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
This commit is contained in:
Emilio Cobos Álvarez 2019-05-09 10:53:50 +00:00
parent 2ed2151b3d
commit 0d5c4481b8
6 changed files with 98 additions and 3 deletions

View file

@ -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
}
}