script_bindings/

cell.rs

/* 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};
pub use std::cell::{Ref, RefCell, RefMut};

use js::context::NoGC;
use js::jsapi::JSTracer;
use malloc_size_of::{MallocConditionalSizeOf, MallocSizeOfOps};

use crate::CustomTraceable;
use crate::assert::{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 struct DomRefCell<T> {
    value: RefCell<T>,
}

impl<T: MallocConditionalSizeOf> MallocConditionalSizeOf for DomRefCell<T> {
    fn conditional_size_of(&self, ops: &mut MallocSizeOfOps) -> usize {
        self.value.borrow().conditional_size_of(ops)
    }
}

// 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.
    #[expect(unsafe_code)]
    pub unsafe fn borrow_for_layout(&self) -> &T {
        assert_in_layout();
        unsafe {
            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.
    #[expect(unsafe_code)]
    #[allow(clippy::mut_from_ref)]
    pub unsafe fn borrow_for_script_deallocation(&self) -> &mut T {
        assert_in_script();
        unsafe { &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.
    #[expect(unsafe_code)]
    #[allow(clippy::mut_from_ref)]
    pub unsafe fn borrow_mut_for_layout(&self) -> &mut T {
        assert_in_layout();
        unsafe { &mut *self.value.as_ptr() }
    }

    /// Mutably borrows the wrapped value.
    ///
    /// The borrow lasts until the returned `RefMut` exits scope. The value
    /// cannot be borrowed while this borrow is active.
    ///
    /// By passing a `&NoGC` we statically prevent GC from being run while the borrow is active,
    /// to prevent panic when tracing (which calls `borrow`).
    ///
    /// # Example
    ///
    /// In simple cases one can use `NoGC` to statically ensure no GC can happen in the whole DOM method:
    ///
    /// ```
    /// use js::context::{JSContext, NoGC};
    /// use script_bindings::cell::DomRefCell;
    /// fn DomMethod(no_gc: &NoGC, cell: &DomRefCell<usize>) {
    ///     let mut mutably_borrowed = cell.safe_borrow_mut(no_gc);
    /// }
    /// ```
    ///
    /// But in more complex cases, method might trigger a GC, and thus require a `&mut JSContext`.
    /// In that case `&JSContext` can be used in place of `NoGC`,
    /// which will make `RefMut` bounded to the lifetime of the `&JSContext`
    /// and thus prevent any GC from happening while it is alive.
    ///
    /// ```
    /// use js::context::{JSContext, NoGC};
    /// use script_bindings::cell::DomRefCell;
    /// fn GC(cx: &mut JSContext) {}
    ///
    /// fn DomMethod(cell: &DomRefCell<usize>, cx: &mut JSContext) {
    ///     {
    ///         let mut mutably_borrowed = cell.safe_borrow_mut(cx);
    ///         // do something with mutably_borrowed
    ///
    ///         // only &JSContext is available here
    ///     } // mutably_borrowed goes out of scope here
    ///     // so one can now use &mut JSContext
    ///     GC(cx);
    /// }
    /// ```
    ///
    /// ```compile_fail
    /// use js::context::{JSContext, NoGC};
    /// use script_bindings::cell::DomRefCell;
    /// fn GC(cx: &mut JSContext) {}
    ///
    /// fn DomMethod(cell: &DomRefCell<usize>, cx: &mut JSContext) {
    ///     {
    ///         let mut mutably_borrowed = cell.safe_borrow_mut(cx);
    ///         // do something with mutably_borrowed
    ///
    ///         // here one cannot use anything that might trigger a GC
    ///         // as that would require &mut JSContext
    ///         // but there is already existing &JSContext bounded at RefMut
    ///         GC(cx);
    ///     } // mutably_borrowed goes out of scope here
    /// }
    /// ```
    ///
    /// # Panics
    ///
    /// Panics if the value is currently borrowed.
    #[track_caller]
    pub fn safe_borrow_mut<'a: 'r, 'no_cx: 'r, 'r>(
        &'a self,
        _no_gc: &'no_cx NoGC,
    ) -> RefMut<'r, T> {
        self.value.borrow_mut()
    }

    /// Mutably borrows the wrapped value.
    ///
    /// The borrow lasts until the returned `RefMut` exits scope. The value
    /// cannot be borrowed while this borrow is active.
    ///
    /// By passing a `&NoGC` we statically prevent GC from being run while the borrow is active,
    /// to prevent panic when tracing (which calls `borrow`).
    ///
    /// Returns `None` if the value is currently borrowed.
    pub fn safe_try_borrow_mut<'a: 'r, 'no_cx: 'r, 'r>(
        &'a self,
        _no_gc: &'no_cx NoGC,
    ) -> Result<RefMut<'r, T>, BorrowMutError> {
        self.value.try_borrow_mut()
    }
}

// Functionality duplicated with `std::cell::RefCell`
// ===================================================
impl<T> DomRefCell<T> {
    /// Create a new `DomRefCell` containing `value`.
    pub 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.
    /// Panics if this is called from anywhere other than the script thread.
    /// Use borrow_for_layout if the borrowed data might used during layout.
    #[track_caller]
    pub fn borrow(&self) -> Ref<'_, T> {
        assert_in_script();
        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.
    /// Panics if this is called from anywhere other than the script thread.
    #[track_caller]
    pub fn borrow_mut(&self) -> RefMut<'_, T> {
        assert_in_script();
        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 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 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 fn take(&self) -> T {
        self.value.take()
    }
}

unsafe impl<T: CustomTraceable> CustomTraceable for DomRefCell<T> {
    unsafe fn trace(&self, trc: *mut JSTracer) {
        unsafe { (*self).borrow().trace(trc) }
    }
}

unsafe impl<T: js::gc::Traceable> js::gc::Traceable for DomRefCell<T> {
    unsafe fn trace(&self, trc: *mut JSTracer) {
        unsafe { (*self).borrow().trace(trc) };
    }
}