Move more bindings code to script_bindings (#35578)

* Move JSContext wrapper to script_bindings.

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

* Move webidl constant bindings to script_bindings.

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

* Move CanGc to script_bindings.

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

* Move Dom<T> and Root<T> types to script_bindings.

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

* Formatting.

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

* Extra docs for new traits.

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

* Fix clippy warnings.

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

---------

Signed-off-by: Josh Matthews <josh@joshmatthews.net>

components/script_bindings/root.rs

@@ -0,0 +1,432 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
+
+use std::cell::{Cell, UnsafeCell};
+use std::hash::{Hash, Hasher};
+use std::ops::Deref;
+use std::{fmt, mem, ptr};
+
+use js::gc::Traceable as JSTraceable;
+use js::jsapi::{JSObject, JSTracer};
+use malloc_size_of::{MallocSizeOf, MallocSizeOfOps};
+use style::thread_state;
+
+use crate::conversions::DerivedFrom;
+use crate::inheritance::Castable;
+use crate::reflector::{DomObject, MutDomObject, Reflector};
+use crate::trace::trace_reflector;
+
+/// A rooted value.
+#[cfg_attr(crown, crown::unrooted_must_root_lint::allow_unrooted_interior)]
+pub struct Root<T: StableTraceObject> {
+    /// The value to root.
+    value: T,
+    /// List that ensures correct dynamic root ordering
+    root_list: *const RootCollection,
+}
+
+impl<T> Root<T>
+where
+    T: StableTraceObject + 'static,
+{
+    /// Create a new stack-bounded root for the provided value.
+    /// It gives out references which cannot outlive this new `Root`.
+    ///
+    /// # Safety
+    /// It must not outlive its associated `RootCollection`.
+    #[cfg_attr(crown, allow(crown::unrooted_must_root))]
+    pub unsafe fn new(value: T) -> Self {
+        unsafe fn add_to_root_list(object: *const dyn JSTraceable) -> *const RootCollection {
+            assert_in_script();
+            STACK_ROOTS.with(|root_list| {
+                let root_list = &*root_list.get().unwrap();
+                root_list.root(object);
+                root_list
+            })
+        }
+
+        let root_list = add_to_root_list(value.stable_trace_object());
+        Root { value, root_list }
+    }
+}
+
+/// `StableTraceObject` represents values that can be rooted through a stable address that will
+/// not change for their whole lifetime.
+/// It is an unsafe trait that requires implementors to ensure certain safety guarantees.
+///
+/// # Safety
+///
+/// Implementors of this trait must ensure that the `trace` method correctly accounts for all
+/// owned and referenced objects, so that the garbage collector can accurately determine which
+/// objects are still in use. Failing to adhere to this contract may result in undefined behavior,
+/// such as use-after-free errors.
+pub unsafe trait StableTraceObject {
+    /// Returns a stable trace object which address won't change for the whole
+    /// lifetime of the value.
+    fn stable_trace_object(&self) -> *const dyn JSTraceable;
+}
+
+unsafe impl<T> StableTraceObject for Dom<T>
+where
+    T: DomObject,
+{
+    fn stable_trace_object(&self) -> *const dyn JSTraceable {
+        // The JSTraceable impl for Reflector doesn't actually do anything,
+        // so we need this shenanigan to actually trace the reflector of the
+        // T pointer in Dom<T>.
+        #[cfg_attr(crown, allow(crown::unrooted_must_root))]
+        struct ReflectorStackRoot(Reflector);
+        unsafe impl JSTraceable for ReflectorStackRoot {
+            unsafe fn trace(&self, tracer: *mut JSTracer) {
+                trace_reflector(tracer, "on stack", &self.0);
+            }
+        }
+        unsafe { &*(self.reflector() as *const Reflector as *const ReflectorStackRoot) }
+    }
+}
+
+unsafe impl<T> StableTraceObject for MaybeUnreflectedDom<T>
+where
+    T: DomObject,
+{
+    fn stable_trace_object(&self) -> *const dyn JSTraceable {
+        // The JSTraceable impl for Reflector doesn't actually do anything,
+        // so we need this shenanigan to actually trace the reflector of the
+        // T pointer in Dom<T>.
+        #[cfg_attr(crown, allow(crown::unrooted_must_root))]
+        struct MaybeUnreflectedStackRoot<T>(T);
+        unsafe impl<T> JSTraceable for MaybeUnreflectedStackRoot<T>
+        where
+            T: DomObject,
+        {
+            unsafe fn trace(&self, tracer: *mut JSTracer) {
+                if self.0.reflector().get_jsobject().is_null() {
+                    self.0.trace(tracer);
+                } else {
+                    trace_reflector(tracer, "on stack", self.0.reflector());
+                }
+            }
+        }
+        unsafe { &*(self.ptr.as_ptr() as *const T as *const MaybeUnreflectedStackRoot<T>) }
+    }
+}
+
+impl<T> Deref for Root<T>
+where
+    T: Deref + StableTraceObject,
+{
+    type Target = <T as Deref>::Target;
+
+    fn deref(&self) -> &Self::Target {
+        assert_in_script();
+        &self.value
+    }
+}
+
+impl<T> Drop for Root<T>
+where
+    T: StableTraceObject,
+{
+    fn drop(&mut self) {
+        unsafe {
+            (*self.root_list).unroot(self.value.stable_trace_object());
+        }
+    }
+}
+
+impl<T: fmt::Debug + StableTraceObject> fmt::Debug for Root<T> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        self.value.fmt(f)
+    }
+}
+
+impl<T: fmt::Debug + DomObject> fmt::Debug for Dom<T> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        (**self).fmt(f)
+    }
+}
+
+/// A traced reference to a DOM object
+///
+/// This type is critical to making garbage collection work with the DOM,
+/// but it is very dangerous; if garbage collection happens with a `Dom<T>`
+/// on the stack, the `Dom<T>` can point to freed memory.
+///
+/// This should only be used as a field in other DOM objects.
+#[cfg_attr(crown, crown::unrooted_must_root_lint::must_root)]
+#[repr(transparent)]
+pub struct Dom<T> {
+    ptr: ptr::NonNull<T>,
+}
+
+// Dom<T> is similar to Rc<T>, in that it's not always clear how to avoid double-counting.
+// For now, we choose not to follow any such pointers.
+impl<T> MallocSizeOf for Dom<T> {
+    fn size_of(&self, _ops: &mut MallocSizeOfOps) -> usize {
+        0
+    }
+}
+
+impl<T> PartialEq for Dom<T> {
+    fn eq(&self, other: &Dom<T>) -> bool {
+        self.ptr.as_ptr() == other.ptr.as_ptr()
+    }
+}
+
+impl<'a, T: DomObject> PartialEq<&'a T> for Dom<T> {
+    fn eq(&self, other: &&'a T) -> bool {
+        *self == Dom::from_ref(*other)
+    }
+}
+
+impl<T> Eq for Dom<T> {}
+
+impl<T> Hash for Dom<T> {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        self.ptr.as_ptr().hash(state)
+    }
+}
+
+impl<T> Clone for Dom<T> {
+    #[inline]
+    #[cfg_attr(crown, allow(crown::unrooted_must_root))]
+    fn clone(&self) -> Self {
+        assert_in_script();
+        Dom { ptr: self.ptr }
+    }
+}
+
+impl<T: DomObject> Dom<T> {
+    /// Create a `Dom<T>` from a `&T`
+    #[cfg_attr(crown, allow(crown::unrooted_must_root))]
+    pub fn from_ref(obj: &T) -> Dom<T> {
+        assert_in_script();
+        Dom {
+            ptr: ptr::NonNull::from(obj),
+        }
+    }
+
+    /// Return a rooted version of this DOM object ([`DomRoot<T>`]) suitable for use on the stack.
+    pub fn as_rooted(&self) -> DomRoot<T> {
+        DomRoot::from_ref(self)
+    }
+
+    pub fn as_ptr(&self) -> *const T {
+        self.ptr.as_ptr()
+    }
+}
+
+impl<T: DomObject> Deref for Dom<T> {
+    type Target = T;
+
+    fn deref(&self) -> &T {
+        assert_in_script();
+        // We can only have &Dom<T> from a rooted thing, so it's safe to deref
+        // it to &T.
+        unsafe { &*self.ptr.as_ptr() }
+    }
+}
+
+unsafe impl<T: DomObject> JSTraceable for Dom<T> {
+    unsafe fn trace(&self, trc: *mut JSTracer) {
+        let trace_string;
+        let trace_info = if cfg!(debug_assertions) {
+            trace_string = format!("for {} on heap", ::std::any::type_name::<T>());
+            &trace_string[..]
+        } else {
+            "for DOM object on heap"
+        };
+        trace_reflector(trc, trace_info, (*self.ptr.as_ptr()).reflector());
+    }
+}
+
+/// A traced reference to a DOM object that may not be reflected yet.
+#[cfg_attr(crown, crown::unrooted_must_root_lint::must_root)]
+pub struct MaybeUnreflectedDom<T> {
+    ptr: ptr::NonNull<T>,
+}
+
+impl<T> MaybeUnreflectedDom<T>
+where
+    T: DomObject,
+{
+    /// Create a new MaybeUnreflectedDom value from the given boxed DOM object.
+    ///
+    /// # Safety
+    /// TODO: unclear why this is marked unsafe.
+    #[cfg_attr(crown, allow(crown::unrooted_must_root))]
+    pub unsafe fn from_box(value: Box<T>) -> Self {
+        Self {
+            ptr: Box::leak(value).into(),
+        }
+    }
+}
+
+impl<T> Root<MaybeUnreflectedDom<T>>
+where
+    T: DomObject,
+{
+    pub fn as_ptr(&self) -> *const T {
+        self.value.ptr.as_ptr()
+    }
+}
+
+impl<T> Root<MaybeUnreflectedDom<T>>
+where
+    T: MutDomObject,
+{
+    /// Treat the given JS object as the reflector of this unreflected object.
+    ///
+    /// # Safety
+    /// obj must point to a valid, non-null JS object.
+    pub unsafe fn reflect_with(self, obj: *mut JSObject) -> DomRoot<T> {
+        let ptr = self.as_ptr();
+        drop(self);
+        let root = DomRoot::from_ref(&*ptr);
+        root.init_reflector(obj);
+        root
+    }
+}
+
+/// A rooted reference to a DOM object.
+pub type DomRoot<T> = Root<Dom<T>>;
+
+impl<T: Castable> DomRoot<T> {
+    /// Cast a DOM object root upwards to one of the interfaces it derives from.
+    pub fn upcast<U>(root: DomRoot<T>) -> DomRoot<U>
+    where
+        U: Castable,
+        T: DerivedFrom<U>,
+    {
+        unsafe { mem::transmute::<DomRoot<T>, DomRoot<U>>(root) }
+    }
+
+    /// Cast a DOM object root downwards to one of the interfaces it might implement.
+    pub fn downcast<U>(root: DomRoot<T>) -> Option<DomRoot<U>>
+    where
+        U: DerivedFrom<T>,
+    {
+        if root.is::<U>() {
+            Some(unsafe { mem::transmute::<DomRoot<T>, DomRoot<U>>(root) })
+        } else {
+            None
+        }
+    }
+}
+
+impl<T: DomObject> DomRoot<T> {
+    /// Generate a new root from a reference
+    pub fn from_ref(unrooted: &T) -> DomRoot<T> {
+        unsafe { DomRoot::new(Dom::from_ref(unrooted)) }
+    }
+
+    /// Create a traced version of this rooted object.
+    ///
+    /// # Safety
+    ///
+    /// This should never be used to create on-stack values. Instead these values should always
+    /// end up as members of other DOM objects.
+    #[cfg_attr(crown, allow(crown::unrooted_must_root))]
+    pub fn as_traced(&self) -> Dom<T> {
+        Dom::from_ref(self)
+    }
+}
+
+impl<T> MallocSizeOf for DomRoot<T>
+where
+    T: DomObject + MallocSizeOf,
+{
+    fn size_of(&self, ops: &mut MallocSizeOfOps) -> usize {
+        (**self).size_of(ops)
+    }
+}
+
+impl<T> PartialEq for DomRoot<T>
+where
+    T: DomObject,
+{
+    fn eq(&self, other: &Self) -> bool {
+        self.value == other.value
+    }
+}
+
+impl<T> Clone for DomRoot<T>
+where
+    T: DomObject,
+{
+    fn clone(&self) -> DomRoot<T> {
+        DomRoot::from_ref(self)
+    }
+}
+
+unsafe impl<T> JSTraceable for DomRoot<T>
+where
+    T: DomObject,
+{
+    unsafe fn trace(&self, _: *mut JSTracer) {
+        // Already traced.
+    }
+}
+
+/// A rooting mechanism for reflectors on the stack.
+/// LIFO is not required.
+///
+/// See also [*Exact Stack Rooting - Storing a GCPointer on the CStack*][cstack].
+///
+/// [cstack]: https://developer.mozilla.org/en-US/docs/Mozilla/Projects/SpiderMonkey/Internals/GC/Exact_Stack_Rooting
+pub struct RootCollection {
+    roots: UnsafeCell<Vec<*const dyn JSTraceable>>,
+}
+
+impl RootCollection {
+    /// Create an empty collection of roots
+    #[allow(clippy::new_without_default)]
+    pub fn new() -> RootCollection {
+        assert_in_script();
+        RootCollection {
+            roots: UnsafeCell::new(vec![]),
+        }
+    }
+
+    /// Starts tracking a trace object.
+    unsafe fn root(&self, object: *const dyn JSTraceable) {
+        assert_in_script();
+        (*self.roots.get()).push(object);
+    }
+
+    /// Stops tracking a trace object, asserting if it isn't found.
+    unsafe fn unroot(&self, object: *const dyn JSTraceable) {
+        assert_in_script();
+        let roots = &mut *self.roots.get();
+        match roots
+            .iter()
+            .rposition(|r| std::ptr::addr_eq(*r as *const (), object as *const ()))
+        {
+            Some(idx) => {
+                roots.remove(idx);
+            },
+            None => panic!("Can't remove a root that was never rooted!"),
+        }
+    }
+}
+
+thread_local!(pub static STACK_ROOTS: Cell<Option<*const RootCollection>> = const { Cell::new(None) });
+
+/// SM Callback that traces the rooted reflectors
+///
+/// # Safety
+/// tracer must point to a valid, non-null JS tracer object.
+pub unsafe fn trace_roots(tracer: *mut JSTracer) {
+    trace!("tracing stack roots");
+    STACK_ROOTS.with(|collection| {
+        let collection = &*(*collection.get().unwrap()).roots.get();
+        for root in collection {
+            (**root).trace(tracer);
+        }
+    });
+}
+
+pub fn assert_in_script() {
+    debug_assert!(thread_state::get().is_script());
+}