script/dom/bindings/
root.rs

1/* This Source Code Form is subject to the terms of the Mozilla Public
2 * License, v. 2.0. If a copy of the MPL was not distributed with this
3 * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
4
5//! Smart pointers for the JS-managed DOM objects.
6//!
7//! The DOM is made up of DOM objects whose lifetime is entirely controlled by
8//! the whims of the SpiderMonkey garbage collector. The types in this module
9//! are designed to ensure that any interactions with said Rust types only
10//! occur on values that will remain alive the entire time.
11//!
12//! Here is a brief overview of the important types:
13//!
14//! - `Root<T>`: a stack-based rooted value.
15//! - `DomRoot<T>`: a stack-based reference to a rooted DOM object.
16//! - `Dom<T>`: a reference to a DOM object that can automatically be traced by
17//!   the GC when encountered as a field of a Rust structure.
18//!
19//! `Dom<T>` does not allow access to their inner value without explicitly
20//! creating a stack-based root via the `root` method. This returns a `DomRoot<T>`,
21//! which causes the JS-owned value to be uncollectable for the duration of the
22//! `Root` object's lifetime. A reference to the object can then be obtained
23//! from the `Root` object. These references are not allowed to outlive their
24//! originating `DomRoot<T>`.
25//!
26
27use std::cell::{OnceCell, UnsafeCell};
28use std::default::Default;
29use std::hash::{Hash, Hasher};
30use std::ops::Deref;
31use std::{mem, ptr};
32
33use js::context::NoGC;
34use js::jsapi::{Heap, JSObject, JSTracer, Value};
35use js::rust::HandleValue;
36use layout_api::TrustedNodeAddress;
37use malloc_size_of::{MallocSizeOf, MallocSizeOfOps};
38use script_bindings::assert::{assert_in_layout, assert_in_script};
39use script_bindings::reflector::DomObject;
40pub(crate) use script_bindings::root::*;
41
42use crate::dom::bindings::conversions::DerivedFrom;
43use crate::dom::bindings::inheritance::Castable;
44use crate::dom::bindings::trace::JSTraceable;
45use crate::dom::node::Node;
46
47pub(crate) trait ToLayout<T> {
48    /// Returns `LayoutDom<T>` containing the same pointer.
49    ///
50    /// # Safety
51    ///
52    /// The `self` parameter to this method must meet all the requirements of [`ptr::NonNull::as_ref`].
53    unsafe fn to_layout(&self) -> LayoutDom<'_, T>;
54}
55
56impl<T: DomObject> ToLayout<T> for Dom<T> {
57    unsafe fn to_layout(&self) -> LayoutDom<'_, T> {
58        assert_in_layout();
59        LayoutDom {
60            value: unsafe { self.as_ptr().as_ref().unwrap() },
61        }
62    }
63}
64
65/// An unrooted reference to a DOM object for use in layout. `Layout*Helpers`
66/// traits must be implemented on this.
67#[cfg_attr(crown, crown::unrooted_must_root_lint::allow_unrooted_interior)]
68#[repr(transparent)]
69pub(crate) struct LayoutDom<'dom, T> {
70    value: &'dom T,
71}
72
73impl<'dom, T> LayoutDom<'dom, T>
74where
75    T: Castable,
76{
77    /// Cast a DOM object root upwards to one of the interfaces it derives from.
78    pub(crate) fn upcast<U>(&self) -> LayoutDom<'dom, U>
79    where
80        U: Castable,
81        T: DerivedFrom<U>,
82    {
83        assert_in_layout();
84        LayoutDom {
85            value: self.value.upcast::<U>(),
86        }
87    }
88
89    /// Cast a DOM object downwards to one of the interfaces it might implement.
90    pub(crate) fn downcast<U>(&self) -> Option<LayoutDom<'dom, U>>
91    where
92        U: DerivedFrom<T>,
93    {
94        assert_in_layout();
95        self.value.downcast::<U>().map(|value| LayoutDom { value })
96    }
97
98    /// Returns whether this inner object is a U.
99    pub(crate) fn is<U>(&self) -> bool
100    where
101        U: DerivedFrom<T>,
102    {
103        assert_in_layout();
104        self.value.is::<U>()
105    }
106
107    /// Get a reference to the internal value.
108    ///
109    /// ## SAFETY
110    /// This function effectively circumvents all the safety provided by `LayoutDom` as it allows
111    /// performing arbitrary (potentially mutating) operations on the value. Use with caution!
112    pub(crate) unsafe fn as_ref(self) -> &'dom T {
113        self.value
114    }
115}
116
117impl<T> LayoutDom<'_, T>
118where
119    T: DomObject,
120{
121    /// Get the reflector.
122    pub(crate) unsafe fn get_jsobject(&self) -> *mut JSObject {
123        assert_in_layout();
124        self.value.reflector().get_jsobject().get()
125    }
126}
127
128impl<T> Copy for LayoutDom<'_, T> {}
129
130impl<T> PartialEq for LayoutDom<'_, T> {
131    fn eq(&self, other: &Self) -> bool {
132        std::ptr::eq(self.value, other.value)
133    }
134}
135
136impl<T> Eq for LayoutDom<'_, T> {}
137
138impl<T> Hash for LayoutDom<'_, T> {
139    fn hash<H: Hasher>(&self, state: &mut H) {
140        (self.value as *const T).hash(state)
141    }
142}
143
144#[expect(clippy::non_canonical_clone_impl)]
145impl<T> Clone for LayoutDom<'_, T> {
146    #[inline]
147    fn clone(&self) -> Self {
148        assert_in_layout();
149        *self
150    }
151}
152
153impl LayoutDom<'_, Node> {
154    /// Create a new JS-owned value wrapped from an address known to be a
155    /// `Node` pointer.
156    pub(crate) unsafe fn from_trusted_node_address(inner: TrustedNodeAddress) -> Self {
157        assert_in_layout();
158        let TrustedNodeAddress(addr) = inner;
159        LayoutDom {
160            value: unsafe { &*(addr as *const Node) },
161        }
162    }
163}
164
165/// A holder that provides interior mutability for GC-managed values such as
166/// `Dom<T>`.  Essentially a `Cell<Dom<T>>`, but safer.
167///
168/// This should only be used as a field in other DOM objects; see warning
169/// on `Dom<T>`.
170#[cfg_attr(crown, crown::unrooted_must_root_lint::must_root)]
171#[derive(JSTraceable)]
172pub(crate) struct MutDom<T: DomObject> {
173    val: UnsafeCell<Dom<T>>,
174}
175
176impl<T: DomObject> MutDom<T> {
177    /// Create a new `MutDom`.
178    pub(crate) fn new(initial: &T) -> MutDom<T> {
179        assert_in_script();
180        MutDom {
181            val: UnsafeCell::new(Dom::from_ref(initial)),
182        }
183    }
184
185    /// Set this `MutDom` to the given value.
186    pub(crate) fn set(&self, val: &T) {
187        assert_in_script();
188        unsafe {
189            *self.val.get() = Dom::from_ref(val);
190        }
191    }
192
193    /// Get the value in this `MutDom`.
194    pub(crate) fn get(&self) -> DomRoot<T> {
195        assert_in_script();
196        unsafe { DomRoot::from_ref(&*ptr::read(self.val.get())) }
197    }
198}
199
200impl<T: DomObject> MallocSizeOf for MutDom<T> {
201    fn size_of(&self, _ops: &mut MallocSizeOfOps) -> usize {
202        // See comment on MallocSizeOf for Dom<T>.
203        0
204    }
205}
206
207impl<T: DomObject> PartialEq for MutDom<T> {
208    fn eq(&self, other: &Self) -> bool {
209        unsafe { *self.val.get() == *other.val.get() }
210    }
211}
212
213impl<T: DomObject + PartialEq> PartialEq<T> for MutDom<T> {
214    fn eq(&self, other: &T) -> bool {
215        unsafe { **self.val.get() == *other }
216    }
217}
218
219/// A struct to make Unrooted Dom objects work. By taking a no_gc as reference, we ensure that the lifetime of this object
220/// is bounded by the lifetime of NoGC which enforces no gc happening.
221#[cfg_attr(crown, crown::unrooted_must_root_lint::allow_unrooted_interior)]
222pub(crate) struct UnrootedDom<'a, T: DomObject> {
223    inner: Dom<T>,
224    no_gc: &'a NoGC,
225}
226
227impl<'a, T: DomObject> UnrootedDom<'a, T> {
228    /// Construct an `UnrootedDom` with the lifetime of `NoGC`. This is safe, as `NoGC` implies no garbage collection will happen
229    #[cfg_attr(crown, expect(crown::unrooted_must_root))]
230    pub(crate) fn from_dom(object: Dom<T>, no_gc: &'a NoGC) -> UnrootedDom<'a, T> {
231        UnrootedDom {
232            inner: object,
233            no_gc,
234        }
235    }
236}
237
238impl<'a, T: DomObject> Deref for UnrootedDom<'a, T> {
239    type Target = Dom<T>;
240
241    fn deref(&self) -> &Self::Target {
242        &self.inner
243    }
244}
245
246/// Safety:
247/// We enforce the same lifetime as the given `UnrootedDom`, so the same
248/// guarantee about no GC happening in this lifetime.
249impl<'a, T: Castable> UnrootedDom<'a, T> {
250    /// Cast a DOM object root upwards to one of the interfaces it derives from.
251    pub fn upcast<U>(dom: UnrootedDom<'a, T>) -> UnrootedDom<'a, U>
252    where
253        U: Castable,
254        T: DerivedFrom<U>,
255    {
256        UnrootedDom {
257            inner: unsafe { mem::transmute::<Dom<T>, Dom<U>>(dom.inner) },
258            no_gc: dom.no_gc,
259        }
260    }
261
262    /// Cast a DOM object root downwards to one of the interfaces it might implement.
263    pub fn downcast<U>(dom: UnrootedDom<'a, T>) -> Option<UnrootedDom<'a, U>>
264    where
265        U: DerivedFrom<T>,
266    {
267        if dom.is::<U>() {
268            Some(UnrootedDom {
269                inner: unsafe { mem::transmute::<Dom<T>, Dom<U>>(dom.inner) },
270                no_gc: dom.no_gc,
271            })
272        } else {
273            None
274        }
275    }
276}
277
278impl<'a, T: DomObject> PartialEq<&T> for UnrootedDom<'a, T> {
279    fn eq(&self, other: &&T) -> bool {
280        self.inner == Dom::from_ref(*other)
281    }
282}
283
284/// A holder that provides interior mutability for GC-managed values such as
285/// `Dom<T>`, with nullability represented by an enclosing Option wrapper.
286/// Essentially a `Cell<Option<Dom<T>>>`, but safer.
287///
288/// This should only be used as a field in other DOM objects; see warning
289/// on `Dom<T>`.
290#[cfg_attr(crown, crown::unrooted_must_root_lint::must_root)]
291#[derive(JSTraceable)]
292pub(crate) struct MutNullableDom<T: DomObject> {
293    ptr: UnsafeCell<Option<Dom<T>>>,
294}
295
296impl<T: DomObject> MutNullableDom<T> {
297    /// Create a new `MutNullableDom`.
298    pub(crate) fn new(initial: Option<&T>) -> MutNullableDom<T> {
299        assert_in_script();
300        MutNullableDom {
301            ptr: UnsafeCell::new(initial.map(Dom::from_ref)),
302        }
303    }
304
305    /// Retrieve a copy of the current inner value. If it is `None`, it is
306    /// initialized with the result of `cb` first.
307    pub(crate) fn or_init<F>(&self, cb: F) -> DomRoot<T>
308    where
309        F: FnOnce() -> DomRoot<T>,
310    {
311        assert_in_script();
312        match self.get() {
313            Some(inner) => inner,
314            None => {
315                let inner = cb();
316                self.set(Some(&inner));
317                inner
318            },
319        }
320    }
321
322    /// Retrieve a copy of the inner optional `Dom<T>` as `LayoutDom<T>`.
323    /// For use by layout, which can't use safe types like Temporary.
324    pub(crate) unsafe fn get_inner_as_layout(&self) -> Option<LayoutDom<'_, T>> {
325        assert_in_layout();
326        unsafe { (*self.ptr.get()).as_ref().map(|js| js.to_layout()) }
327    }
328
329    /// Get a rooted value out of this object
330    pub(crate) fn get(&self) -> Option<DomRoot<T>> {
331        assert_in_script();
332        unsafe { ptr::read(self.ptr.get()).map(|o| DomRoot::from_ref(&*o)) }
333    }
334
335    /// Get the `DomObject` without rooting it. Constructing an UnrootedDom. This is safe
336    /// as we take a reference to NoGC and bound the lifetime by NoGC bound. This implies that
337    /// while the `UnrootedDom` is alive we do not have a GC run.
338    #[cfg_attr(crown, expect(crown::unrooted_must_root))]
339    pub(crate) fn get_unrooted<'a>(&self, no_gc: &'a NoGC) -> Option<UnrootedDom<'a, T>> {
340        assert_in_script();
341        let ptr = unsafe { ptr::read(self.ptr.get()) };
342        ptr.map(|o| Dom::from_ref(&*o))
343            .map(|dom| UnrootedDom { inner: dom, no_gc })
344    }
345
346    /// Set this `MutNullableDom` to the given value.
347    pub(crate) fn set(&self, val: Option<&T>) {
348        assert_in_script();
349        unsafe {
350            *self.ptr.get() = val.map(|p| Dom::from_ref(p));
351        }
352    }
353
354    /// Gets the current value out of this object and sets it to `None`.
355    pub(crate) fn take(&self) -> Option<DomRoot<T>> {
356        let value = self.get();
357        self.set(None);
358        value
359    }
360
361    /// Sets the current value of this [`MutNullableDom`] to `None`.
362    pub(crate) fn clear(&self) {
363        self.set(None)
364    }
365
366    /// Runs the given callback on the object if it's not null.
367    pub(crate) fn if_is_some<F, R>(&self, cb: F) -> Option<&R>
368    where
369        F: FnOnce(&T) -> &R,
370    {
371        unsafe {
372            if let Some(ref value) = *self.ptr.get() {
373                Some(cb(value))
374            } else {
375                None
376            }
377        }
378    }
379}
380
381impl<T: DomObject> PartialEq for MutNullableDom<T> {
382    fn eq(&self, other: &Self) -> bool {
383        unsafe { *self.ptr.get() == *other.ptr.get() }
384    }
385}
386
387impl<T: DomObject> PartialEq<Option<&T>> for MutNullableDom<T> {
388    fn eq(&self, other: &Option<&T>) -> bool {
389        unsafe { *self.ptr.get() == other.map(Dom::from_ref) }
390    }
391}
392
393impl<T: DomObject> Default for MutNullableDom<T> {
394    fn default() -> MutNullableDom<T> {
395        assert_in_script();
396        MutNullableDom {
397            ptr: UnsafeCell::new(None),
398        }
399    }
400}
401
402impl<T: DomObject> MallocSizeOf for MutNullableDom<T> {
403    fn size_of(&self, _ops: &mut MallocSizeOfOps) -> usize {
404        // See comment on MallocSizeOf for Dom<T>.
405        0
406    }
407}
408
409/// A holder that allows to lazily initialize the value only once
410/// `Dom<T>`, using OnceCell
411/// Essentially a `OnceCell<Dom<T>>`.
412///
413/// This should only be used as a field in other DOM objects; see warning
414/// on `Dom<T>`.
415#[cfg_attr(crown, crown::unrooted_must_root_lint::must_root)]
416pub(crate) struct DomOnceCell<T: DomObject> {
417    ptr: OnceCell<Dom<T>>,
418}
419
420impl<T> DomOnceCell<T>
421where
422    T: DomObject,
423{
424    /// Retrieve a copy of the current inner value. If it is `None`, it is
425    /// initialized with the result of `cb` first.
426    pub(crate) fn init_once<F>(&self, cb: F) -> &T
427    where
428        F: FnOnce() -> DomRoot<T>,
429    {
430        assert_in_script();
431        self.ptr.get_or_init(|| Dom::from_ref(&cb()))
432    }
433}
434
435impl<T: DomObject> Default for DomOnceCell<T> {
436    fn default() -> DomOnceCell<T> {
437        assert_in_script();
438        DomOnceCell {
439            ptr: OnceCell::new(),
440        }
441    }
442}
443
444impl<T: DomObject> MallocSizeOf for DomOnceCell<T> {
445    fn size_of(&self, _ops: &mut MallocSizeOfOps) -> usize {
446        // See comment on MallocSizeOf for Dom<T>.
447        0
448    }
449}
450
451unsafe impl<T: DomObject> JSTraceable for DomOnceCell<T> {
452    unsafe fn trace(&self, trc: *mut JSTracer) {
453        if let Some(ptr) = self.ptr.get() {
454            unsafe { ptr.trace(trc) };
455        }
456    }
457}
458
459impl<'dom, T> LayoutDom<'dom, T>
460where
461    T: 'dom + DomObject,
462{
463    /// Returns a reference to the interior of this JS object. The fact
464    /// that this is unsafe is what necessitates the layout wrappers.
465    pub(crate) fn unsafe_get(self) -> &'dom T {
466        assert_in_layout();
467        self.value
468    }
469
470    /// Transforms a slice of `Dom<T>` into a slice of `LayoutDom<T>`.
471    // FIXME(nox): This should probably be done through a ToLayout trait.
472    pub(crate) unsafe fn to_layout_slice(slice: &'dom [Dom<T>]) -> &'dom [LayoutDom<'dom, T>] {
473        // This doesn't compile if Dom and LayoutDom don't have the same
474        // representation.
475        let _ = mem::transmute::<Dom<T>, LayoutDom<T>>;
476        unsafe { &*(slice as *const [Dom<T>] as *const [LayoutDom<T>]) }
477    }
478}
479
480/// Converts a rooted `Heap<Value>` into a `HandleValue`.
481///
482/// This is only safe if the `Heap` is rooted (e.g., held inside a `Dom`-managed struct),
483/// and the `#[must_root]` crown lint is active to enforce rooting at compile time.
484/// Avoids repeating unsafe `from_raw` calls at each usage site.
485pub trait AsHandleValue<'a> {
486    fn as_handle_value(&'a self) -> HandleValue<'a>;
487}
488
489impl<'a> AsHandleValue<'a> for Heap<Value> {
490    #[cfg_attr(crown, crown::unrooted_must_root_lint::must_root)]
491    fn as_handle_value(&'a self) -> HandleValue<'a> {
492        // SAFETY: `self` is assumed to be rooted, and `handle()` ties
493        // the lifetime to `&self`, which the compiler can enforce.
494        unsafe { HandleValue::from_marked_location(self.ptr.get() as *const _) }
495    }
496}
script/dom/bindings/root.rs

script/dom/bindings/
root.rs
