script/dom/bindings/cell.rs
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/* 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/. */
//! A shareable mutable container for the DOM.
use std::cell::{BorrowError, BorrowMutError};
#[cfg(not(feature = "refcell_backtrace"))]
pub(crate) use std::cell::{Ref, RefCell, RefMut};
#[cfg(feature = "refcell_backtrace")]
pub(crate) use accountable_refcell::{ref_filter_map, Ref, RefCell, RefMut};
#[cfg(not(feature = "refcell_backtrace"))]
pub(crate) use ref_filter_map::ref_filter_map;
use crate::dom::bindings::root::{assert_in_layout, assert_in_script};
/// A mutable field in the DOM.
///
/// This extends the API of `std::cell::RefCell` to allow unsafe access in
/// certain situations, with dynamic checking in debug builds.
#[derive(Clone, Debug, Default, MallocSizeOf, PartialEq)]
pub(crate) struct DomRefCell<T> {
value: RefCell<T>,
}
// Functionality specific to Servo's `DomRefCell` type
// ===================================================
impl<T> DomRefCell<T> {
/// Return a reference to the contents. For use in layout only.
///
/// # Safety
///
/// Unlike RefCell::borrow, this method is unsafe because it does not return a Ref, thus leaving
/// the borrow flag untouched. Mutably borrowing the RefCell while the reference returned by
/// this method is alive is undefined behaviour.
///
/// # Panics
///
/// Panics if this is called from anywhere other than the layout thread
///
/// Panics if the value is currently mutably borrowed.
#[allow(unsafe_code)]
pub(crate) unsafe fn borrow_for_layout(&self) -> &T {
assert_in_layout();
self.value
.try_borrow_unguarded()
.expect("cell is mutably borrowed")
}
/// Borrow the contents for the purpose of script deallocation.
///
/// # Safety
///
/// Unlike RefCell::borrow, this method is unsafe because it does not return a Ref, thus leaving
/// the borrow flag untouched. Mutably borrowing the RefCell while the reference returned by
/// this method is alive is undefined behaviour.
///
/// # Panics
///
/// Panics if this is called from anywhere other than the script thread.
#[allow(unsafe_code, clippy::mut_from_ref)]
pub(crate) unsafe fn borrow_for_script_deallocation(&self) -> &mut T {
assert_in_script();
&mut *self.value.as_ptr()
}
/// Mutably borrow a cell for layout. Ideally this would use
/// `RefCell::try_borrow_mut_unguarded` but that doesn't exist yet.
///
/// # Safety
///
/// Unlike RefCell::borrow, this method is unsafe because it does not return a Ref, thus leaving
/// the borrow flag untouched. Mutably borrowing the RefCell while the reference returned by
/// this method is alive is undefined behaviour.
///
/// # Panics
///
/// Panics if this is called from anywhere other than the layout thread.
#[allow(unsafe_code, clippy::mut_from_ref)]
pub(crate) unsafe fn borrow_mut_for_layout(&self) -> &mut T {
assert_in_layout();
&mut *self.value.as_ptr()
}
}
// Functionality duplicated with `std::cell::RefCell`
// ===================================================
impl<T> DomRefCell<T> {
/// Create a new `DomRefCell` containing `value`.
pub(crate) fn new(value: T) -> DomRefCell<T> {
DomRefCell {
value: RefCell::new(value),
}
}
/// Immutably borrows the wrapped value.
///
/// The borrow lasts until the returned `Ref` exits scope. Multiple
/// immutable borrows can be taken out at the same time.
///
/// # Panics
///
/// Panics if the value is currently mutably borrowed.
#[track_caller]
pub(crate) fn borrow(&self) -> Ref<T> {
self.value.borrow()
}
/// Mutably borrows the wrapped value.
///
/// The borrow lasts until the returned `RefMut` exits scope. The value
/// cannot be borrowed while this borrow is active.
///
/// # Panics
///
/// Panics if the value is currently borrowed.
#[track_caller]
pub(crate) fn borrow_mut(&self) -> RefMut<T> {
self.value.borrow_mut()
}
/// Attempts to immutably borrow the wrapped value.
///
/// The borrow lasts until the returned `Ref` exits scope. Multiple
/// immutable borrows can be taken out at the same time.
///
/// Returns `None` if the value is currently mutably borrowed.
///
/// # Panics
///
/// Panics if this is called off the script thread.
pub(crate) fn try_borrow(&self) -> Result<Ref<T>, BorrowError> {
assert_in_script();
self.value.try_borrow()
}
/// Mutably borrows the wrapped value.
///
/// The borrow lasts until the returned `RefMut` exits scope. The value
/// cannot be borrowed while this borrow is active.
///
/// Returns `None` if the value is currently borrowed.
///
/// # Panics
///
/// Panics if this is called off the script thread.
pub(crate) fn try_borrow_mut(&self) -> Result<RefMut<T>, BorrowMutError> {
assert_in_script();
self.value.try_borrow_mut()
}
}
impl<T: Default> DomRefCell<T> {
/// Takes the wrapped value, leaving `Default::default()` in its place.
///
/// # Panics
///
/// Panics if the value is currently borrowed.
pub(crate) fn take(&self) -> T {
self.value.take()
}
}