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

pub(crate) use internal::Beef;
pub(crate) use internal::Capacity;

pub(crate) mod internal {
    use alloc::borrow::ToOwned;
    use alloc::string::String;
    use alloc::vec::Vec;
    use core::mem::ManuallyDrop;
    use core::ptr::{slice_from_raw_parts, NonNull};

    pub trait Capacity {
        type Field: Copy;
        type NonZero: Copy;

        fn len(fat: usize) -> usize;

        fn empty(len: usize) -> (usize, Self::Field);

        fn store(len: usize, capacity: usize) -> (usize, Self::Field);

        fn unpack(fat: usize, capacity: Self::NonZero) -> (usize, usize);

        fn maybe(fat: usize, capacity: Self::Field) -> Option<Self::NonZero>;
    }

    /// Helper trait required by `Cow<T>` to extract capacity of owned
    /// variant of `T`, and manage conversions.
    ///
    /// This can be only implemented on types that match requirements:
    ///
    /// + `T::Owned` has a `capacity`, which is an extra word that is absent in `T`.
    /// + `T::Owned` with `capacity` of `0` does not allocate memory.
    /// + `T::Owned` can be reconstructed from `*mut T` borrowed out of it, plus capacity.
    pub unsafe trait Beef: ToOwned {
        type PointerT;

        fn ref_into_parts<U>(&self) -> (NonNull<Self::PointerT>, usize, U::Field)
        where
            U: Capacity;

        unsafe fn ref_from_parts<U>(ptr: NonNull<Self::PointerT>, len: usize) -> *const Self
        where
            U: Capacity;

        /// Convert `T::Owned` to `NonNull<T>` and capacity.
        /// Return `None` for `0` capacity.
        fn owned_into_parts<U>(owned: Self::Owned) -> (NonNull<Self::PointerT>, usize, U::Field)
        where
            U: Capacity;

        /// Rebuild `T::Owned` from `NonNull<T>` and `capacity`. This can be done by the likes
        /// of [`Vec::from_raw_parts`](https://doc.rust-lang.org/std/vec/struct.Vec.html#method.from_raw_parts).
        unsafe fn owned_from_parts<U>(
            ptr: NonNull<Self::PointerT>,
            fat: usize,
            capacity: U::NonZero,
        ) -> Self::Owned
        where
            U: Capacity;
    }

    unsafe impl Beef for str {
        type PointerT = u8;

        #[inline]
        fn ref_into_parts<U>(&self) -> (NonNull<u8>, usize, U::Field)
        where
            U: Capacity,
        {
            let (fat, cap) = U::empty(self.len());

            // A note on soundness:
            //
            // We are casting *const T to *mut T, however for all borrowed values
            // this raw pointer is only ever dereferenced back to &T.
            (
                unsafe { NonNull::new_unchecked(self.as_ptr() as *mut u8) },
                fat,
                cap,
            )
        }

        #[inline]
        unsafe fn ref_from_parts<U>(ptr: NonNull<u8>, fat: usize) -> *const str
        where
            U: Capacity,
        {
            slice_from_raw_parts(ptr.as_ptr(), U::len(fat)) as *const str
        }

        #[inline]
        fn owned_into_parts<U>(owned: String) -> (NonNull<u8>, usize, U::Field)
        where
            U: Capacity,
        {
            // Convert to `String::into_raw_parts` once stabilized
            let mut owned = ManuallyDrop::new(owned);
            let (fat, cap) = U::store(owned.len(), owned.capacity());

            (
                unsafe { NonNull::new_unchecked(owned.as_mut_ptr()) },
                fat,
                cap,
            )
        }

        #[inline]
        unsafe fn owned_from_parts<U>(ptr: NonNull<u8>, fat: usize, capacity: U::NonZero) -> String
        where
            U: Capacity,
        {
            let (len, cap) = U::unpack(fat, capacity);

            String::from_utf8_unchecked(Vec::from_raw_parts(ptr.as_ptr(), len, cap))
        }
    }

    unsafe impl<T: Clone> Beef for [T] {
        type PointerT = T;

        #[inline]
        fn ref_into_parts<U>(&self) -> (NonNull<T>, usize, U::Field)
        where
            U: Capacity,
        {
            let (fat, cap) = U::empty(self.len());

            // A note on soundness:
            //
            // We are casting *const T to *mut T, however for all borrowed values
            // this raw pointer is only ever dereferenced back to &T.
            (
                unsafe { NonNull::new_unchecked(self.as_ptr() as *mut T) },
                fat,
                cap,
            )
        }

        #[inline]
        unsafe fn ref_from_parts<U>(ptr: NonNull<T>, fat: usize) -> *const [T]
        where
            U: Capacity,
        {
            slice_from_raw_parts(ptr.as_ptr(), U::len(fat))
        }

        #[inline]
        fn owned_into_parts<U>(owned: Vec<T>) -> (NonNull<T>, usize, U::Field)
        where
            U: Capacity,
        {
            // Convert to `Vec::into_raw_parts` once stabilized
            let mut owned = ManuallyDrop::new(owned);
            let (fat, cap) = U::store(owned.len(), owned.capacity());

            (
                unsafe { NonNull::new_unchecked(owned.as_mut_ptr()) },
                fat,
                cap,
            )
        }

        #[inline]
        unsafe fn owned_from_parts<U>(ptr: NonNull<T>, fat: usize, capacity: U::NonZero) -> Vec<T>
        where
            U: Capacity,
        {
            let (len, cap) = U::unpack(fat, capacity);

            Vec::from_raw_parts(ptr.as_ptr(), len, cap)
        }
    }
}