1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
// Based on
// https://github.com/matthieu-m/rfc2580/blob/b58d1d3cba0d4b5e859d3617ea2d0943aaa31329/examples/thin.rs
// by matthieu-m
use crate::alloc::{self, Layout, LayoutError};
use core::fmt::{self, Debug, Display, Formatter};
use core::marker::{PhantomData, Unsize};
use core::mem;
use core::ops::{Deref, DerefMut};
use core::ptr::Pointee;
use core::ptr::{self, NonNull};

/// ThinBox.
///
/// A thin pointer for heap allocation, regardless of T.
///
/// # Examples
///
/// ```
/// #![feature(thin_box)]
/// use std::boxed::ThinBox;
///
/// let five = ThinBox::new(5);
/// let thin_slice = ThinBox::<[i32]>::new_unsize([1, 2, 3, 4]);
///
/// use std::mem::{size_of, size_of_val};
/// let size_of_ptr = size_of::<*const ()>();
/// assert_eq!(size_of_ptr, size_of_val(&five));
/// assert_eq!(size_of_ptr, size_of_val(&thin_slice));
/// ```
#[unstable(feature = "thin_box", issue = "92791")]
pub struct ThinBox<T: ?Sized> {
    ptr: WithHeader<<T as Pointee>::Metadata>,
    _marker: PhantomData<T>,
}

#[unstable(feature = "thin_box", issue = "92791")]
impl<T> ThinBox<T> {
    /// Moves a type to the heap with its `Metadata` stored in the heap allocation instead of on
    /// the stack.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(thin_box)]
    /// use std::boxed::ThinBox;
    ///
    /// let five = ThinBox::new(5);
    /// ```
    #[cfg(not(no_global_oom_handling))]
    pub fn new(value: T) -> Self {
        let meta = ptr::metadata(&value);
        let ptr = WithHeader::new(meta, value);
        ThinBox { ptr, _marker: PhantomData }
    }
}

#[unstable(feature = "thin_box", issue = "92791")]
impl<Dyn: ?Sized> ThinBox<Dyn> {
    /// Moves a type to the heap with its `Metadata` stored in the heap allocation instead of on
    /// the stack.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(thin_box)]
    /// use std::boxed::ThinBox;
    ///
    /// let thin_slice = ThinBox::<[i32]>::new_unsize([1, 2, 3, 4]);
    /// ```
    #[cfg(not(no_global_oom_handling))]
    pub fn new_unsize<T>(value: T) -> Self
    where
        T: Unsize<Dyn>,
    {
        let meta = ptr::metadata(&value as &Dyn);
        let ptr = WithHeader::new(meta, value);
        ThinBox { ptr, _marker: PhantomData }
    }
}

#[unstable(feature = "thin_box", issue = "92791")]
impl<T: ?Sized + Debug> Debug for ThinBox<T> {
    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
        Debug::fmt(self.deref(), f)
    }
}

#[unstable(feature = "thin_box", issue = "92791")]
impl<T: ?Sized + Display> Display for ThinBox<T> {
    fn fmt(&self, f: &mut Formatter<'_>) -> fmt::Result {
        Display::fmt(self.deref(), f)
    }
}

#[unstable(feature = "thin_box", issue = "92791")]
impl<T: ?Sized> Deref for ThinBox<T> {
    type Target = T;

    fn deref(&self) -> &T {
        let value = self.data();
        let metadata = self.meta();
        let pointer = ptr::from_raw_parts(value as *const (), metadata);
        unsafe { &*pointer }
    }
}

#[unstable(feature = "thin_box", issue = "92791")]
impl<T: ?Sized> DerefMut for ThinBox<T> {
    fn deref_mut(&mut self) -> &mut T {
        let value = self.data();
        let metadata = self.meta();
        let pointer = ptr::from_raw_parts_mut::<T>(value as *mut (), metadata);
        unsafe { &mut *pointer }
    }
}

#[unstable(feature = "thin_box", issue = "92791")]
impl<T: ?Sized> Drop for ThinBox<T> {
    fn drop(&mut self) {
        unsafe {
            let value = self.deref_mut();
            let value = value as *mut T;
            self.ptr.drop::<T>(value);
        }
    }
}

#[unstable(feature = "thin_box", issue = "92791")]
impl<T: ?Sized> ThinBox<T> {
    fn meta(&self) -> <T as Pointee>::Metadata {
        //  Safety:
        //  -   NonNull and valid.
        unsafe { *self.ptr.header() }
    }

    fn data(&self) -> *mut u8 {
        self.ptr.value()
    }
}

/// A pointer to type-erased data, guaranteed to have a header `H` before the pointed-to location.
struct WithHeader<H>(NonNull<u8>, PhantomData<H>);

impl<H> WithHeader<H> {
    #[cfg(not(no_global_oom_handling))]
    fn new<T>(header: H, value: T) -> WithHeader<H> {
        let value_layout = Layout::new::<T>();
        let Ok((layout, value_offset)) = Self::alloc_layout(value_layout) else {
            // We pass an empty layout here because we do not know which layout caused the
            // arithmetic overflow in `Layout::extend` and `handle_alloc_error` takes `Layout` as
            // its argument rather than `Result<Layout, LayoutError>`, also this function has been
            // stable since 1.28 ._.
            //
            // On the other hand, look at this gorgeous turbofish!
            alloc::handle_alloc_error(Layout::new::<()>());
        };

        unsafe {
            let ptr = alloc::alloc(layout);

            if ptr.is_null() {
                alloc::handle_alloc_error(layout);
            }
            //  Safety:
            //  -   The size is at least `aligned_header_size`.
            let ptr = ptr.add(value_offset) as *mut _;

            let ptr = NonNull::new_unchecked(ptr);

            let result = WithHeader(ptr, PhantomData);
            ptr::write(result.header(), header);
            ptr::write(result.value().cast(), value);

            result
        }
    }

    //  Safety:
    //  -   Assumes that `value` can be dereferenced.
    unsafe fn drop<T: ?Sized>(&self, value: *mut T) {
        unsafe {
            // SAFETY: Layout must have been computable if we're in drop
            let (layout, value_offset) =
                Self::alloc_layout(Layout::for_value_raw(value)).unwrap_unchecked();

            ptr::drop_in_place::<T>(value);
            // We only drop the value because the Pointee trait requires that the metadata is copy
            // aka trivially droppable
            alloc::dealloc(self.0.as_ptr().sub(value_offset), layout);
        }
    }

    fn header(&self) -> *mut H {
        //  Safety:
        //  - At least `size_of::<H>()` bytes are allocated ahead of the pointer.
        //  - We know that H will be aligned because the middle pointer is aligned to the greater
        //    of the alignment of the header and the data and the header size includes the padding
        //    needed to align the header. Subtracting the header size from the aligned data pointer
        //    will always result in an aligned header pointer, it just may not point to the
        //    beginning of the allocation.
        unsafe { self.0.as_ptr().sub(Self::header_size()) as *mut H }
    }

    fn value(&self) -> *mut u8 {
        self.0.as_ptr()
    }

    const fn header_size() -> usize {
        mem::size_of::<H>()
    }

    fn alloc_layout(value_layout: Layout) -> Result<(Layout, usize), LayoutError> {
        Layout::new::<H>().extend(value_layout)
    }
}