style: Introduce ArcSlice, a small wrapper over ThinArc but without an explicit header.

We could make the header PhantomData or something, but then we wouldn't be able
to bind to C++, since C++ doesn't have ZSTs. So add a canary instead to add a
runtime check of stuff being sane.

Differential Revision: https://phabricator.services.mozilla.com/D30133

components/servo_arc/lib.rs

@@ -784,6 +784,13 @@ pub struct ThinArc<H, T> {
     phantom: PhantomData<(H, T)>,
 }
 
+
+impl<H: fmt::Debug, T: fmt::Debug> fmt::Debug for ThinArc<H, T> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        fmt::Debug::fmt(self.deref(), f)
+    }
+}
+
 unsafe impl<H: Sync + Send, T: Sync + Send> Send for ThinArc<H, T> {}
 unsafe impl<H: Sync + Send, T: Sync + Send> Sync for ThinArc<H, T> {}
 
@@ -856,8 +863,12 @@ impl<H, T> ThinArc<H, T> {
     }
 
     /// Returns the address on the heap of the ThinArc itself -- not the T
-    /// within it -- for memory reporting.
-    ///
+    /// within it -- for memory reporting, and bindings.
+    #[inline]
+    pub fn ptr(&self) -> *const c_void {
+        self.ptr.as_ptr() as *const ArcInner<T> as *const c_void
+    }
+
     /// If this is a static ThinArc, this returns null.
     #[inline]
     pub fn heap_ptr(&self) -> *const c_void {
@@ -866,7 +877,7 @@ impl<H, T> ThinArc<H, T> {
         if is_static {
             ptr::null()
         } else {
-            self.ptr.as_ptr() as *const ArcInner<T> as *const c_void
+            self.ptr()
         }
     }
 }

components/style/lib.rs

@@ -188,6 +188,7 @@ pub use html5ever::Prefix;
 #[cfg(feature = "servo")]
 pub use servo_atoms::Atom;
 
+pub use style_traits::arc_slice::ArcSlice;
 pub use style_traits::owned_slice::OwnedSlice;
 
 /// The CSS properties supported by the style system.

components/style/values/animated/mod.rs

@@ -462,3 +462,17 @@ where
         Ok(v.into_boxed_slice())
     }
 }
+
+impl<T> ToAnimatedZero for crate::ArcSlice<T>
+where
+    T: ToAnimatedZero,
+{
+    #[inline]
+    fn to_animated_zero(&self) -> Result<Self, ()> {
+        let v = self
+            .iter()
+            .map(|v| v.to_animated_zero())
+            .collect::<Result<Vec<_>, _>>()?;
+        Ok(crate::ArcSlice::from_iter(v.into_iter()))
+    }
+}

components/style_traits/arc_slice.rs

@@ -0,0 +1,66 @@
+/* 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 thin atomically-reference-counted slice.
+
+use servo_arc::ThinArc;
+use std::mem;
+use std::ops::Deref;
+use std::ptr::NonNull;
+
+/// A canary that we stash in ArcSlices.
+///
+/// Given we cannot use a zero-sized-type for the header, since well, C++
+/// doesn't have zsts, and we want to use cbindgen for this type, we may as well
+/// assert some sanity at runtime.
+const ARC_SLICE_CANARY: u32 = 0xf3f3f3f3;
+
+/// A wrapper type for a refcounted slice using ThinArc.
+///
+/// cbindgen:derive-eq=false
+/// cbindgen:derive-neq=false
+#[repr(C)]
+#[derive(Debug, Clone, PartialEq, Eq, ToShmem)]
+pub struct ArcSlice<T>(#[shmem(field_bound)] ThinArc<u32, T>);
+
+impl<T> Deref for ArcSlice<T> {
+    type Target = [T];
+
+    #[inline]
+    fn deref(&self) -> &Self::Target {
+        debug_assert_eq!(self.0.header.header, ARC_SLICE_CANARY);
+        &self.0.slice
+    }
+}
+
+/// The inner pointer of an ArcSlice<T>, to be sent via FFI.
+/// The type of the pointer is a bit of a lie, we just want to preserve the type
+/// but these pointers cannot be constructed outside of this crate, so we're
+/// good.
+#[repr(C)]
+pub struct ForgottenArcSlicePtr<T>(NonNull<T>);
+
+impl<T> ArcSlice<T> {
+    /// Creates an Arc for a slice using the given iterator to generate the
+    /// slice.
+    #[inline]
+    pub fn from_iter<I>(items: I) -> Self
+    where
+        I: Iterator<Item = T> + ExactSizeIterator,
+    {
+        ArcSlice(ThinArc::from_header_and_iter(ARC_SLICE_CANARY, items))
+    }
+
+    /// Creates a value that can be passed via FFI, and forgets this value
+    /// altogether.
+    #[inline]
+    #[allow(unsafe_code)]
+    pub fn forget(self) -> ForgottenArcSlicePtr<T> {
+        let ret = unsafe {
+            ForgottenArcSlicePtr(NonNull::new_unchecked(self.0.ptr() as *const _ as *mut _))
+        };
+        mem::forget(self);
+        ret
+    }
+}

components/style_traits/lib.rs

@@ -84,6 +84,7 @@ pub enum CSSPixel {}
 //   / hidpi_ratio => DeviceIndependentPixel
 //     / desktop_zoom => CSSPixel
 
+pub mod arc_slice;
 pub mod specified_value_info;
 #[macro_use]
 pub mod values;

components/style_traits/specified_value_info.rs

@@ -4,6 +4,7 @@
 
 //! Value information for devtools.
 
+use crate::arc_slice::ArcSlice;
 use servo_arc::Arc;
 use std::ops::Range;
 use std::sync::Arc as StdArc;
@@ -116,6 +117,7 @@ impl_generic_specified_value_info!(Option<T>);
 impl_generic_specified_value_info!(Vec<T>);
 impl_generic_specified_value_info!(Arc<T>);
 impl_generic_specified_value_info!(StdArc<T>);
+impl_generic_specified_value_info!(ArcSlice<T>);
 impl_generic_specified_value_info!(Range<Idx>);
 
 impl<T1, T2> SpecifiedValueInfo for (T1, T2)