script/dom/bindings/
cell.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//! A shareable mutable container for the DOM.
6
7use std::cell::{BorrowError, BorrowMutError};
8#[cfg(not(feature = "refcell_backtrace"))]
9pub(crate) use std::cell::{Ref, RefCell, RefMut};
10
11#[cfg(feature = "refcell_backtrace")]
12pub(crate) use accountable_refcell::{Ref, RefCell, RefMut};
13use malloc_size_of::{MallocConditionalSizeOf, MallocSizeOfOps};
14
15use crate::dom::bindings::root::{assert_in_layout, assert_in_script};
16
17/// A mutable field in the DOM.
18///
19/// This extends the API of `std::cell::RefCell` to allow unsafe access in
20/// certain situations, with dynamic checking in debug builds.
21#[derive(Clone, Debug, Default, MallocSizeOf, PartialEq)]
22pub(crate) struct DomRefCell<T> {
23    value: RefCell<T>,
24}
25
26impl<T: MallocConditionalSizeOf> MallocConditionalSizeOf for DomRefCell<T> {
27    fn conditional_size_of(&self, ops: &mut MallocSizeOfOps) -> usize {
28        self.value.borrow().conditional_size_of(ops)
29    }
30}
31
32// Functionality specific to Servo's `DomRefCell` type
33// ===================================================
34
35impl<T> DomRefCell<T> {
36    /// Return a reference to the contents.  For use in layout only.
37    ///
38    /// # Safety
39    ///
40    /// Unlike RefCell::borrow, this method is unsafe because it does not return a Ref, thus leaving
41    /// the borrow flag untouched. Mutably borrowing the RefCell while the reference returned by
42    /// this method is alive is undefined behaviour.
43    ///
44    /// # Panics
45    ///
46    /// Panics if this is called from anywhere other than the layout thread
47    ///
48    /// Panics if the value is currently mutably borrowed.
49    #[expect(unsafe_code)]
50    pub(crate) unsafe fn borrow_for_layout(&self) -> &T {
51        assert_in_layout();
52        unsafe {
53            self.value
54                .try_borrow_unguarded()
55                .expect("cell is mutably borrowed")
56        }
57    }
58
59    /// Borrow the contents for the purpose of script deallocation.
60    ///
61    /// # Safety
62    ///
63    /// Unlike RefCell::borrow, this method is unsafe because it does not return a Ref, thus leaving
64    /// the borrow flag untouched. Mutably borrowing the RefCell while the reference returned by
65    /// this method is alive is undefined behaviour.
66    ///
67    /// # Panics
68    ///
69    /// Panics if this is called from anywhere other than the script thread.
70    #[expect(unsafe_code)]
71    #[allow(clippy::mut_from_ref)]
72    pub(crate) unsafe fn borrow_for_script_deallocation(&self) -> &mut T {
73        assert_in_script();
74        unsafe { &mut *self.value.as_ptr() }
75    }
76
77    /// Mutably borrow a cell for layout. Ideally this would use
78    /// `RefCell::try_borrow_mut_unguarded` but that doesn't exist yet.
79    ///
80    /// # Safety
81    ///
82    /// Unlike RefCell::borrow, this method is unsafe because it does not return a Ref, thus leaving
83    /// the borrow flag untouched. Mutably borrowing the RefCell while the reference returned by
84    /// this method is alive is undefined behaviour.
85    ///
86    /// # Panics
87    ///
88    /// Panics if this is called from anywhere other than the layout thread.
89    #[expect(unsafe_code)]
90    #[allow(clippy::mut_from_ref)]
91    pub(crate) unsafe fn borrow_mut_for_layout(&self) -> &mut T {
92        assert_in_layout();
93        unsafe { &mut *self.value.as_ptr() }
94    }
95}
96
97// Functionality duplicated with `std::cell::RefCell`
98// ===================================================
99impl<T> DomRefCell<T> {
100    /// Create a new `DomRefCell` containing `value`.
101    pub(crate) fn new(value: T) -> DomRefCell<T> {
102        DomRefCell {
103            value: RefCell::new(value),
104        }
105    }
106
107    /// Immutably borrows the wrapped value.
108    ///
109    /// The borrow lasts until the returned `Ref` exits scope. Multiple
110    /// immutable borrows can be taken out at the same time.
111    ///
112    /// # Panics
113    ///
114    /// Panics if the value is currently mutably borrowed.
115    #[track_caller]
116    pub(crate) fn borrow(&self) -> Ref<'_, T> {
117        self.value.borrow()
118    }
119
120    /// Mutably borrows the wrapped value.
121    ///
122    /// The borrow lasts until the returned `RefMut` exits scope. The value
123    /// cannot be borrowed while this borrow is active.
124    ///
125    /// # Panics
126    ///
127    /// Panics if the value is currently borrowed.
128    #[track_caller]
129    pub(crate) fn borrow_mut(&self) -> RefMut<'_, T> {
130        self.value.borrow_mut()
131    }
132
133    /// Attempts to immutably borrow the wrapped value.
134    ///
135    /// The borrow lasts until the returned `Ref` exits scope. Multiple
136    /// immutable borrows can be taken out at the same time.
137    ///
138    /// Returns `None` if the value is currently mutably borrowed.
139    ///
140    /// # Panics
141    ///
142    /// Panics if this is called off the script thread.
143    pub(crate) fn try_borrow(&self) -> Result<Ref<'_, T>, BorrowError> {
144        assert_in_script();
145        self.value.try_borrow()
146    }
147
148    /// Mutably borrows the wrapped value.
149    ///
150    /// The borrow lasts until the returned `RefMut` exits scope. The value
151    /// cannot be borrowed while this borrow is active.
152    ///
153    /// Returns `None` if the value is currently borrowed.
154    ///
155    /// # Panics
156    ///
157    /// Panics if this is called off the script thread.
158    pub(crate) fn try_borrow_mut(&self) -> Result<RefMut<'_, T>, BorrowMutError> {
159        assert_in_script();
160        self.value.try_borrow_mut()
161    }
162}
163
164impl<T: Default> DomRefCell<T> {
165    /// Takes the wrapped value, leaving `Default::default()` in its place.
166    ///
167    /// # Panics
168    ///
169    /// Panics if the value is currently borrowed.
170    pub(crate) fn take(&self) -> T {
171        self.value.take()
172    }
173}
script/dom/bindings/cell.rs

script/dom/bindings/
cell.rs
