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
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
#![unstable(feature = "ptr_metadata", issue = "81513")]

use crate::fmt;
use crate::hash::{Hash, Hasher};

/// Provides the pointer metadata type of any pointed-to type.
///
/// # Pointer metadata
///
/// Raw pointer types and reference types in Rust can be thought of as made of two parts:
/// a data pointer that contains the memory address of the value, and some metadata.
///
/// For statically-sized types (that implement the `Sized` traits)
/// as well as for `extern` types,
/// pointers are said to be “thin”: metadata is zero-sized and its type is `()`.
///
/// Pointers to [dynamically-sized types][dst] are said to be “wide” or “fat”,
/// they have non-zero-sized metadata:
///
/// * For structs whose last field is a DST, metadata is the metadata for the last field
/// * For the `str` type, metadata is the length in bytes as `usize`
/// * For slice types like `[T]`, metadata is the length in items as `usize`
/// * For trait objects like `dyn SomeTrait`, metadata is [`DynMetadata<Self>`][DynMetadata]
///   (e.g. `DynMetadata<dyn SomeTrait>`)
///
/// In the future, the Rust language may gain new kinds of types
/// that have different pointer metadata.
///
/// [dst]: https://doc.rust-lang.org/nomicon/exotic-sizes.html#dynamically-sized-types-dsts
///
///
/// # The `Pointee` trait
///
/// The point of this trait is its `Metadata` associated type,
/// which is `()` or `usize` or `DynMetadata<_>` as described above.
/// It is automatically implemented for every type.
/// It can be assumed to be implemented in a generic context, even without a corresponding bound.
///
///
/// # Usage
///
/// Raw pointers can be decomposed into the data address and metadata components
/// with their [`to_raw_parts`] method.
///
/// Alternatively, metadata alone can be extracted with the [`metadata`] function.
/// A reference can be passed to [`metadata`] and implicitly coerced.
///
/// A (possibly-wide) pointer can be put back together from its address and metadata
/// with [`from_raw_parts`] or [`from_raw_parts_mut`].
///
/// [`to_raw_parts`]: *const::to_raw_parts
#[lang = "pointee_trait"]
pub trait Pointee {
    /// The type for metadata in pointers and references to `Self`.
    #[lang = "metadata_type"]
    // NOTE: Keep trait bounds in `static_assert_expected_bounds_for_metadata`
    // in `library/core/src/ptr/metadata.rs`
    // in sync with those here:
    type Metadata: Copy + Send + Sync + Ord + Hash + Unpin;
}

/// Pointers to types implementing this trait alias are “thin”.
///
/// This includes statically-`Sized` types and `extern` types.
///
/// # Example
///
/// ```rust
/// #![feature(ptr_metadata)]
///
/// fn this_never_panics<T: std::ptr::Thin>() {
///     assert_eq!(std::mem::size_of::<&T>(), std::mem::size_of::<usize>())
/// }
/// ```
#[unstable(feature = "ptr_metadata", issue = "81513")]
// NOTE: don’t stabilize this before trait aliases are stable in the language?
pub trait Thin = Pointee<Metadata = ()>;

/// Extract the metadata component of a pointer.
///
/// Values of type `*mut T`, `&T`, or `&mut T` can be passed directly to this function
/// as they implicitly coerce to `*const T`.
///
/// # Example
///
/// ```
/// #![feature(ptr_metadata)]
///
/// assert_eq!(std::ptr::metadata("foo"), 3_usize);
/// ```
#[rustc_const_unstable(feature = "ptr_metadata", issue = "81513")]
#[inline]
pub const fn metadata<T: ?Sized>(ptr: *const T) -> <T as Pointee>::Metadata {
    // SAFETY: Accessing the value from the `PtrRepr` union is safe since *const T
    // and PtrComponents<T> have the same memory layouts. Only std can make this
    // guarantee.
    unsafe { PtrRepr { const_ptr: ptr }.components.metadata }
}

/// Forms a (possibly-wide) raw pointer from a data address and metadata.
///
/// This function is safe but the returned pointer is not necessarily safe to dereference.
/// For slices, see the documentation of [`slice::from_raw_parts`] for safety requirements.
/// For trait objects, the metadata must come from a pointer to the same underlying erased type.
///
/// [`slice::from_raw_parts`]: crate::slice::from_raw_parts
#[unstable(feature = "ptr_metadata", issue = "81513")]
#[rustc_const_unstable(feature = "ptr_metadata", issue = "81513")]
#[inline]
pub const fn from_raw_parts<T: ?Sized>(
    data_address: *const (),
    metadata: <T as Pointee>::Metadata,
) -> *const T {
    // SAFETY: Accessing the value from the `PtrRepr` union is safe since *const T
    // and PtrComponents<T> have the same memory layouts. Only std can make this
    // guarantee.
    unsafe { PtrRepr { components: PtrComponents { data_address, metadata } }.const_ptr }
}

/// Performs the same functionality as [`from_raw_parts`], except that a
/// raw `*mut` pointer is returned, as opposed to a raw `*const` pointer.
///
/// See the documentation of [`from_raw_parts`] for more details.
#[unstable(feature = "ptr_metadata", issue = "81513")]
#[rustc_const_unstable(feature = "ptr_metadata", issue = "81513")]
#[inline]
pub const fn from_raw_parts_mut<T: ?Sized>(
    data_address: *mut (),
    metadata: <T as Pointee>::Metadata,
) -> *mut T {
    // SAFETY: Accessing the value from the `PtrRepr` union is safe since *const T
    // and PtrComponents<T> have the same memory layouts. Only std can make this
    // guarantee.
    unsafe { PtrRepr { components: PtrComponents { data_address, metadata } }.mut_ptr }
}

#[repr(C)]
pub(crate) union PtrRepr<T: ?Sized> {
    pub(crate) const_ptr: *const T,
    pub(crate) mut_ptr: *mut T,
    pub(crate) components: PtrComponents<T>,
}

#[repr(C)]
pub(crate) struct PtrComponents<T: ?Sized> {
    pub(crate) data_address: *const (),
    pub(crate) metadata: <T as Pointee>::Metadata,
}

// Manual impl needed to avoid `T: Copy` bound.
impl<T: ?Sized> Copy for PtrComponents<T> {}

// Manual impl needed to avoid `T: Clone` bound.
impl<T: ?Sized> Clone for PtrComponents<T> {
    fn clone(&self) -> Self {
        *self
    }
}

/// The metadata for a `Dyn = dyn SomeTrait` trait object type.
///
/// It is a pointer to a vtable (virtual call table)
/// that represents all the necessary information
/// to manipulate the concrete type stored inside a trait object.
/// The vtable notably it contains:
///
/// * type size
/// * type alignment
/// * a pointer to the type’s `drop_in_place` impl (may be a no-op for plain-old-data)
/// * pointers to all the methods for the type’s implementation of the trait
///
/// Note that the first three are special because they’re necessary to allocate, drop,
/// and deallocate any trait object.
///
/// It is possible to name this struct with a type parameter that is not a `dyn` trait object
/// (for example `DynMetadata<u64>`) but not to obtain a meaningful value of that struct.
#[lang = "dyn_metadata"]
pub struct DynMetadata<Dyn: ?Sized> {
    vtable_ptr: &'static VTable,
    phantom: crate::marker::PhantomData<Dyn>,
}

#[cfg(not(bootstrap))]
extern "C" {
    /// Opaque type for accessing vtables.
    ///
    /// Private implementation detail of `DynMetadata::size_of` etc.
    /// There is conceptually not actually any Abstract Machine memory behind this pointer.
    type VTable;
}

/// The common prefix of all vtables. It is followed by function pointers for trait methods.
///
/// Private implementation detail of `DynMetadata::size_of` etc.
#[repr(C)]
#[cfg(bootstrap)]
struct VTable {
    drop_in_place: fn(*mut ()),
    size_of: usize,
    align_of: usize,
}

impl<Dyn: ?Sized> DynMetadata<Dyn> {
    /// Returns the size of the type associated with this vtable.
    #[inline]
    pub fn size_of(self) -> usize {
        // Note that "size stored in vtable" is *not* the same as "result of size_of_val_raw".
        // Consider a reference like `&(i32, dyn Send)`: the vtable will only store the size of the
        // `Send` part!
        #[cfg(bootstrap)]
        return self.vtable_ptr.size_of;
        #[cfg(not(bootstrap))]
        // SAFETY: DynMetadata always contains a valid vtable pointer
        return unsafe {
            crate::intrinsics::vtable_size(self.vtable_ptr as *const VTable as *const ())
        };
    }

    /// Returns the alignment of the type associated with this vtable.
    #[inline]
    pub fn align_of(self) -> usize {
        #[cfg(bootstrap)]
        return self.vtable_ptr.align_of;
        #[cfg(not(bootstrap))]
        // SAFETY: DynMetadata always contains a valid vtable pointer
        return unsafe {
            crate::intrinsics::vtable_align(self.vtable_ptr as *const VTable as *const ())
        };
    }

    /// Returns the size and alignment together as a `Layout`
    #[inline]
    pub fn layout(self) -> crate::alloc::Layout {
        // SAFETY: the compiler emitted this vtable for a concrete Rust type which
        // is known to have a valid layout. Same rationale as in `Layout::for_value`.
        unsafe { crate::alloc::Layout::from_size_align_unchecked(self.size_of(), self.align_of()) }
    }
}

unsafe impl<Dyn: ?Sized> Send for DynMetadata<Dyn> {}
unsafe impl<Dyn: ?Sized> Sync for DynMetadata<Dyn> {}

impl<Dyn: ?Sized> fmt::Debug for DynMetadata<Dyn> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_tuple("DynMetadata").field(&(self.vtable_ptr as *const VTable)).finish()
    }
}

// Manual impls needed to avoid `Dyn: $Trait` bounds.

impl<Dyn: ?Sized> Unpin for DynMetadata<Dyn> {}

impl<Dyn: ?Sized> Copy for DynMetadata<Dyn> {}

impl<Dyn: ?Sized> Clone for DynMetadata<Dyn> {
    #[inline]
    fn clone(&self) -> Self {
        *self
    }
}

impl<Dyn: ?Sized> Eq for DynMetadata<Dyn> {}

impl<Dyn: ?Sized> PartialEq for DynMetadata<Dyn> {
    #[inline]
    fn eq(&self, other: &Self) -> bool {
        crate::ptr::eq::<VTable>(self.vtable_ptr, other.vtable_ptr)
    }
}

impl<Dyn: ?Sized> Ord for DynMetadata<Dyn> {
    #[inline]
    fn cmp(&self, other: &Self) -> crate::cmp::Ordering {
        (self.vtable_ptr as *const VTable).cmp(&(other.vtable_ptr as *const VTable))
    }
}

impl<Dyn: ?Sized> PartialOrd for DynMetadata<Dyn> {
    #[inline]
    fn partial_cmp(&self, other: &Self) -> Option<crate::cmp::Ordering> {
        Some(self.cmp(other))
    }
}

impl<Dyn: ?Sized> Hash for DynMetadata<Dyn> {
    #[inline]
    fn hash<H: Hasher>(&self, hasher: &mut H) {
        crate::ptr::hash::<VTable, _>(self.vtable_ptr, hasher)
    }
}