JsResult<JsValue>`.
+    ///
+    /// # Errors
+    /// Any parsing errors that may occur during the conversion, or any
+    /// error that happened during the call to a function.
+    fn try_into_js_result(self, context: &mut Context) -> JsResult<JsValue>;
+}
+
+mod try_into_js_result_impls;
+
 /// A utility trait to make working with function arguments easier.
 pub trait JsArgs {
     /// Utility function to `get` a parameter from a `[JsValue]` or default to `JsValue::Undefined`
diff --git a/core/engine/src/try_into_js_result_impls.rs b/core/engine/src/try_into_js_result_impls.rs
new file mode 100644
index 00000000000..e4fe117ec7d
--- /dev/null
+++ b/core/engine/src/try_into_js_result_impls.rs
@@ -0,0 +1,33 @@
+//! Declare implementations of [`TryIntoJsResult`] trait for various types.
+
+use crate::{Context, JsResult, JsValue, TryIntoJsResult};
+
+impl<T> TryIntoJsResult for T
+where
+    T: Into<JsValue>,
+{
+    fn try_into_js_result(self, _cx: &mut Context) -> JsResult<JsValue> {
+        Ok(self.into())
+    }
+}
+
+impl<T> TryIntoJsResult for Option<T>
+where
+    T: TryIntoJsResult,
+{
+    fn try_into_js_result(self, cx: &mut Context) -> JsResult<JsValue> {
+        match self {
+            Some(value) => value.try_into_js_result(cx),
+            None => Ok(JsValue::undefined()),
+        }
+    }
+}
+
+impl<T> TryIntoJsResult for JsResult<T>
+where
+    T: TryIntoJsResult,
+{
+    fn try_into_js_result(self, cx: &mut Context) -> JsResult<JsValue> {
+        self.and_then(|value| value.try_into_js_result(cx))
+    }
+}
diff --git a/core/interop/src/into_js_function_impls.rs b/core/interop/src/into_js_function_impls.rs
index 83fade587d4..5eb5a571dac 100644
--- a/core/interop/src/into_js_function_impls.rs
+++ b/core/interop/src/into_js_function_impls.rs
@@ -2,10 +2,10 @@
 
 use std::cell::RefCell;
 
-use boa_engine::{js_string, Context, JsError, NativeFunction};
+use boa_engine::{js_string, Context, JsError, NativeFunction, TryIntoJsResult};
 
 use crate::private::IntoJsFunctionSealed;
-use crate::{IntoJsFunctionCopied, TryFromJsArgument, TryIntoJsResult, UnsafeIntoJsFunction};
+use crate::{IntoJsFunctionCopied, TryFromJsArgument, UnsafeIntoJsFunction};
 
 /// A token to represent the context argument in the function signature.
 /// This should not be used directly and has no external meaning.
diff --git a/core/interop/src/lib.rs b/core/interop/src/lib.rs
index 5bc8b72b4bc..809df0ee8b3 100644
--- a/core/interop/src/lib.rs
+++ b/core/interop/src/lib.rs
@@ -108,22 +108,6 @@ pub trait IntoJsFunctionCopied<Args, Ret>: private::IntoJsFunctionSealed<Args, R
     fn into_js_function_copied(self, context: &mut Context) -> NativeFunction;
 }
 
-/// Create a [`JsResult`] from a Rust value. This trait is used to
-/// convert Rust types to JS types, including [`JsResult`] of
-/// Rust values and [`JsValue`]s. It is used to convert the
-/// return value of a function in [`UnsafeIntoJsFunction`] and
-/// [`IntoJsFunctionCopied`].
-pub trait TryIntoJsResult {
-    /// Try to convert a Rust value into a `JsResult<JsValue>`.
-    ///
-    /// # Errors
-    /// Any parsing errors that may occur during the conversion, or any
-    /// error that happened during the call to a function.
-    fn try_into_js_result(self, context: &mut Context) -> JsResult<JsValue>;
-}
-
-mod try_into_js_result_impls;
-
 /// Create a Rust value from a JS argument. This trait is used to
 /// convert arguments from JS to Rust types. It allows support
 /// for optional arguments or rest arguments.
@@ -349,7 +333,7 @@ fn can_throw_exception() {
     let module = vec![(
         js_string!("doTheThrow"),
         IntoJsFunctionCopied::into_js_function_copied(
-            |message: JsValue| -> JsResult<()> { JsResult::Err(JsError::from_opaque(message)) },
+            |message: JsValue| -> JsResult<()> { Err(JsError::from_opaque(message)) },
             &mut context,
         ),
     )]
diff --git a/core/interop/src/try_into_js_result_impls.rs b/core/interop/src/try_into_js_result_impls.rs
deleted file mode 100644
index 1be5e1e92e8..00000000000
--- a/core/interop/src/try_into_js_result_impls.rs
+++ /dev/null
@@ -1,55 +0,0 @@
-//! Declare implementations of [`TryIntoJsResult`] trait for various types.
-//! We cannot rely on a generic implementation based on `TryIntoJs` due
-//! to a limitation of the Rust type system which prevents generalization
-//! of traits based on an upstream crate.
-
-use crate::TryIntoJsResult;
-use boa_engine::{Context, JsBigInt, JsResult, JsString, JsSymbol, JsValue};
-
-impl<T: TryIntoJsResult> TryIntoJsResult for Option<T> {
-    fn try_into_js_result(self, context: &mut Context) -> JsResult<JsValue> {
-        match self {
-            Some(value) => value.try_into_js_result(context),
-            None => Ok(JsValue::undefined()),
-        }
-    }
-}
-
-impl<T: TryIntoJsResult> TryIntoJsResult for JsResult<T> {
-    fn try_into_js_result(self, context: &mut Context) -> JsResult<JsValue> {
-        self.and_then(|value| value.try_into_js_result(context))
-    }
-}
-
-macro_rules! impl_try_into_js_result {
-    ($($t: ty),*) => {
-        $(
-            impl TryIntoJsResult for $t {
-                fn try_into_js_result(self, _context: &mut Context) -> JsResult<JsValue> {
-                    Ok(JsValue::from(self))
-                }
-            }
-        )*
-    };
-}
-
-impl_try_into_js_result!(
-    bool,
-    char,
-    i8,
-    i16,
-    i32,
-    i64,
-    u8,
-    u16,
-    u32,
-    u64,
-    usize,
-    f32,
-    f64,
-    JsBigInt,
-    JsString,
-    JsSymbol,
-    JsValue,
-    ()
-);

From 35fea4e5ff5543f2b50a70534144dae89d18583e Mon Sep 17 00:00:00 2001
From: Hans Larsen <hans@larsen.online>
Date: Sun, 7 Apr 2024 21:00:13 -0700
Subject: [PATCH 12/22] Move JsRest to be at the end only of arguments and add
 JsAll

---
 core/interop/src/into_js_function_impls.rs | 156 +++++++++++++++------
 core/interop/src/lib.rs                    | 119 ++++++++++++++--
 2 files changed, 219 insertions(+), 56 deletions(-)

diff --git a/core/interop/src/into_js_function_impls.rs b/core/interop/src/into_js_function_impls.rs
index 5eb5a571dac..93854adf6f4 100644
--- a/core/interop/src/into_js_function_impls.rs
+++ b/core/interop/src/into_js_function_impls.rs
@@ -5,7 +5,7 @@ use std::cell::RefCell;
 use boa_engine::{js_string, Context, JsError, NativeFunction, TryIntoJsResult};
 
 use crate::private::IntoJsFunctionSealed;
-use crate::{IntoJsFunctionCopied, TryFromJsArgument, UnsafeIntoJsFunction};
+use crate::{IntoJsFunctionCopied, JsRest, TryFromJsArgument, UnsafeIntoJsFunction};
 
 /// A token to represent the context argument in the function signature.
 /// This should not be used directly and has no external meaning.
@@ -21,13 +21,27 @@ macro_rules! impl_into_js_function {
             T: FnMut($($t,)*) -> R + 'static
         {}
 
-        impl<$($t,)* R, T> IntoJsFunctionSealed<($($t,)* ContextArgToken), R> for T
+        impl<$($t,)* R, T> IntoJsFunctionSealed<($($t,)* ContextArgToken,), R> for T
         where
             $($t: TryFromJsArgument + 'static,)*
             R: TryIntoJsResult,
             T: FnMut($($t,)* &mut Context) -> R + 'static
         {}
 
+        impl<$($t,)* R, T> IntoJsFunctionSealed<($($t,)* JsRest,), R> for T
+        where
+            $($t: TryFromJsArgument + 'static,)*
+            R: TryIntoJsResult,
+            T: FnMut($($t,)* JsRest) -> R + 'static
+        {}
+
+        impl<$($t,)* R, T> IntoJsFunctionSealed<($($t,)* JsRest, ContextArgToken), R> for T
+        where
+            $($t: TryFromJsArgument + 'static,)*
+            R: TryIntoJsResult,
+            T: FnMut($($t,)* JsRest, &mut Context) -> R + 'static
+        {}
+
         impl<$($t,)* R, T> UnsafeIntoJsFunction<($($t,)*), R> for T
         where
             $($t: TryFromJsArgument + 'static,)*
@@ -44,7 +58,33 @@ macro_rules! impl_into_js_function {
                             let ($id, rest) = $t::try_from_js_argument(this, rest, ctx)?;
                         )*
                         match s.try_borrow_mut() {
-                            Ok(mut r) => r( $($id),* ).try_into_js_result(ctx),
+                            Ok(mut r) => r( $($id,)* ).try_into_js_result(ctx),
+                            Err(_) => {
+                                Err(JsError::from_opaque(js_string!("recursive calls to this function not supported").into()))
+                            }
+                        }
+                    })
+                }
+            }
+        }
+
+        impl<$($t,)* R, T> UnsafeIntoJsFunction<($($t,)* JsRest,), R> for T
+        where
+            $($t: TryFromJsArgument + 'static,)*
+            R: TryIntoJsResult,
+            T: FnMut($($t,)* JsRest) -> R + 'static,
+        {
+            #[allow(unused_variables)]
+            unsafe fn into_js_function_unsafe(self, _context: &mut Context) -> NativeFunction {
+                let s = RefCell::new(self);
+                unsafe {
+                    NativeFunction::from_closure(move |this, args, ctx| {
+                        let rest = args;
+                        $(
+                            let ($id, rest) = $t::try_from_js_argument(this, rest, ctx)?;
+                        )*
+                        match s.try_borrow_mut() {
+                            Ok(mut r) => r( $($id,)* rest.into() ).try_into_js_result(ctx),
                             Err(_) => {
                                 Err(JsError::from_opaque(js_string!("recursive calls to this function not supported").into()))
                             }
@@ -54,7 +94,7 @@ macro_rules! impl_into_js_function {
             }
         }
 
-        impl<$($t,)* R, T> UnsafeIntoJsFunction<($($t,)* ContextArgToken), R> for T
+        impl<$($t,)* R, T> UnsafeIntoJsFunction<($($t,)* ContextArgToken,), R> for T
         where
             $($t: TryFromJsArgument + 'static,)*
             R: TryIntoJsResult,
@@ -76,6 +116,28 @@ macro_rules! impl_into_js_function {
             }
         }
 
+        impl<$($t,)* R, T> UnsafeIntoJsFunction<($($t,)* JsRest, ContextArgToken), R> for T
+        where
+            $($t: TryFromJsArgument + 'static,)*
+            R: TryIntoJsResult,
+            T: FnMut($($t,)* JsRest, &mut Context) -> R + 'static,
+        {
+            #[allow(unused_variables)]
+            unsafe fn into_js_function_unsafe(self, _context: &mut Context) -> NativeFunction {
+                let s = RefCell::new(self);
+                unsafe {
+                    NativeFunction::from_closure(move |this, args, ctx| {
+                        let rest = args;
+                        $(
+                            let ($id, rest) = $t::try_from_js_argument(this, rest, ctx)?;
+                        )*
+                        let r = s.borrow_mut()( $($id,)* rest.into(), ctx);
+                        r.try_into_js_result(ctx)
+                    })
+                }
+            }
+        }
+
         // Safe versions for `Fn(..) -> ...`.
         impl<$($t,)* R, T> IntoJsFunctionCopied<($($t,)*), R> for T
         where
@@ -92,18 +154,18 @@ macro_rules! impl_into_js_function {
                         $(
                             let ($id, rest) = $t::try_from_js_argument(this, rest, ctx)?;
                         )*
-                        let r = s( $($id),* );
+                        let r = s( $($id,)* );
                         r.try_into_js_result(ctx)
                     })
                 }
             }
         }
 
-        impl<$($t,)* R, T> IntoJsFunctionCopied<($($t,)* ContextArgToken), R> for T
+        impl<$($t,)* R, T> IntoJsFunctionCopied<($($t,)* JsRest,), R> for T
         where
             $($t: TryFromJsArgument + 'static,)*
             R: TryIntoJsResult,
-            T: Fn($($t,)* &mut Context) -> R + 'static + Copy,
+            T: Fn($($t,)* JsRest) -> R + 'static + Copy,
         {
             #[allow(unused_variables)]
             fn into_js_function_copied(self, _context: &mut Context) -> NativeFunction {
@@ -114,58 +176,64 @@ macro_rules! impl_into_js_function {
                         $(
                             let ($id, rest) = $t::try_from_js_argument(this, rest, ctx)?;
                         )*
-                        let r = s( $($id,)* ctx);
+                        let r = s( $($id,)* rest.into() );
                         r.try_into_js_result(ctx)
                     })
                 }
             }
         }
-    };
-}
-
-impl<R, T> IntoJsFunctionSealed<(), R> for T
-where
-    R: TryIntoJsResult,
-    T: FnMut() -> R + 'static,
-{
-}
 
-impl<R, T> UnsafeIntoJsFunction<(), R> for T
-where
-    R: TryIntoJsResult,
-    T: FnMut() -> R + 'static,
-{
-    unsafe fn into_js_function_unsafe(self, _context: &mut Context) -> NativeFunction {
-        let s = RefCell::new(self);
-        unsafe {
-            NativeFunction::from_closure(move |_this, _args, ctx| {
-                let r = s.borrow_mut()();
-                r.try_into_js_result(ctx)
-            })
+        impl<$($t,)* R, T> IntoJsFunctionCopied<($($t,)* ContextArgToken,), R> for T
+        where
+            $($t: TryFromJsArgument + 'static,)*
+            R: TryIntoJsResult,
+            T: Fn($($t,)* &mut Context) -> R + 'static + Copy,
+        {
+            #[allow(unused_variables)]
+            fn into_js_function_copied(self, _context: &mut Context) -> NativeFunction {
+                let s = self;
+                unsafe {
+                    NativeFunction::from_closure(move |this, args, ctx| {
+                        let rest = args;
+                        $(
+                            let ($id, rest) = $t::try_from_js_argument(this, rest, ctx)?;
+                        )*
+                        let r = s( $($id,)* ctx);
+                        r.try_into_js_result(ctx)
+                    })
+                }
+            }
         }
-    }
-}
 
-impl<R, T> IntoJsFunctionCopied<(), R> for T
-where
-    R: TryIntoJsResult,
-    T: Fn() -> R + 'static + Copy,
-{
-    fn into_js_function_copied(self, _context: &mut Context) -> NativeFunction {
-        let s = self;
-        unsafe {
-            NativeFunction::from_closure(move |_this, _args, ctx| {
-                let r = s();
-                r.try_into_js_result(ctx)
-            })
+        impl<$($t,)* R, T> IntoJsFunctionCopied<($($t,)* JsRest, ContextArgToken), R> for T
+        where
+            $($t: TryFromJsArgument + 'static,)*
+            R: TryIntoJsResult,
+            T: Fn($($t,)* JsRest, &mut Context) -> R + 'static + Copy,
+        {
+            #[allow(unused_variables)]
+            fn into_js_function_copied(self, _context: &mut Context) -> NativeFunction {
+                let s = self;
+                unsafe {
+                    NativeFunction::from_closure(move |this, args, ctx| {
+                        let rest = args;
+                        $(
+                            let ($id, rest) = $t::try_from_js_argument(this, rest, ctx)?;
+                        )*
+                        let r = s( $($id,)* rest.into(), ctx);
+                        r.try_into_js_result(ctx)
+                    })
+                }
+            }
         }
-    }
+    };
 }
 
 // Currently implemented up to 12 arguments. The empty argument list
 // is implemented separately above.
 // Consider that JsRest and JsThis are part of this list, but Context
 // is not, as it is a special specialization of the template.
+impl_into_js_function!();
 impl_into_js_function!(a: A);
 impl_into_js_function!(a: A, b: B);
 impl_into_js_function!(a: A, b: B, c: C);
diff --git a/core/interop/src/lib.rs b/core/interop/src/lib.rs
index 809df0ee8b3..1ff4eca8da5 100644
--- a/core/interop/src/lib.rs
+++ b/core/interop/src/lib.rs
@@ -100,7 +100,11 @@ pub trait UnsafeIntoJsFunction<Args, Ret>: private::IntoJsFunctionSealed<Args, R
 /// # let mut context = Context::default();
 /// let f = |a: i32, b: i32| a + b;
 /// let f = f.into_js_function_copied(&mut context);
-/// let result = f.call(&JsValue::undefined(), &[JsValue::from(1), JsValue::from(2)], &mut context).unwrap();
+/// let result = f.call(
+///     &JsValue::undefined(),
+///     &[JsValue::from(1), JsValue::from(2)],
+///     &mut context
+/// ).unwrap();
 /// assert_eq!(result, JsValue::new(3));
 /// ```
 pub trait IntoJsFunctionCopied<Args, Ret>: private::IntoJsFunctionSealed<Args, Ret> + Copy {
@@ -139,38 +143,65 @@ impl<T: TryFromJs> TryFromJsArgument for T {
 
 /// An argument that when used in a JS function will empty the list
 /// of JS arguments as `JsValue`s. This can be used for having the
-/// rest of the arguments in a function.
+/// rest of the arguments in a function. It should be the last
+/// argument of your function, before the `Context` argument if any.
+///
+/// For example,
+/// ```
+/// # use boa_engine::{Context, JsValue};
+/// # use boa_interop::{IntoJsFunctionCopied, JsRest};
+/// # let mut context = Context::default();
+/// let sums = (|args: JsRest, context: &mut Context| -> i32 {
+///    args.iter().map(|i| i.try_js_into::<i32>(context).unwrap()).sum::<i32>()
+/// }).into_js_function_copied(&mut context);
+///
+/// let result = sums.call(
+///     &JsValue::undefined(),
+///     &[JsValue::from(1), JsValue::from(2), JsValue::from(3)],
+///     &mut context
+/// ).unwrap();
+/// assert_eq!(result, JsValue::new(6));
+/// ```
 #[derive(Debug, Clone)]
 pub struct JsRest(pub Vec<JsValue>);
 
 #[allow(unused)]
 impl JsRest {
     /// Consumes the `JsRest` and returns the inner list of `JsValue`.
-    fn into_inner(self) -> Vec<JsValue> {
+    #[must_use]
+    pub fn into_inner(self) -> Vec<JsValue> {
         self.0
     }
 
     /// Returns an iterator over the inner list of `JsValue`.
-    fn iter(&self) -> impl Iterator<Item = &JsValue> {
+    pub fn iter(&self) -> impl Iterator<Item = &JsValue> {
         self.0.iter()
     }
 
     /// Returns a mutable iterator over the inner list of `JsValue`.
-    fn iter_mut(&mut self) -> impl Iterator<Item = &mut JsValue> {
+    pub fn iter_mut(&mut self) -> impl Iterator<Item = &mut JsValue> {
         self.0.iter_mut()
     }
 
     /// Returns the length of the inner list of `JsValue`.
-    fn len(&self) -> usize {
+    #[must_use]
+    pub fn len(&self) -> usize {
         self.0.len()
     }
 
     /// Returns `true` if the inner list of `JsValue` is empty.
-    fn is_empty(&self) -> bool {
+    #[must_use]
+    pub fn is_empty(&self) -> bool {
         self.0.is_empty()
     }
 }
 
+impl From<&[JsValue]> for JsRest {
+    fn from(values: &[JsValue]) -> Self {
+        Self(values.to_vec())
+    }
+}
+
 impl IntoIterator for JsRest {
     type Item = JsValue;
     type IntoIter = std::vec::IntoIter<JsValue>;
@@ -180,13 +211,77 @@ impl IntoIterator for JsRest {
     }
 }
 
-impl TryFromJsArgument for JsRest {
+/// An argument that when used in a JS function will capture all
+/// the arguments that can be converted to `T`. The first argument
+/// that cannot be converted to `T` will stop the conversion.
+///
+/// For example,
+/// ```
+/// # use boa_engine::{Context, JsValue};
+/// # use boa_interop::{IntoJsFunctionCopied, JsAll};
+/// # let mut context = Context::default();
+/// let sums = (|args: JsAll<i32>, context: &mut Context| -> i32 {
+///    args.iter().sum()
+/// }).into_js_function_copied(&mut context);
+///
+/// let result = sums.call(
+///     &JsValue::undefined(),
+///     &[JsValue::from(1), JsValue::from(2), JsValue::from(3), JsValue::Boolean(true), JsValue::from(4)],
+///     &mut context
+/// ).unwrap();
+/// assert_eq!(result, JsValue::new(6));
+/// ```
+#[derive(Debug, Clone)]
+pub struct JsAll<T: TryFromJs>(pub Vec<T>);
+
+impl<T: TryFromJs> JsAll<T> {
+    /// Consumes the `JsAll` and returns the inner list of `T`.
+    #[must_use]
+    pub fn into_inner(self) -> Vec<T> {
+        self.0
+    }
+
+    /// Returns an iterator over the inner list of `T`.
+    pub fn iter(&self) -> impl Iterator<Item = &T> {
+        self.0.iter()
+    }
+
+    /// Returns a mutable iterator over the inner list of `T`.
+    pub fn iter_mut(&mut self) -> impl Iterator<Item = &mut T> {
+        self.0.iter_mut()
+    }
+
+    /// Returns the length of the inner list of `T`.
+    #[must_use]
+    pub fn len(&self) -> usize {
+        self.0.len()
+    }
+
+    /// Returns `true` if the inner list of `T` is empty.
+    #[must_use]
+    pub fn is_empty(&self) -> bool {
+        self.0.is_empty()
+    }
+}
+
+impl<T: TryFromJs> TryFromJsArgument for JsAll<T> {
     fn try_from_js_argument<'a>(
-        _: &'a JsValue,
-        rest: &'a [JsValue],
-        _context: &mut Context,
+        _this: &'a JsValue,
+        mut rest: &'a [JsValue],
+        context: &mut Context,
     ) -> JsResult<(Self, &'a [JsValue])> {
-        Ok((JsRest(rest.to_vec()), &[]))
+        let mut values = Vec::new();
+
+        while !rest.is_empty() {
+            match rest[0].try_js_into(context) {
+                Ok(value) => {
+                    values.push(value);
+                    rest = &rest[1..];
+                }
+                Err(_) => break,
+            }
+        }
+        Ok((JsAll(values), rest))
     }
 }
 

From deda29b354578c72bc5ed27497a02c98599b25a2 Mon Sep 17 00:00:00 2001
From: Hans Larsen <hans@larsen.online>
Date: Mon, 8 Apr 2024 21:54:42 -0700
Subject: [PATCH 13/22] use from_copy_closure and remove unsafe

---
 core/interop/src/into_js_function_impls.rs | 72 ++++++++++------------
 1 file changed, 32 insertions(+), 40 deletions(-)

diff --git a/core/interop/src/into_js_function_impls.rs b/core/interop/src/into_js_function_impls.rs
index 93854adf6f4..d0d3093195f 100644
--- a/core/interop/src/into_js_function_impls.rs
+++ b/core/interop/src/into_js_function_impls.rs
@@ -148,16 +148,14 @@ macro_rules! impl_into_js_function {
             #[allow(unused_variables)]
             fn into_js_function_copied(self, _context: &mut Context) -> NativeFunction {
                 let s = self;
-                unsafe {
-                    NativeFunction::from_closure(move |this, args, ctx| {
-                        let rest = args;
-                        $(
-                            let ($id, rest) = $t::try_from_js_argument(this, rest, ctx)?;
-                        )*
-                        let r = s( $($id,)* );
-                        r.try_into_js_result(ctx)
-                    })
-                }
+                NativeFunction::from_copy_closure(move |this, args, ctx| {
+                    let rest = args;
+                    $(
+                        let ($id, rest) = $t::try_from_js_argument(this, rest, ctx)?;
+                    )*
+                    let r = s( $($id,)* );
+                    r.try_into_js_result(ctx)
+                })
             }
         }
 
@@ -170,16 +168,14 @@ macro_rules! impl_into_js_function {
             #[allow(unused_variables)]
             fn into_js_function_copied(self, _context: &mut Context) -> NativeFunction {
                 let s = self;
-                unsafe {
-                    NativeFunction::from_closure(move |this, args, ctx| {
-                        let rest = args;
-                        $(
-                            let ($id, rest) = $t::try_from_js_argument(this, rest, ctx)?;
-                        )*
-                        let r = s( $($id,)* rest.into() );
-                        r.try_into_js_result(ctx)
-                    })
-                }
+                NativeFunction::from_copy_closure(move |this, args, ctx| {
+                    let rest = args;
+                    $(
+                        let ($id, rest) = $t::try_from_js_argument(this, rest, ctx)?;
+                    )*
+                    let r = s( $($id,)* rest.into() );
+                    r.try_into_js_result(ctx)
+                })
             }
         }
 
@@ -192,16 +188,14 @@ macro_rules! impl_into_js_function {
             #[allow(unused_variables)]
             fn into_js_function_copied(self, _context: &mut Context) -> NativeFunction {
                 let s = self;
-                unsafe {
-                    NativeFunction::from_closure(move |this, args, ctx| {
-                        let rest = args;
-                        $(
-                            let ($id, rest) = $t::try_from_js_argument(this, rest, ctx)?;
-                        )*
-                        let r = s( $($id,)* ctx);
-                        r.try_into_js_result(ctx)
-                    })
-                }
+                NativeFunction::from_copy_closure(move |this, args, ctx| {
+                    let rest = args;
+                    $(
+                        let ($id, rest) = $t::try_from_js_argument(this, rest, ctx)?;
+                    )*
+                    let r = s( $($id,)* ctx);
+                    r.try_into_js_result(ctx)
+                })
             }
         }
 
@@ -214,16 +208,14 @@ macro_rules! impl_into_js_function {
             #[allow(unused_variables)]
             fn into_js_function_copied(self, _context: &mut Context) -> NativeFunction {
                 let s = self;
-                unsafe {
-                    NativeFunction::from_closure(move |this, args, ctx| {
-                        let rest = args;
-                        $(
-                            let ($id, rest) = $t::try_from_js_argument(this, rest, ctx)?;
-                        )*
-                        let r = s( $($id,)* rest.into(), ctx);
-                        r.try_into_js_result(ctx)
-                    })
-                }
+                NativeFunction::from_copy_closure(move |this, args, ctx| {
+                    let rest = args;
+                    $(
+                        let ($id, rest) = $t::try_from_js_argument(this, rest, ctx)?;
+                    )*
+                    let r = s( $($id,)* rest.into(), ctx);
+                    r.try_into_js_result(ctx)
+                })
             }
         }
     };

From f6dd8a3fe365530d21ff4e4b8e3f741231d7cdc2 Mon Sep 17 00:00:00 2001
From: Hans Larsen <hans@larsen.online>
Date: Tue, 9 Apr 2024 12:20:45 -0700
Subject: [PATCH 14/22] Add a HostDefined struct to grab host defined in the
 argument list

---
 core/interop/src/lib.rs | 44 ++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 43 insertions(+), 1 deletion(-)

diff --git a/core/interop/src/lib.rs b/core/interop/src/lib.rs
index 1ff4eca8da5..9cddca53359 100644
--- a/core/interop/src/lib.rs
+++ b/core/interop/src/lib.rs
@@ -1,8 +1,10 @@
 //! Interop utilities between Boa and its host.
 
+use boa_engine::{
+    Context, js_string, JsError, JsResult, JsString, JsValue, Module, NativeFunction, NativeObject,
+};
 use boa_engine::module::SyntheticModuleInitializer;
 use boa_engine::value::TryFromJs;
-use boa_engine::{Context, JsResult, JsString, JsValue, Module, NativeFunction};
 
 pub mod loaders;
 
@@ -301,6 +303,46 @@ impl<T: TryFromJs> TryFromJsArgument for JsThis<T> {
     }
 }
 
+/// Captures the host_defined value as a JS function argument, based on
+/// its type. This will clone the HostDefined value.
+///
+/// For example,
+/// ```
+/// # use boa_engine::{Context, Finalize, JsData, JsValue, Trace};
+/// use boa_interop::{IntoJsFunctionCopied, HostDefined};
+///
+/// #[derive(Clone, Debug, Finalize, JsData, Trace)]
+/// struct CustomHostDefinedStruct {
+///    #[unsafe_ignore_trace]
+///    pub counter: usize,
+/// }
+/// let mut context = Context::default();
+/// context.realm().host_defined_mut().insert(CustomHostDefinedStruct { counter: 123 });
+/// let f = (|HostDefined(host): HostDefined<CustomHostDefinedStruct>| {
+///   host.counter + 1
+/// }).into_js_function_copied(&mut context);
+///
+/// assert_eq!(f.call(&JsValue::undefined(), &[], &mut context), Ok(JsValue::new(124)));
+///
+/// ```
+#[derive(Debug, Clone)]
+pub struct HostDefined<T>(pub T);
+
+impl<T: NativeObject + Clone> TryFromJsArgument for HostDefined<T> {
+    fn try_from_js_argument<'a>(
+        _this: &'a JsValue,
+        rest: &'a [JsValue],
+        context: &mut Context,
+    ) -> JsResult<(Self, &'a [JsValue])> {
+        match context.realm().host_defined().get::<T>() {
+            Some(value) => Ok((HostDefined(value.clone()), rest)),
+            None => Err(JsError::from_opaque(
+                js_string!("Host defined value not found").into(),
+            )),
+        }
+    }
+}
+
 // Implement `IntoJsFunction` for functions with a various list of
 // arguments.
 mod into_js_function_impls;

From fc0c7dcaa16ac29a83fb9b0ab4ebda7d191546cc Mon Sep 17 00:00:00 2001
From: Hans Larsen <hans@larsen.online>
Date: Tue, 9 Apr 2024 12:24:40 -0700
Subject: [PATCH 15/22] cargo fmt and clippy

---
 core/interop/src/lib.rs | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/core/interop/src/lib.rs b/core/interop/src/lib.rs
index 9cddca53359..6162e0fd468 100644
--- a/core/interop/src/lib.rs
+++ b/core/interop/src/lib.rs
@@ -1,10 +1,10 @@
 //! Interop utilities between Boa and its host.
 
-use boa_engine::{
-    Context, js_string, JsError, JsResult, JsString, JsValue, Module, NativeFunction, NativeObject,
-};
 use boa_engine::module::SyntheticModuleInitializer;
 use boa_engine::value::TryFromJs;
+use boa_engine::{
+    js_string, Context, JsError, JsResult, JsString, JsValue, Module, NativeFunction, NativeObject,
+};
 
 pub mod loaders;
 
@@ -303,8 +303,8 @@ impl<T: TryFromJs> TryFromJsArgument for JsThis<T> {
     }
 }
 
-/// Captures the host_defined value as a JS function argument, based on
-/// its type. This will clone the HostDefined value.
+/// Captures the `host_defined` value as a JS function argument, based on
+/// its type. This will clone the `HostDefined` value.
 ///
 /// For example,
 /// ```

From cbaee86597b7a7f08e65e3f4310109fefbd87721 Mon Sep 17 00:00:00 2001
From: Hans Larsen <hans@larsen.online>
Date: Tue, 9 Apr 2024 12:38:37 -0700
Subject: [PATCH 16/22] Explain why we need Clone

---
 core/interop/src/lib.rs | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/core/interop/src/lib.rs b/core/interop/src/lib.rs
index 6162e0fd468..22241107b99 100644
--- a/core/interop/src/lib.rs
+++ b/core/interop/src/lib.rs
@@ -306,6 +306,9 @@ impl<T: TryFromJs> TryFromJsArgument for JsThis<T> {
 /// Captures the `host_defined` value as a JS function argument, based on
 /// its type. This will clone the `HostDefined` value.
 ///
+/// The host defined type must implement [`Clone`], otherwise the borrow
+/// checker would not be able to ensure the safety of the operation.
+///
 /// For example,
 /// ```
 /// # use boa_engine::{Context, Finalize, JsData, JsValue, Trace};

From 644937bb4edd5905f8c667bd34d08bac39120c7d Mon Sep 17 00:00:00 2001
From: Hans Larsen <hans@larsen.online>
Date: Tue, 9 Apr 2024 12:48:43 -0700
Subject: [PATCH 17/22] Remove the vector from JsRest<> and use a reference

---
 core/interop/src/into_js_function_impls.rs | 48 ++++++++++----------
 core/interop/src/lib.rs                    | 53 +++++++++++-----------
 2 files changed, 51 insertions(+), 50 deletions(-)

diff --git a/core/interop/src/into_js_function_impls.rs b/core/interop/src/into_js_function_impls.rs
index d0d3093195f..772a9914f85 100644
--- a/core/interop/src/into_js_function_impls.rs
+++ b/core/interop/src/into_js_function_impls.rs
@@ -16,35 +16,35 @@ macro_rules! impl_into_js_function {
     ($($id: ident: $t: ident),*) => {
         impl<$($t,)* R, T> IntoJsFunctionSealed<($($t,)*), R> for T
         where
-            $($t: TryFromJsArgument + 'static,)*
+            $($t: for<'a> TryFromJsArgument<'a> + 'static,)*
             R: TryIntoJsResult,
             T: FnMut($($t,)*) -> R + 'static
         {}
 
         impl<$($t,)* R, T> IntoJsFunctionSealed<($($t,)* ContextArgToken,), R> for T
         where
-            $($t: TryFromJsArgument + 'static,)*
+            $($t: for<'a> TryFromJsArgument<'a> + 'static,)*
             R: TryIntoJsResult,
             T: FnMut($($t,)* &mut Context) -> R + 'static
         {}
 
-        impl<$($t,)* R, T> IntoJsFunctionSealed<($($t,)* JsRest,), R> for T
+        impl<$($t,)* R, T> IntoJsFunctionSealed<($($t,)* JsRest<'_>,), R> for T
         where
-            $($t: TryFromJsArgument + 'static,)*
+            $($t: for<'a> TryFromJsArgument<'a> + 'static,)*
             R: TryIntoJsResult,
-            T: FnMut($($t,)* JsRest) -> R + 'static
+            T: FnMut($($t,)* JsRest<'_>) -> R + 'static
         {}
 
-        impl<$($t,)* R, T> IntoJsFunctionSealed<($($t,)* JsRest, ContextArgToken), R> for T
+        impl<$($t,)* R, T> IntoJsFunctionSealed<($($t,)* JsRest<'_>, ContextArgToken), R> for T
         where
-            $($t: TryFromJsArgument + 'static,)*
+            $($t: for<'a> TryFromJsArgument<'a> + 'static,)*
             R: TryIntoJsResult,
-            T: FnMut($($t,)* JsRest, &mut Context) -> R + 'static
+            T: FnMut($($t,)* JsRest<'_>, &mut Context) -> R + 'static
         {}
 
         impl<$($t,)* R, T> UnsafeIntoJsFunction<($($t,)*), R> for T
         where
-            $($t: TryFromJsArgument + 'static,)*
+            $($t: for<'a> TryFromJsArgument<'a> + 'static,)*
             R: TryIntoJsResult,
             T: FnMut($($t,)*) -> R + 'static,
         {
@@ -68,11 +68,11 @@ macro_rules! impl_into_js_function {
             }
         }
 
-        impl<$($t,)* R, T> UnsafeIntoJsFunction<($($t,)* JsRest,), R> for T
+        impl<$($t,)* R, T> UnsafeIntoJsFunction<($($t,)* JsRest<'_>,), R> for T
         where
-            $($t: TryFromJsArgument + 'static,)*
+            $($t: for<'a> TryFromJsArgument<'a> + 'static,)*
             R: TryIntoJsResult,
-            T: FnMut($($t,)* JsRest) -> R + 'static,
+            T: FnMut($($t,)* JsRest<'_>) -> R + 'static,
         {
             #[allow(unused_variables)]
             unsafe fn into_js_function_unsafe(self, _context: &mut Context) -> NativeFunction {
@@ -96,7 +96,7 @@ macro_rules! impl_into_js_function {
 
         impl<$($t,)* R, T> UnsafeIntoJsFunction<($($t,)* ContextArgToken,), R> for T
         where
-            $($t: TryFromJsArgument + 'static,)*
+            $($t: for<'a> TryFromJsArgument<'a> + 'static,)*
             R: TryIntoJsResult,
             T: FnMut($($t,)* &mut Context) -> R + 'static,
         {
@@ -116,11 +116,11 @@ macro_rules! impl_into_js_function {
             }
         }
 
-        impl<$($t,)* R, T> UnsafeIntoJsFunction<($($t,)* JsRest, ContextArgToken), R> for T
+        impl<$($t,)* R, T> UnsafeIntoJsFunction<($($t,)* JsRest<'_>, ContextArgToken), R> for T
         where
-            $($t: TryFromJsArgument + 'static,)*
+            $($t: for<'a> TryFromJsArgument<'a> + 'static,)*
             R: TryIntoJsResult,
-            T: FnMut($($t,)* JsRest, &mut Context) -> R + 'static,
+            T: FnMut($($t,)* JsRest<'_>, &mut Context) -> R + 'static,
         {
             #[allow(unused_variables)]
             unsafe fn into_js_function_unsafe(self, _context: &mut Context) -> NativeFunction {
@@ -141,7 +141,7 @@ macro_rules! impl_into_js_function {
         // Safe versions for `Fn(..) -> ...`.
         impl<$($t,)* R, T> IntoJsFunctionCopied<($($t,)*), R> for T
         where
-            $($t: TryFromJsArgument + 'static,)*
+            $($t: for<'a> TryFromJsArgument<'a> + 'static,)*
             R: TryIntoJsResult,
             T: Fn($($t,)*) -> R + 'static + Copy,
         {
@@ -159,11 +159,11 @@ macro_rules! impl_into_js_function {
             }
         }
 
-        impl<$($t,)* R, T> IntoJsFunctionCopied<($($t,)* JsRest,), R> for T
+        impl<$($t,)* R, T> IntoJsFunctionCopied<($($t,)* JsRest<'_>,), R> for T
         where
-            $($t: TryFromJsArgument + 'static,)*
+            $($t: for<'a> TryFromJsArgument<'a> + 'static,)*
             R: TryIntoJsResult,
-            T: Fn($($t,)* JsRest) -> R + 'static + Copy,
+            T: Fn($($t,)* JsRest<'_>) -> R + 'static + Copy,
         {
             #[allow(unused_variables)]
             fn into_js_function_copied(self, _context: &mut Context) -> NativeFunction {
@@ -181,7 +181,7 @@ macro_rules! impl_into_js_function {
 
         impl<$($t,)* R, T> IntoJsFunctionCopied<($($t,)* ContextArgToken,), R> for T
         where
-            $($t: TryFromJsArgument + 'static,)*
+            $($t: for<'a> TryFromJsArgument<'a> + 'static,)*
             R: TryIntoJsResult,
             T: Fn($($t,)* &mut Context) -> R + 'static + Copy,
         {
@@ -199,11 +199,11 @@ macro_rules! impl_into_js_function {
             }
         }
 
-        impl<$($t,)* R, T> IntoJsFunctionCopied<($($t,)* JsRest, ContextArgToken), R> for T
+        impl<$($t,)* R, T> IntoJsFunctionCopied<($($t,)* JsRest<'_>, ContextArgToken), R> for T
         where
-            $($t: TryFromJsArgument + 'static,)*
+            $($t: for<'a> TryFromJsArgument<'a> + 'static,)*
             R: TryIntoJsResult,
-            T: Fn($($t,)* JsRest, &mut Context) -> R + 'static + Copy,
+            T: Fn($($t,)* JsRest<'_>, &mut Context) -> R + 'static + Copy,
         {
             #[allow(unused_variables)]
             fn into_js_function_copied(self, _context: &mut Context) -> NativeFunction {
diff --git a/core/interop/src/lib.rs b/core/interop/src/lib.rs
index 22241107b99..9da1bbe0f40 100644
--- a/core/interop/src/lib.rs
+++ b/core/interop/src/lib.rs
@@ -117,21 +117,21 @@ pub trait IntoJsFunctionCopied<Args, Ret>: private::IntoJsFunctionSealed<Args, R
 /// Create a Rust value from a JS argument. This trait is used to
 /// convert arguments from JS to Rust types. It allows support
 /// for optional arguments or rest arguments.
-pub trait TryFromJsArgument: Sized {
+pub trait TryFromJsArgument<'a>: Sized {
     /// Try to convert a JS argument into a Rust value, returning the
     /// value and the rest of the arguments to be parsed.
     ///
     /// # Errors
     /// Any parsing errors that may occur during the conversion.
-    fn try_from_js_argument<'a>(
+    fn try_from_js_argument(
         this: &'a JsValue,
         rest: &'a [JsValue],
         context: &mut Context,
     ) -> JsResult<(Self, &'a [JsValue])>;
 }
 
-impl<T: TryFromJs> TryFromJsArgument for T {
-    fn try_from_js_argument<'a>(
+impl<'a, T: TryFromJs> TryFromJsArgument<'a> for T {
+    fn try_from_js_argument(
         _: &'a JsValue,
         rest: &'a [JsValue],
         context: &mut Context,
@@ -165,26 +165,27 @@ impl<T: TryFromJs> TryFromJsArgument for T {
 /// assert_eq!(result, JsValue::new(6));
 /// ```
 #[derive(Debug, Clone)]
-pub struct JsRest(pub Vec<JsValue>);
+pub struct JsRest<'a>(pub &'a [JsValue]);
 
 #[allow(unused)]
-impl JsRest {
+impl<'a> JsRest<'a> {
     /// Consumes the `JsRest` and returns the inner list of `JsValue`.
     #[must_use]
-    pub fn into_inner(self) -> Vec<JsValue> {
+    pub fn into_inner(self) -> &'a [JsValue] {
         self.0
     }
 
+    /// Transforms the `JsRest` into a `Vec<JsValue>`.
+    #[must_use]
+    pub fn to_vec(self) -> Vec<JsValue> {
+        self.0.to_vec()
+    }
+
     /// Returns an iterator over the inner list of `JsValue`.
     pub fn iter(&self) -> impl Iterator<Item = &JsValue> {
         self.0.iter()
     }
 
-    /// Returns a mutable iterator over the inner list of `JsValue`.
-    pub fn iter_mut(&mut self) -> impl Iterator<Item = &mut JsValue> {
-        self.0.iter_mut()
-    }
-
     /// Returns the length of the inner list of `JsValue`.
     #[must_use]
     pub fn len(&self) -> usize {
@@ -198,18 +199,18 @@ impl JsRest {
     }
 }
 
-impl From<&[JsValue]> for JsRest {
-    fn from(values: &[JsValue]) -> Self {
-        Self(values.to_vec())
+impl<'a> From<&'a [JsValue]> for JsRest<'a> {
+    fn from(values: &'a [JsValue]) -> Self {
+        Self(values)
     }
 }
 
-impl IntoIterator for JsRest {
-    type Item = JsValue;
-    type IntoIter = std::vec::IntoIter<JsValue>;
+impl<'a> IntoIterator for JsRest<'a> {
+    type Item = &'a JsValue;
+    type IntoIter = std::slice::Iter<'a, JsValue>;
 
     fn into_iter(self) -> Self::IntoIter {
-        self.into_inner().into_iter()
+        self.into_inner().iter()
     }
 }
 
@@ -266,8 +267,8 @@ impl<T: TryFromJs> JsAll<T> {
     }
 }
 
-impl<T: TryFromJs> TryFromJsArgument for JsAll<T> {
-    fn try_from_js_argument<'a>(
+impl<'a, T: TryFromJs> TryFromJsArgument<'a> for JsAll<T> {
+    fn try_from_js_argument(
         _this: &'a JsValue,
         mut rest: &'a [JsValue],
         context: &mut Context,
@@ -293,8 +294,8 @@ impl<T: TryFromJs> TryFromJsArgument for JsAll<T> {
 #[derive(Debug, Clone)]
 pub struct JsThis<T: TryFromJs>(pub T);
 
-impl<T: TryFromJs> TryFromJsArgument for JsThis<T> {
-    fn try_from_js_argument<'a>(
+impl<'a, T: TryFromJs> TryFromJsArgument<'a> for JsThis<T> {
+    fn try_from_js_argument(
         this: &'a JsValue,
         rest: &'a [JsValue],
         context: &mut Context,
@@ -331,8 +332,8 @@ impl<T: TryFromJs> TryFromJsArgument for JsThis<T> {
 #[derive(Debug, Clone)]
 pub struct HostDefined<T>(pub T);
 
-impl<T: NativeObject + Clone> TryFromJsArgument for HostDefined<T> {
-    fn try_from_js_argument<'a>(
+impl<'a, T: NativeObject + Clone> TryFromJsArgument<'a> for HostDefined<T> {
+    fn try_from_js_argument(
         _this: &'a JsValue,
         rest: &'a [JsValue],
         context: &mut Context,
@@ -398,7 +399,7 @@ pub fn into_js_module() {
                 UnsafeIntoJsFunction::into_js_function_unsafe(
                     {
                         let counter = dad_count.clone();
-                        move |args: JsRest, context: &mut Context| {
+                        move |args: JsRest<'_>, context: &mut Context| {
                             *counter.borrow_mut() += args
                                 .into_iter()
                                 .map(|i| i.try_js_into::<i32>(context).unwrap())

From 2c2e1fb0607fe122a3718192ad94c3e7a32e214b Mon Sep 17 00:00:00 2001
From: Hans Larsen <hans@larsen.online>
Date: Wed, 10 Apr 2024 11:19:13 -0700
Subject: [PATCH 18/22] Add HostDefined to context as Data, and rename the
 injector in boa_interop

---
 core/engine/src/context/mod.rs | 31 +++++++++++++++++++++-
 core/interop/src/lib.rs        | 47 ++++++++++++++++++----------------
 2 files changed, 55 insertions(+), 23 deletions(-)

diff --git a/core/engine/src/context/mod.rs b/core/engine/src/context/mod.rs
index c69ebd5ec86..32df528b35d 100644
--- a/core/engine/src/context/mod.rs
+++ b/core/engine/src/context/mod.rs
@@ -25,7 +25,7 @@ use crate::{
     realm::Realm,
     script::Script,
     vm::{ActiveRunnable, CallFrame, Vm},
-    JsNativeError, JsResult, JsString, JsValue, Source,
+    HostDefined, JsNativeError, JsResult, JsString, JsValue, NativeObject, Source,
 };
 
 use self::intrinsics::StandardConstructor;
@@ -115,6 +115,8 @@ pub struct Context {
 
     /// Unique identifier for each parser instance used during the context lifetime.
     parser_identifier: u32,
+
+    data: HostDefined,
 }
 
 impl std::fmt::Debug for Context {
@@ -585,6 +587,32 @@ impl Context {
     pub fn can_block(&self) -> bool {
         self.can_block
     }
+
+    /// Insert a type into the context-specific [`HostDefined`] field.
+    #[inline]
+    pub fn insert_data<T: NativeObject>(&mut self, value: T) -> Option<Box<T>> {
+        self.data.insert(value)
+    }
+
+    /// Check if the context-specific [`HostDefined`] has type T.
+    #[inline]
+    #[must_use]
+    pub fn has_data<T: NativeObject>(&self) -> bool {
+        self.data.has::<T>()
+    }
+
+    /// Remove type T from the context-specific [`HostDefined`], if it exists.
+    #[inline]
+    pub fn remove_data<T: NativeObject>(&mut self) -> Option<Box<T>> {
+        self.data.remove::<T>()
+    }
+
+    /// Get type T from the context-specific [`HostDefined`], if it exists.
+    #[inline]
+    #[must_use]
+    pub fn get_data<T: NativeObject>(&self) -> Option<&T> {
+        self.data.get::<T>()
+    }
 }
 
 // ==== Private API ====
@@ -1070,6 +1098,7 @@ impl ContextBuilder {
             root_shape,
             parser_identifier: 0,
             can_block: self.can_block,
+            data: HostDefined::default(),
         };
 
         builtins::set_default_global_bindings(&mut context)?;
diff --git a/core/interop/src/lib.rs b/core/interop/src/lib.rs
index 9da1bbe0f40..cd7e1a48347 100644
--- a/core/interop/src/lib.rs
+++ b/core/interop/src/lib.rs
@@ -304,16 +304,18 @@ impl<'a, T: TryFromJs> TryFromJsArgument<'a> for JsThis<T> {
     }
 }
 
-/// Captures the `host_defined` value as a JS function argument, based on
-/// its type. This will clone the `HostDefined` value.
+/// Captures a [`ContextData`] data from the [`Context`] as a JS function argument,
+/// based on its type.
 ///
 /// The host defined type must implement [`Clone`], otherwise the borrow
-/// checker would not be able to ensure the safety of the operation.
+/// checker would not be able to ensure the safety of the context while
+/// making the function call. Because of this, it is recommended to use
+/// types that are cheap to clone.
 ///
 /// For example,
 /// ```
 /// # use boa_engine::{Context, Finalize, JsData, JsValue, Trace};
-/// use boa_interop::{IntoJsFunctionCopied, HostDefined};
+/// use boa_interop::{IntoJsFunctionCopied, ContextData};
 ///
 /// #[derive(Clone, Debug, Finalize, JsData, Trace)]
 /// struct CustomHostDefinedStruct {
@@ -321,25 +323,24 @@ impl<'a, T: TryFromJs> TryFromJsArgument<'a> for JsThis<T> {
 ///    pub counter: usize,
 /// }
 /// let mut context = Context::default();
-/// context.realm().host_defined_mut().insert(CustomHostDefinedStruct { counter: 123 });
-/// let f = (|HostDefined(host): HostDefined<CustomHostDefinedStruct>| {
+/// context.insert_data(CustomHostDefinedStruct { counter: 123 });
+/// let f = (|ContextData(host): ContextData<CustomHostDefinedStruct>| {
 ///   host.counter + 1
 /// }).into_js_function_copied(&mut context);
 ///
 /// assert_eq!(f.call(&JsValue::undefined(), &[], &mut context), Ok(JsValue::new(124)));
-///
 /// ```
 #[derive(Debug, Clone)]
-pub struct HostDefined<T>(pub T);
+pub struct ContextData<T: Clone>(pub T);
 
-impl<'a, T: NativeObject + Clone> TryFromJsArgument<'a> for HostDefined<T> {
+impl<'a, T: NativeObject + Clone> TryFromJsArgument<'a> for ContextData<T> {
     fn try_from_js_argument(
         _this: &'a JsValue,
         rest: &'a [JsValue],
         context: &mut Context,
     ) -> JsResult<(Self, &'a [JsValue])> {
-        match context.realm().host_defined().get::<T>() {
-            Some(value) => Ok((HostDefined(value.clone()), rest)),
+        match context.get_data::<T>() {
+            Some(value) => Ok((ContextData(value.clone()), rest)),
             None => Err(JsError::from_opaque(
                 js_string!("Host defined value not found").into(),
             )),
@@ -355,9 +356,12 @@ mod into_js_function_impls;
 #[allow(clippy::missing_panics_doc)]
 pub fn into_js_module() {
     use boa_engine::{js_string, JsValue, Source};
+    use boa_gc::{Gc, GcRefCell};
     use std::cell::RefCell;
     use std::rc::Rc;
 
+    type ResultType = Gc<GcRefCell<JsValue>>;
+
     let loader = Rc::new(loaders::HashMapModuleLoader::new());
     let mut context = Context::builder()
         .module_loader(loader.clone())
@@ -367,7 +371,9 @@ pub fn into_js_module() {
     let foo_count = Rc::new(RefCell::new(0));
     let bar_count = Rc::new(RefCell::new(0));
     let dad_count = Rc::new(RefCell::new(0));
-    let result = Rc::new(RefCell::new(JsValue::undefined()));
+
+    context.insert_data(Gc::new(GcRefCell::new(JsValue::undefined())));
+
     let module = unsafe {
         vec![
             (
@@ -411,15 +417,10 @@ pub fn into_js_module() {
             ),
             (
                 js_string!("send"),
-                UnsafeIntoJsFunction::into_js_function_unsafe(
-                    {
-                        let result = result.clone();
-                        move |value: JsValue| {
-                            *result.borrow_mut() = value;
-                        }
-                    },
-                    &mut context,
-                ),
+                (move |value: JsValue, ContextData(result): ContextData<ResultType>| {
+                    *result.borrow_mut() = value;
+                })
+                .into_js_function_copied(&mut context),
             ),
         ]
     }
@@ -454,10 +455,12 @@ pub fn into_js_module() {
         promise_result.state()
     );
 
+    let result = context.get_data::<ResultType>().unwrap().borrow().clone();
+
     assert_eq!(*foo_count.borrow(), 2);
     assert_eq!(*bar_count.borrow(), 15);
     assert_eq!(*dad_count.borrow(), 24);
-    assert_eq!(result.borrow().clone().try_js_into(&mut context), Ok(1u32));
+    assert_eq!(result.try_js_into(&mut context), Ok(1u32));
 }
 
 #[test]

From fffb5266c078977f84fe1b3491aabc4569be8ee1 Mon Sep 17 00:00:00 2001
From: Hans Larsen <hans@larsen.online>
Date: Thu, 11 Apr 2024 08:18:39 -0700
Subject: [PATCH 19/22] Use TypeError instead if the type was not found in
 context

---
 core/interop/src/lib.rs | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/core/interop/src/lib.rs b/core/interop/src/lib.rs
index cd7e1a48347..9e3e0d79f19 100644
--- a/core/interop/src/lib.rs
+++ b/core/interop/src/lib.rs
@@ -3,7 +3,8 @@
 use boa_engine::module::SyntheticModuleInitializer;
 use boa_engine::value::TryFromJs;
 use boa_engine::{
-    js_string, Context, JsError, JsResult, JsString, JsValue, Module, NativeFunction, NativeObject,
+    js_string, Context, JsError, JsNativeError, JsResult, JsString, JsValue, Module,
+    NativeFunction, NativeObject,
 };
 
 pub mod loaders;
@@ -341,8 +342,8 @@ impl<'a, T: NativeObject + Clone> TryFromJsArgument<'a> for ContextData<T> {
     ) -> JsResult<(Self, &'a [JsValue])> {
         match context.get_data::<T>() {
             Some(value) => Ok((ContextData(value.clone()), rest)),
-            None => Err(JsError::from_opaque(
-                js_string!("Host defined value not found").into(),
+            None => Err(JsError::from_native(
+                JsNativeError::typ().with_cause("Context data not found"),
             )),
         }
     }

From 05f178190c938186e16a6909b71007b03bbfa2ab Mon Sep 17 00:00:00 2001
From: Hans Larsen <hans@larsen.online>
Date: Thu, 11 Apr 2024 13:34:05 -0700
Subject: [PATCH 20/22] Update core/interop/src/lib.rs
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

Co-authored-by: José Julián Espina <jedel0124@gmail.com>
---
 core/interop/src/lib.rs | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/core/interop/src/lib.rs b/core/interop/src/lib.rs
index 9e3e0d79f19..715392aae71 100644
--- a/core/interop/src/lib.rs
+++ b/core/interop/src/lib.rs
@@ -342,9 +342,10 @@ impl<'a, T: NativeObject + Clone> TryFromJsArgument<'a> for ContextData<T> {
     ) -> JsResult<(Self, &'a [JsValue])> {
         match context.get_data::<T>() {
             Some(value) => Ok((ContextData(value.clone()), rest)),
-            None => Err(JsError::from_native(
-                JsNativeError::typ().with_cause("Context data not found"),
-            )),
+            None => Err(JsNativeError::typ()
+                .with_message("Context data not found")
+                .into(),
+            ),
         }
     }
 }

From 73e71f2f0bbab3fe889b59af7254159ea279f831 Mon Sep 17 00:00:00 2001
From: Hans Larsen <hans@larsen.online>
Date: Thu, 11 Apr 2024 13:36:11 -0700
Subject: [PATCH 21/22] cargo fmt

---
 core/interop/src/lib.rs | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/core/interop/src/lib.rs b/core/interop/src/lib.rs
index 715392aae71..6f9083f23ad 100644
--- a/core/interop/src/lib.rs
+++ b/core/interop/src/lib.rs
@@ -3,7 +3,7 @@
 use boa_engine::module::SyntheticModuleInitializer;
 use boa_engine::value::TryFromJs;
 use boa_engine::{
-    js_string, Context, JsError, JsNativeError, JsResult, JsString, JsValue, Module,
+    Context, JsNativeError, JsResult, JsString, JsValue, Module,
     NativeFunction, NativeObject,
 };
 
@@ -344,8 +344,7 @@ impl<'a, T: NativeObject + Clone> TryFromJsArgument<'a> for ContextData<T> {
             Some(value) => Ok((ContextData(value.clone()), rest)),
             None => Err(JsNativeError::typ()
                 .with_message("Context data not found")
-                .into(),
-            ),
+                .into()),
         }
     }
 }

From 911052e57d80c86c11893d5d87b5039df2eda10d Mon Sep 17 00:00:00 2001
From: Hans Larsen <hans@larsen.online>
Date: Thu, 11 Apr 2024 13:39:01 -0700
Subject: [PATCH 22/22] cargo fmt

---
 core/interop/src/lib.rs | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/core/interop/src/lib.rs b/core/interop/src/lib.rs
index 6f9083f23ad..b03d8adc213 100644
--- a/core/interop/src/lib.rs
+++ b/core/interop/src/lib.rs
@@ -3,8 +3,7 @@
 use boa_engine::module::SyntheticModuleInitializer;
 use boa_engine::value::TryFromJs;
 use boa_engine::{
-    Context, JsNativeError, JsResult, JsString, JsValue, Module,
-    NativeFunction, NativeObject,
+    Context, JsNativeError, JsResult, JsString, JsValue, Module, NativeFunction, NativeObject,
 };
 
 pub mod loaders;