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diff --git a/components/to_shmem/lib.rs b/components/to_shmem/lib.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/. */
+
+//! Trait for cloning data into a shared memory buffer.
+//!
+//! This module contains the SharedMemoryBuilder type and ToShmem trait.
+//!
+//! We put them here (and not in style_traits) so that we can derive ToShmem
+//! from the selectors and style crates.
+
+#![crate_name = "to_shmem"]
+#![crate_type = "rlib"]
+
+use std::alloc::Layout;
+#[cfg(debug_assertions)]
+use std::any::TypeId;
+use std::isize;
+#[cfg(debug_assertions)]
+use std::collections::HashSet;
+use std::mem::{self, ManuallyDrop};
+#[cfg(debug_assertions)]
+use std::os::raw::c_void;
+use std::ptr::{self, NonNull};
+
+// Various pointer arithmetic functions in this file can be replaced with
+// functions on `Layout` once they have stabilized:
+//
+// https://github.com/rust-lang/rust/issues/55724
+
+/// A builder object that transforms and copies values into a fixed size buffer.
+pub struct SharedMemoryBuilder {
+    /// The buffer into which values will be copied.
+    buffer: *mut u8,
+    /// The size of the buffer.
+    capacity: usize,
+    /// The current position in the buffer, where the next value will be written
+    /// at.
+    index: usize,
+    /// Pointers to every sharable value that we store in the shared memory
+    /// buffer.  We use this to assert against encountering the same value
+    /// twice, e.g. through another Arc reference, so that we don't
+    /// inadvertently store duplicate copies of values.
+    #[cfg(debug_assertions)]
+    shared_values: HashSet<*const c_void>,
+    /// Types of values that we may duplicate in the shared memory buffer when
+    /// there are shared references to them, such as in Arcs.
+    #[cfg(debug_assertions)]
+    allowed_duplication_types: HashSet<TypeId>,
+}
+
+/// Amount of padding needed after `size` bytes to ensure that the following
+/// address will satisfy `align`.
+fn padding_needed_for(size: usize, align: usize) -> usize {
+    padded_size(size, align).wrapping_sub(size)
+}
+
+/// Rounds up `size` so that the following address will satisfy `align`.
+fn padded_size(size: usize, align: usize) -> usize {
+    size.wrapping_add(align).wrapping_sub(1) & !align.wrapping_sub(1)
+}
+
+impl SharedMemoryBuilder {
+    /// Creates a new SharedMemoryBuilder using the specified buffer.
+    pub unsafe fn new(buffer: *mut u8, capacity: usize) -> SharedMemoryBuilder {
+        SharedMemoryBuilder {
+            buffer,
+            capacity,
+            index: 0,
+            #[cfg(debug_assertions)]
+            shared_values: HashSet::new(),
+            #[cfg(debug_assertions)]
+            allowed_duplication_types: HashSet::new(),
+        }
+    }
+
+    /// Notes a type as being allowed for duplication when being copied to the
+    /// shared memory buffer, such as Arcs referencing the same value.
+    #[inline]
+    pub fn add_allowed_duplication_type<T: 'static>(&mut self) {
+        #[cfg(debug_assertions)]
+        self.allowed_duplication_types.insert(TypeId::of::<T>());
+    }
+
+    /// Returns the number of bytes currently used in the buffer.
+    #[inline]
+    pub fn len(&self) -> usize {
+        self.index
+    }
+
+    /// Writes a value into the shared memory buffer and returns a pointer to
+    /// it in the buffer.
+    ///
+    /// The value is cloned and converted into a form suitable for placing into
+    /// a shared memory buffer by calling ToShmem::to_shmem on it.
+    ///
+    /// Panics if there is insufficient space in the buffer.
+    pub fn write<T: ToShmem>(&mut self, value: &T) -> *mut T {
+        // Reserve space for the value.
+        let dest: *mut T = self.alloc_value();
+
+        // Make a clone of the value with all of its heap allocations
+        // placed in the shared memory buffer.
+        let value = value.to_shmem(self);
+
+        unsafe {
+            // Copy the value into the buffer.
+            ptr::write(dest, ManuallyDrop::into_inner(value));
+        }
+
+        // Return a pointer to the shared value.
+        dest
+    }
+
+    /// Reserves space in the shared memory buffer to fit a value of type T,
+    /// and returns a pointer to that reserved space.
+    ///
+    /// Panics if there is insufficient space in the buffer.
+    pub fn alloc_value<T>(&mut self) -> *mut T {
+        self.alloc(Layout::new::<T>())
+    }
+
+    /// Reserves space in the shared memory buffer to fit an array of values of
+    /// type T, and returns a pointer to that reserved space.
+    ///
+    /// Panics if there is insufficient space in the buffer.
+    pub fn alloc_array<T>(&mut self, len: usize) -> *mut T {
+        if len == 0 {
+            return NonNull::dangling().as_ptr();
+        }
+
+        let size = mem::size_of::<T>();
+        let align = mem::align_of::<T>();
+
+        self.alloc(Layout::from_size_align(padded_size(size, align) * len, align).unwrap())
+    }
+
+    /// Reserves space in the shared memory buffer that conforms to the
+    /// specified layout, and returns a pointer to that reserved space.
+    ///
+    /// Panics if there is insufficient space in the buffer.
+    pub fn alloc<T>(&mut self, layout: Layout) -> *mut T {
+        // Amount of padding to align the value.
+        //
+        // The addition can't overflow, since self.index <= self.capacity, and
+        // for us to have successfully allocated the buffer, `buffer + capacity`
+        // can't overflow.
+        let padding = padding_needed_for(self.buffer as usize + self.index, layout.align());
+
+        // Reserve space for the padding.
+        let start = self.index.checked_add(padding).unwrap();
+        assert!(start <= std::isize::MAX as usize);  // for the cast below
+
+        // Reserve space for the value.
+        let end = start.checked_add(layout.size()).unwrap();
+        assert!(end <= self.capacity);
+
+        self.index = end;
+        unsafe { self.buffer.offset(start as isize) as *mut T }
+    }
+}
+
+/// A type that can be copied into a SharedMemoryBuilder.
+pub trait ToShmem: Sized {
+    /// Clones this value into a form suitable for writing into a
+    /// SharedMemoryBuilder.
+    ///
+    /// If this value owns any heap allocations, they should be written into
+    /// `builder` so that the return value of this function can point to the
+    /// copy in the shared memory buffer.
+    ///
+    /// The return type is wrapped in ManuallyDrop to make it harder to
+    /// accidentally invoke the destructor of the value that is produced.
+    fn to_shmem(&self, builder: &mut SharedMemoryBuilder) -> ManuallyDrop<Self>;
+}