Add a JsCast trait specified in [RFC 2]

[RFC 2]: https://github.com/rustwasm/rfcs/pull/2

src/cast.rs

@@ -0,0 +1,165 @@
+use JsValue;
+
+/// A trait for checked and unchecked casting between JS types.
+///
+/// Specified [in an RFC][rfc] this trait is intended to provide support for
+/// casting JS values between differnet types of one another. In JS there aren't
+/// many static types but we've ascribed JS values with static types in Rust,
+/// yet they often need to be switched to other types temporarily! This trait
+/// provides both checked and unchecked casting into various kinds of values.
+///
+/// This trait is automatically implemented for any type imported in a
+/// `#[wasm_bindgen]` `extern` block.
+///
+/// [rfc]: https://github.com/rustwasm/rfcs/pull/2
+pub trait JsCast
+where
+    Self: AsRef<JsValue> + AsMut<JsValue> + Into<JsValue>,
+{
+    /// Test whether this JS value is an instance of the type `T`.
+    ///
+    /// This commit performs a dynamic check (at runtime) using the JS
+    /// `instanceof` operator. This method returns `self instanceof T`.
+    fn is_instance_of<T>(&self) -> bool
+    where
+        T: JsCast,
+    {
+        T::instanceof(self.as_ref())
+    }
+
+    /// Performs a dynamic cast (checked at runtime) of this value into the
+    /// target type `T`.
+    ///
+    /// This method will return `Err(self)` is `self.is_instance_of::<T>()`
+    /// returns `false`, and otherwise it will return `Ok(T)` manufactured with
+    /// an unchecked cast (verified safe via the `instanceof` operation).
+    fn dyn_into<T>(self) -> Result<T, Self>
+    where
+        T: JsCast,
+    {
+        if self.is_instance_of::<T>() {
+            Ok(self.unchecked_into())
+        } else {
+            Err(self)
+        }
+    }
+
+    /// Performs a dynamic cast (checked at runtime) of this value into the
+    /// target type `T`.
+    ///
+    /// This method will return `None` is `self.is_instance_of::<T>()`
+    /// returns `false`, and otherwise it will return `Some(&T)` manufactured
+    /// with an unchecked cast (verified safe via the `instanceof` operation).
+    fn dyn_ref<T>(&self) -> Option<&T>
+    where
+        T: JsCast,
+    {
+        if self.is_instance_of::<T>() {
+            Some(self.unchecked_ref())
+        } else {
+            None
+        }
+    }
+
+    /// Performs a dynamic cast (checked at runtime) of this value into the
+    /// target type `T`.
+    ///
+    /// This method will return `None` is `self.is_instance_of::<T>()`
+    /// returns `false`, and otherwise it will return `Some(&mut T)`
+    /// manufactured with an unchecked cast (verified safe via the `instanceof`
+    /// operation).
+    fn dyn_mut<T>(&mut self) -> Option<&mut T>
+    where
+        T: JsCast,
+    {
+        if self.is_instance_of::<T>() {
+            Some(self.unchecked_mut())
+        } else {
+            None
+        }
+    }
+
+    /// Performs a zero-cost unchecked cast into the specified type.
+    ///
+    /// This method will convert the `self` value to the type `T`, where both
+    /// `self` and `T` are simple wrappers around `JsValue`. This method **does
+    /// not check whether `self` is an instance of `T`**. If used incorrectly
+    /// then this method may cause runtime exceptions in both Rust and JS, this
+    /// shoudl be used with caution.
+    fn unchecked_into<T>(self) -> T
+    where
+        T: JsCast,
+    {
+        T::unchecked_from_js(self.into())
+    }
+
+    /// Performs a zero-cost unchecked cast into a reference to the specified
+    /// type.
+    ///
+    /// This method will convert the `self` value to the type `T`, where both
+    /// `self` and `T` are simple wrappers around `JsValue`. This method **does
+    /// not check whether `self` is an instance of `T`**. If used incorrectly
+    /// then this method may cause runtime exceptions in both Rust and JS, this
+    /// shoudl be used with caution.
+    ///
+    /// This method, unlike `unchecked_into`, does not consume ownership of
+    /// `self` and instead works over a shared reference.
+    fn unchecked_ref<T>(&self) -> &T
+    where
+        T: JsCast,
+    {
+        T::unchecked_from_js_ref(self.as_ref())
+    }
+
+    /// Performs a zero-cost unchecked cast into a mutable reference to the
+    /// specified type.
+    ///
+    /// This method will convert the `self` value to the type `T`, where both
+    /// `self` and `T` are simple wrappers around `JsValue`. This method **does
+    /// not check whether `self` is an instance of `T`**. If used incorrectly
+    /// then this method may cause runtime exceptions in both Rust and JS, this
+    /// shoudl be used with caution.
+    ///
+    /// This method, unlike `unchecked_into`, does not consume ownership of
+    /// `self` and instead works over a utable reference.
+    fn unchecked_mut<T>(&mut self) -> &mut T
+    where
+        T: JsCast,
+    {
+        T::unchecked_from_js_mut(self.as_mut())
+    }
+
+    /// Performs a dynamic `instanceof` check to see whether the `JsValue`
+    /// provided is an instance of this type.
+    ///
+    /// This is intended to be an internal implementation detail, you likely
+    /// won't need to call this.
+    fn instanceof(val: &JsValue) -> bool;
+
+    /// Performs a zero-cost unchecked conversion from a `JsValue` into an
+    /// instance of `Self`
+    ///
+    /// This is intended to be an internal implementation detail, you likely
+    /// won't need to call this.
+    fn unchecked_from_js(val: JsValue) -> Self;
+
+    /// Performs a zero-cost unchecked conversion from a `&JsValue` into an
+    /// instance of `&Self`.
+    ///
+    /// Note the safety of this method, which basically means that `Self` must
+    /// be a newtype wrapper around `JsValue`.
+    ///
+    /// This is intended to be an internal implementation detail, you likely
+    /// won't need to call this.
+    fn unchecked_from_js_ref(val: &JsValue) -> &Self;
+
+    /// Performs a zero-cost unchecked conversion from a `&mut JsValue` into an
+    /// instance of `&mut Self`.
+    ///
+    /// Note the safety of this method, which basically means that `Self` must
+    /// be a newtype wrapper around `JsValue`.
+    ///
+    /// This is intended to be an internal implementation detail, you likely
+    /// won't need to call this.
+    fn unchecked_from_js_mut(val: &mut JsValue) -> &mut Self;
+}

src/lib.rs

@@ -46,6 +46,9 @@ pub mod prelude {
 pub mod convert;
 pub mod describe;
 
+mod cast;
+pub use cast::JsCast;
+
 if_std! {
     extern crate std;
     use std::prelude::v1::*;
@@ -347,6 +350,22 @@ impl From<bool> for JsValue {
     }
 }
 
+impl JsCast for JsValue {
+    // everything is a `JsValue`!
+    fn instanceof(_val: &JsValue) -> bool { true }
+    fn unchecked_from_js(val: JsValue) -> Self { val }
+    fn unchecked_from_js_ref(val: &JsValue) -> &Self { val }
+    fn unchecked_from_js_mut(val: &mut JsValue) -> &mut Self { val }
+}
+
+impl AsMut<JsValue> for JsValue {
+    fn as_mut(&mut self) -> &mut JsValue { self }
+}
+
+impl AsRef<JsValue> for JsValue {
+    fn as_ref(&self) -> &JsValue { self }
+}
+
 macro_rules! numbers {
     ($($n:ident)*) => ($(
         impl PartialEq<$n> for JsValue {