style_traits/
arc_slice.rs

1/* This Source Code Form is subject to the terms of the Mozilla Public
2 * License, v. 2.0. If a copy of the MPL was not distributed with this
3 * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
4
5//! A thin atomically-reference-counted slice.
6
7use serde::de::{Deserialize, Deserializer};
8use serde::ser::{Serialize, Serializer};
9use servo_arc::ThinArc;
10use std::ops::Deref;
11use std::ptr::NonNull;
12use std::sync::LazyLock;
13use std::{
14    hash::{Hash, Hasher},
15    iter, mem,
16};
17
18use malloc_size_of::{MallocSizeOf, MallocSizeOfOps, MallocUnconditionalSizeOf};
19
20/// A canary that we stash in ArcSlices.
21///
22/// Given we cannot use a zero-sized-type for the header, since well, C++
23/// doesn't have zsts, and we want to use cbindgen for this type, we may as well
24/// assert some sanity at runtime.
25///
26/// We use an u64, to guarantee that we can use a single singleton for every
27/// empty slice, even if the types they hold are aligned differently.
28const ARC_SLICE_CANARY: u64 = 0xf3f3f3f3f3f3f3f3;
29
30/// A wrapper type for a refcounted slice using ThinArc.
31#[repr(C)]
32#[derive(Debug, Eq, PartialEq, ToShmem)]
33pub struct ArcSlice<T>(#[shmem(field_bound)] ThinArc<u64, T>);
34
35impl<T> Deref for ArcSlice<T> {
36    type Target = [T];
37
38    #[inline]
39    fn deref(&self) -> &Self::Target {
40        debug_assert_eq!(self.0.header, ARC_SLICE_CANARY);
41        self.0.slice()
42    }
43}
44
45impl<T> Clone for ArcSlice<T> {
46    fn clone(&self) -> Self {
47        ArcSlice(self.0.clone())
48    }
49}
50
51// ThinArc doesn't support alignments greater than align_of::<u64>.
52static EMPTY_ARC_SLICE: LazyLock<ArcSlice<u64>> =
53    LazyLock::new(|| ArcSlice::from_iter_leaked(iter::empty()));
54
55impl<T> Default for ArcSlice<T> {
56    #[allow(unsafe_code)]
57    fn default() -> Self {
58        debug_assert!(
59            mem::align_of::<T>() <= mem::align_of::<u64>(),
60            "Need to increase the alignment of EMPTY_ARC_SLICE"
61        );
62        unsafe {
63            let empty: ArcSlice<_> = EMPTY_ARC_SLICE.clone();
64            let empty: Self = mem::transmute(empty);
65            debug_assert_eq!(empty.len(), 0);
66            empty
67        }
68    }
69}
70
71impl<T: Serialize> Serialize for ArcSlice<T> {
72    fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>
73    where
74        S: Serializer,
75    {
76        self.deref().serialize(serializer)
77    }
78}
79
80impl<'de, T: Deserialize<'de>> Deserialize<'de> for ArcSlice<T> {
81    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
82    where
83        D: Deserializer<'de>,
84    {
85        let r = Vec::deserialize(deserializer)?;
86        Ok(ArcSlice::from_iter(r.into_iter()))
87    }
88}
89
90impl<T> ArcSlice<T> {
91    /// Creates an Arc for a slice using the given iterator to generate the
92    /// slice.
93    #[inline]
94    pub fn from_iter<I>(items: I) -> Self
95    where
96        I: Iterator<Item = T> + ExactSizeIterator,
97    {
98        if items.len() == 0 {
99            return Self::default();
100        }
101        ArcSlice(ThinArc::from_header_and_iter(ARC_SLICE_CANARY, items))
102    }
103
104    /// Creates an Arc for a slice using the given iterator to generate the
105    /// slice, and marks the arc as intentionally leaked from the refcount
106    /// logging point of view.
107    #[inline]
108    pub fn from_iter_leaked<I>(items: I) -> Self
109    where
110        I: Iterator<Item = T> + ExactSizeIterator,
111    {
112        let arc = ThinArc::from_header_and_iter(ARC_SLICE_CANARY, items);
113        arc.mark_as_intentionally_leaked();
114        ArcSlice(arc)
115    }
116
117    /// Creates a value that can be passed via FFI, and forgets this value
118    /// altogether.
119    #[inline]
120    pub fn forget(self) -> ForgottenArcSlicePtr<T> {
121        let ret = ForgottenArcSlicePtr(self.0.raw_ptr().cast());
122        mem::forget(self);
123        ret
124    }
125
126    /// Leaks an empty arc slice pointer, and returns it. Only to be used to
127    /// construct ArcSlices from FFI.
128    #[inline]
129    pub fn leaked_empty_ptr() -> *mut std::os::raw::c_void {
130        let empty: ArcSlice<_> = EMPTY_ARC_SLICE.clone();
131        let ptr = empty.0.raw_ptr();
132        std::mem::forget(empty);
133        ptr.cast().as_ptr()
134    }
135
136    /// Returns whether there's only one reference to this ArcSlice.
137    pub fn is_unique(&self) -> bool {
138        self.0.is_unique()
139    }
140}
141
142impl<T: MallocSizeOf> MallocUnconditionalSizeOf for ArcSlice<T> {
143    #[allow(unsafe_code)]
144    fn unconditional_size_of(&self, ops: &mut MallocSizeOfOps) -> usize {
145        let mut size = unsafe { ops.malloc_size_of(self.0.heap_ptr()) };
146        for el in self.iter() {
147            size += el.size_of(ops);
148        }
149        size
150    }
151}
152
153impl<T: Hash> Hash for ArcSlice<T> {
154    fn hash<H: Hasher>(&self, state: &mut H) {
155        T::hash_slice(&**self, state)
156    }
157}
158
159/// The inner pointer of an ArcSlice<T>, to be sent via FFI.
160/// The type of the pointer is a bit of a lie, we just want to preserve the type
161/// but these pointers cannot be constructed outside of this crate, so we're
162/// good.
163#[repr(C)]
164pub struct ForgottenArcSlicePtr<T>(NonNull<T>);
style_traits/arc_slice.rs

style_traits/
arc_slice.rs
