core/ptr/
mod.rs

1//! Manually manage memory through raw pointers.
2//!
3//! *[See also the pointer primitive types](pointer).*
4//!
5//! # Safety
6//!
7//! Many functions in this module take raw pointers as arguments and read from or write to them. For
8//! this to be safe, these pointers must be *valid* for the given access. Whether a pointer is valid
9//! depends on the operation it is used for (read or write), and the extent of the memory that is
10//! accessed (i.e., how many bytes are read/written) -- it makes no sense to ask "is this pointer
11//! valid"; one has to ask "is this pointer valid for a given access". Most functions use `*mut T`
12//! and `*const T` to access only a single value, in which case the documentation omits the size and
13//! implicitly assumes it to be `size_of::<T>()` bytes.
14//!
15//! The precise rules for validity are not determined yet. The guarantees that are
16//! provided at this point are very minimal:
17//!
18//! * For memory accesses of [size zero][zst], *every* pointer is valid, including the [null]
19//!   pointer. The following points are only concerned with non-zero-sized accesses.
20//! * A [null] pointer is *never* valid.
21//! * For a pointer to be valid, it is necessary, but not always sufficient, that the pointer be
22//!   *dereferenceable*. The [provenance] of the pointer is used to determine which [allocation]
23//!   it is derived from; a pointer is dereferenceable if the memory range of the given size
24//!   starting at the pointer is entirely contained within the bounds of that allocation. Note
25//!   that in Rust, every (stack-allocated) variable is considered a separate allocation.
26//! * All accesses performed by functions in this module are *non-atomic* in the sense
27//!   of [atomic operations] used to synchronize between threads. This means it is
28//!   undefined behavior to perform two concurrent accesses to the same location from different
29//!   threads unless both accesses only read from memory. Notice that this explicitly
30//!   includes [`read_volatile`] and [`write_volatile`]: Volatile accesses cannot
31//!   be used for inter-thread synchronization, regardless of whether they are acting on
32//!   Rust memory or not.
33//! * The result of casting a reference to a pointer is valid for as long as the
34//!   underlying allocation is live and no reference (just raw pointers) is used to
35//!   access the same memory. That is, reference and pointer accesses cannot be
36//!   interleaved.
37//!
38//! These axioms, along with careful use of [`offset`] for pointer arithmetic,
39//! are enough to correctly implement many useful things in unsafe code. Stronger guarantees
40//! will be provided eventually, as the [aliasing] rules are being determined. For more
41//! information, see the [book] as well as the section in the reference devoted
42//! to [undefined behavior][ub].
43//!
44//! We say that a pointer is "dangling" if it is not valid for any non-zero-sized accesses. This
45//! means out-of-bounds pointers, pointers to freed memory, null pointers, and pointers created with
46//! [`NonNull::dangling`] are all dangling.
47//!
48//! ## Alignment
49//!
50//! Valid raw pointers as defined above are not necessarily properly aligned (where
51//! "proper" alignment is defined by the pointee type, i.e., `*const T` must be
52//! aligned to `align_of::<T>()`). However, most functions require their
53//! arguments to be properly aligned, and will explicitly state
54//! this requirement in their documentation. Notable exceptions to this are
55//! [`read_unaligned`] and [`write_unaligned`].
56//!
57//! When a function requires proper alignment, it does so even if the access
58//! has size 0, i.e., even if memory is not actually touched. Consider using
59//! [`NonNull::dangling`] in such cases.
60//!
61//! ## Pointer to reference conversion
62//!
63//! When converting a pointer to a reference (e.g. via `&*ptr` or `&mut *ptr`),
64//! there are several rules that must be followed:
65//!
66//! * The pointer must be properly aligned.
67//!
68//! * It must be non-null.
69//!
70//! * It must be "dereferenceable" in the sense defined above.
71//!
72//! * The pointer must point to a [valid value] of type `T`.
73//!
74//! * You must enforce Rust's aliasing rules. The exact aliasing rules are not decided yet, so we
75//!   only give a rough overview here. The rules also depend on whether a mutable or a shared
76//!   reference is being created.
77//!   * When creating a mutable reference, then while this reference exists, the memory it points to
78//!     must not get accessed (read or written) through any other pointer or reference not derived
79//!     from this reference.
80//!   * When creating a shared reference, then while this reference exists, the memory it points to
81//!     must not get mutated (except inside `UnsafeCell`).
82//!
83//! If a pointer follows all of these rules, it is said to be
84//! *convertible to a (mutable or shared) reference*.
85// ^ we use this term instead of saying that the produced reference must
86// be valid, as the validity of a reference is easily confused for the
87// validity of the thing it refers to, and while the two concepts are
88// closely related, they are not identical.
89//!
90//! These rules apply even if the result is unused!
91//! (The part about being initialized is not yet fully decided, but until
92//! it is, the only safe approach is to ensure that they are indeed initialized.)
93//!
94//! An example of the implications of the above rules is that an expression such
95//! as `unsafe { &*(0 as *const u8) }` is Immediate Undefined Behavior.
96//!
97//! [valid value]: ../../reference/behavior-considered-undefined.html#invalid-values
98//!
99//! ## Allocation
100//!
101//! <a id="allocated-object"></a> <!-- keep old URLs working -->
102//!
103//! An *allocation* is a subset of program memory which is addressable
104//! from Rust, and within which pointer arithmetic is possible. Examples of
105//! allocations include heap allocations, stack-allocated variables,
106//! statics, and consts. The safety preconditions of some Rust operations -
107//! such as `offset` and field projections (`expr.field`) - are defined in
108//! terms of the allocations on which they operate.
109//!
110//! An allocation has a base address, a size, and a set of memory
111//! addresses. It is possible for an allocation to have zero size, but
112//! such an allocation will still have a base address. The base address
113//! of an allocation is not necessarily unique. While it is currently the
114//! case that an allocation always has a set of memory addresses which is
115//! fully contiguous (i.e., has no "holes"), there is no guarantee that this
116//! will not change in the future.
117//!
118//! Allocations must behave like "normal" memory: in particular, reads must not have
119//! side-effects, and writes must become visible to other threads using the usual synchronization
120//! primitives.
121//!
122//! For any allocation with `base` address, `size`, and a set of
123//! `addresses`, the following are guaranteed:
124//! - For all addresses `a` in `addresses`, `a` is in the range `base .. (base +
125//!   size)` (note that this requires `a < base + size`, not `a <= base + size`)
126//! - `base` is not equal to [`null()`] (i.e., the address with the numerical
127//!   value 0)
128//! - `base + size <= usize::MAX`
129//! - `size <= isize::MAX`
130//!
131//! As a consequence of these guarantees, given any address `a` within the set
132//! of addresses of an allocation:
133//! - It is guaranteed that `a - base` does not overflow `isize`
134//! - It is guaranteed that `a - base` is non-negative
135//! - It is guaranteed that, given `o = a - base` (i.e., the offset of `a` within
136//!   the allocation), `base + o` will not wrap around the address space (in
137//!   other words, will not overflow `usize`)
138//!
139//! [`null()`]: null
140//!
141//! # Provenance
142//!
143//! Pointers are not *simply* an "integer" or "address". For instance, it's uncontroversial
144//! to say that a Use After Free is clearly Undefined Behavior, even if you "get lucky"
145//! and the freed memory gets reallocated before your read/write (in fact this is the
146//! worst-case scenario, UAFs would be much less concerning if this didn't happen!).
147//! As another example, consider that [`wrapping_offset`] is documented to "remember"
148//! the allocation that the original pointer points to, even if it is offset far
149//! outside the memory range occupied by that allocation.
150//! To rationalize claims like this, pointers need to somehow be *more* than just their addresses:
151//! they must have **provenance**.
152//!
153//! A pointer value in Rust semantically contains the following information:
154//!
155//! * The **address** it points to, which can be represented by a `usize`.
156//! * The **provenance** it has, defining the memory it has permission to access. Provenance can be
157//!   absent, in which case the pointer does not have permission to access any memory.
158//!
159//! The exact structure of provenance is not yet specified, but the permission defined by a
160//! pointer's provenance have a *spatial* component, a *temporal* component, and a *mutability*
161//! component:
162//!
163//! * Spatial: The set of memory addresses that the pointer is allowed to access.
164//! * Temporal: The timespan during which the pointer is allowed to access those memory addresses.
165//! * Mutability: Whether the pointer may only access the memory for reads, or also access it for
166//!   writes. Note that this can interact with the other components, e.g. a pointer might permit
167//!   mutation only for a subset of addresses, or only for a subset of its maximal timespan.
168//!
169//! When an [allocation] is created, it has a unique Original Pointer. For alloc
170//! APIs this is literally the pointer the call returns, and for local variables and statics,
171//! this is the name of the variable/static. (This is mildly overloading the term "pointer"
172//! for the sake of brevity/exposition.)
173//!
174//! The Original Pointer for an allocation has provenance that constrains the *spatial*
175//! permissions of this pointer to the memory range of the allocation, and the *temporal*
176//! permissions to the lifetime of the allocation. Provenance is implicitly inherited by all
177//! pointers transitively derived from the Original Pointer through operations like [`offset`],
178//! borrowing, and pointer casts. Some operations may *shrink* the permissions of the derived
179//! provenance, limiting how much memory it can access or how long it's valid for (i.e. borrowing a
180//! subfield and subslicing can shrink the spatial component of provenance, and all borrowing can
181//! shrink the temporal component of provenance). However, no operation can ever *grow* the
182//! permissions of the derived provenance: even if you "know" there is a larger allocation, you
183//! can't derive a pointer with a larger provenance. Similarly, you cannot "recombine" two
184//! contiguous provenances back into one (i.e. with a `fn merge(&[T], &[T]) -> &[T]`).
185//!
186//! A reference to a place always has provenance over at least the memory that place occupies.
187//! A reference to a slice always has provenance over at least the range that slice describes.
188//! Whether and when exactly the provenance of a reference gets "shrunk" to *exactly* fit
189//! the memory it points to is not yet determined.
190//!
191//! A *shared* reference only ever has provenance that permits reading from memory,
192//! and never permits writes, except inside [`UnsafeCell`].
193//!
194//! Provenance can affect whether a program has undefined behavior:
195//!
196//! * It is undefined behavior to access memory through a pointer that does not have provenance over
197//!   that memory. Note that a pointer "at the end" of its provenance is not actually outside its
198//!   provenance, it just has 0 bytes it can load/store. Zero-sized accesses do not require any
199//!   provenance since they access an empty range of memory.
200//!
201//! * It is undefined behavior to [`offset`] a pointer across a memory range that is not contained
202//!   in the allocation it is derived from, or to [`offset_from`] two pointers not derived
203//!   from the same allocation. Provenance is used to say what exactly "derived from" even
204//!   means: the lineage of a pointer is traced back to the Original Pointer it descends from, and
205//!   that identifies the relevant allocation. In particular, it's always UB to offset a
206//!   pointer derived from something that is now deallocated, except if the offset is 0.
207//!
208//! But it *is* still sound to:
209//!
210//! * Create a pointer without provenance from just an address (see [`without_provenance`]). Such a
211//!   pointer cannot be used for memory accesses (except for zero-sized accesses). This can still be
212//!   useful for sentinel values like `null` *or* to represent a tagged pointer that will never be
213//!   dereferenceable. In general, it is always sound for an integer to pretend to be a pointer "for
214//!   fun" as long as you don't use operations on it which require it to be valid (non-zero-sized
215//!   offset, read, write, etc).
216//!
217//! * Forge an allocation of size zero at any sufficiently aligned non-null address.
218//!   i.e. the usual "ZSTs are fake, do what you want" rules apply.
219//!
220//! * [`wrapping_offset`] a pointer outside its provenance. This includes pointers
221//!   which have "no" provenance. In particular, this makes it sound to do pointer tagging tricks.
222//!
223//! * Compare arbitrary pointers by address. Pointer comparison ignores provenance and addresses
224//!   *are* just integers, so there is always a coherent answer, even if the pointers are dangling
225//!   or from different provenances. Note that if you get "lucky" and notice that a pointer at the
226//!   end of one allocation is the "same" address as the start of another allocation,
227//!   anything you do with that fact is *probably* going to be gibberish. The scope of that
228//!   gibberish is kept under control by the fact that the two pointers *still* aren't allowed to
229//!   access the other's allocation (bytes), because they still have different provenance.
230//!
231//! Note that the full definition of provenance in Rust is not decided yet, as this interacts
232//! with the as-yet undecided [aliasing] rules.
233//!
234//! ## Pointers Vs Integers
235//!
236//! From this discussion, it becomes very clear that a `usize` *cannot* accurately represent a pointer,
237//! and converting from a pointer to a `usize` is generally an operation which *only* extracts the
238//! address. Converting this address back into pointer requires somehow answering the question:
239//! which provenance should the resulting pointer have?
240//!
241//! Rust provides two ways of dealing with this situation: *Strict Provenance* and *Exposed Provenance*.
242//!
243//! Note that a pointer *can* represent a `usize` (via [`without_provenance`]), so the right type to
244//! use in situations where a value is "sometimes a pointer and sometimes a bare `usize`" is a
245//! pointer type.
246//!
247//! ## Strict Provenance
248//!
249//! "Strict Provenance" refers to a set of APIs designed to make working with provenance more
250//! explicit. They are intended as substitutes for casting a pointer to an integer and back.
251//!
252//! Entirely avoiding integer-to-pointer casts successfully side-steps the inherent ambiguity of
253//! that operation. This benefits compiler optimizations, and it is pretty much a requirement for
254//! using tools like [Miri] and architectures like [CHERI] that aim to detect and diagnose pointer
255//! misuse.
256//!
257//! The key insight to making programming without integer-to-pointer casts *at all* viable is the
258//! [`with_addr`] method:
259//!
260//! ```text
261//!     /// Creates a new pointer with the given address.
262//!     ///
263//!     /// This performs the same operation as an `addr as ptr` cast, but copies
264//!     /// the *provenance* of `self` to the new pointer.
265//!     /// This allows us to dynamically preserve and propagate this important
266//!     /// information in a way that is otherwise impossible with a unary cast.
267//!     ///
268//!     /// This is equivalent to using `wrapping_offset` to offset `self` to the
269//!     /// given address, and therefore has all the same capabilities and restrictions.
270//!     pub fn with_addr(self, addr: usize) -> Self;
271//! ```
272//!
273//! So you're still able to drop down to the address representation and do whatever
274//! clever bit tricks you want *as long as* you're able to keep around a pointer
275//! into the allocation you care about that can "reconstitute" the provenance.
276//! Usually this is very easy, because you only are taking a pointer, messing with the address,
277//! and then immediately converting back to a pointer. To make this use case more ergonomic,
278//! we provide the [`map_addr`] method.
279//!
280//! To help make it clear that code is "following" Strict Provenance semantics, we also provide an
281//! [`addr`] method which promises that the returned address is not part of a
282//! pointer-integer-pointer roundtrip. In the future we may provide a lint for pointer<->integer
283//! casts to help you audit if your code conforms to strict provenance.
284//!
285//! ### Using Strict Provenance
286//!
287//! Most code needs no changes to conform to strict provenance, as the only really concerning
288//! operation is casts from `usize` to a pointer. For code which *does* cast a `usize` to a pointer,
289//! the scope of the change depends on exactly what you're doing.
290//!
291//! In general, you just need to make sure that if you want to convert a `usize` address to a
292//! pointer and then use that pointer to read/write memory, you need to keep around a pointer
293//! that has sufficient provenance to perform that read/write itself. In this way all of your
294//! casts from an address to a pointer are essentially just applying offsets/indexing.
295//!
296//! This is generally trivial to do for simple cases like tagged pointers *as long as you
297//! represent the tagged pointer as an actual pointer and not a `usize`*. For instance:
298//!
299//! ```
300//! unsafe {
301//!     // A flag we want to pack into our pointer
302//!     static HAS_DATA: usize = 0x1;
303//!     static FLAG_MASK: usize = !HAS_DATA;
304//!
305//!     // Our value, which must have enough alignment to have spare least-significant-bits.
306//!     let my_precious_data: u32 = 17;
307//!     assert!(align_of::<u32>() > 1);
308//!
309//!     // Create a tagged pointer
310//!     let ptr = &my_precious_data as *const u32;
311//!     let tagged = ptr.map_addr(|addr| addr | HAS_DATA);
312//!
313//!     // Check the flag:
314//!     if tagged.addr() & HAS_DATA != 0 {
315//!         // Untag and read the pointer
316//!         let data = *tagged.map_addr(|addr| addr & FLAG_MASK);
317//!         assert_eq!(data, 17);
318//!     } else {
319//!         unreachable!()
320//!     }
321//! }
322//! ```
323//!
324//! (Yes, if you've been using [`AtomicUsize`] for pointers in concurrent datastructures, you should
325//! be using [`AtomicPtr`] instead. If that messes up the way you atomically manipulate pointers,
326//! we would like to know why, and what needs to be done to fix it.)
327//!
328//! Situations where a valid pointer *must* be created from just an address, such as baremetal code
329//! accessing a memory-mapped interface at a fixed address, cannot currently be handled with strict
330//! provenance APIs and should use [exposed provenance](#exposed-provenance).
331//!
332//! ## Exposed Provenance
333//!
334//! As discussed above, integer-to-pointer casts are not possible with Strict Provenance APIs.
335//! This is by design: the goal of Strict Provenance is to provide a clear specification that we are
336//! confident can be formalized unambiguously and can be subject to precise formal reasoning.
337//! Integer-to-pointer casts do not (currently) have such a clear specification.
338//!
339//! However, there exist situations where integer-to-pointer casts cannot be avoided, or
340//! where avoiding them would require major refactoring. Legacy platform APIs also regularly assume
341//! that `usize` can capture all the information that makes up a pointer.
342//! Bare-metal platforms can also require the synthesis of a pointer "out of thin air" without
343//! anywhere to obtain proper provenance from.
344//!
345//! Rust's model for dealing with integer-to-pointer casts is called *Exposed Provenance*. However,
346//! the semantics of Exposed Provenance are on much less solid footing than Strict Provenance, and
347//! at this point it is not yet clear whether a satisfying unambiguous semantics can be defined for
348//! Exposed Provenance. (If that sounds bad, be reassured that other popular languages that provide
349//! integer-to-pointer casts are not faring any better.) Furthermore, Exposed Provenance will not
350//! work (well) with tools like [Miri] and [CHERI].
351//!
352//! Exposed Provenance is provided by the [`expose_provenance`] and [`with_exposed_provenance`] methods,
353//! which are equivalent to `as` casts between pointers and integers.
354//! - [`expose_provenance`] is a lot like [`addr`], but additionally adds the provenance of the
355//!   pointer to a global list of 'exposed' provenances. (This list is purely conceptual, it exists
356//!   for the purpose of specifying Rust but is not materialized in actual executions, except in
357//!   tools like [Miri].)
358//!   Memory which is outside the control of the Rust abstract machine (MMIO registers, for example)
359//!   is always considered to be exposed, so long as this memory is disjoint from memory that will
360//!   be used by the abstract machine such as the stack, heap, and statics.
361//! - [`with_exposed_provenance`] can be used to construct a pointer with one of these previously
362//!   'exposed' provenances. [`with_exposed_provenance`] takes only `addr: usize` as arguments, so
363//!   unlike in [`with_addr`] there is no indication of what the correct provenance for the returned
364//!   pointer is -- and that is exactly what makes integer-to-pointer casts so tricky to rigorously
365//!   specify! The compiler will do its best to pick the right provenance for you, but currently we
366//!   cannot provide any guarantees about which provenance the resulting pointer will have. Only one
367//!   thing is clear: if there is *no* previously 'exposed' provenance that justifies the way the
368//!   returned pointer will be used, the program has undefined behavior.
369//!
370//! If at all possible, we encourage code to be ported to [Strict Provenance] APIs, thus avoiding
371//! the need for Exposed Provenance. Maximizing the amount of such code is a major win for avoiding
372//! specification complexity and to facilitate adoption of tools like [CHERI] and [Miri] that can be
373//! a big help in increasing the confidence in (unsafe) Rust code. However, we acknowledge that this
374//! is not always possible, and offer Exposed Provenance as a way to explicit "opt out" of the
375//! well-defined semantics of Strict Provenance, and "opt in" to the unclear semantics of
376//! integer-to-pointer casts.
377//!
378//! [aliasing]: ../../nomicon/aliasing.html
379//! [allocation]: #allocation
380//! [provenance]: #provenance
381//! [book]: ../../book/ch19-01-unsafe-rust.html#dereferencing-a-raw-pointer
382//! [ub]: ../../reference/behavior-considered-undefined.html
383//! [zst]: ../../nomicon/exotic-sizes.html#zero-sized-types-zsts
384//! [atomic operations]: crate::sync::atomic
385//! [`offset`]: pointer::offset
386//! [`offset_from`]: pointer::offset_from
387//! [`wrapping_offset`]: pointer::wrapping_offset
388//! [`with_addr`]: pointer::with_addr
389//! [`map_addr`]: pointer::map_addr
390//! [`addr`]: pointer::addr
391//! [`AtomicUsize`]: crate::sync::atomic::AtomicUsize
392//! [`AtomicPtr`]: crate::sync::atomic::AtomicPtr
393//! [`expose_provenance`]: pointer::expose_provenance
394//! [`with_exposed_provenance`]: with_exposed_provenance
395//! [Miri]: https://github.com/rust-lang/miri
396//! [CHERI]: https://www.cl.cam.ac.uk/research/security/ctsrd/cheri/
397//! [Strict Provenance]: #strict-provenance
398//! [`UnsafeCell`]: core::cell::UnsafeCell
399
400#![stable(feature = "rust1", since = "1.0.0")]
401// There are many unsafe functions taking pointers that don't dereference them.
402#![allow(clippy::not_unsafe_ptr_arg_deref)]
403
404use crate::cmp::Ordering;
405use crate::intrinsics::const_eval_select;
406use crate::marker::{FnPtr, PointeeSized};
407use crate::mem::{self, MaybeUninit, SizedTypeProperties};
408use crate::num::NonZero;
409use crate::{fmt, hash, intrinsics, ub_checks};
410
411mod alignment;
412#[unstable(feature = "ptr_alignment_type", issue = "102070")]
413pub use alignment::Alignment;
414
415mod metadata;
416#[unstable(feature = "ptr_metadata", issue = "81513")]
417pub use metadata::{DynMetadata, Pointee, Thin, from_raw_parts, from_raw_parts_mut, metadata};
418
419mod non_null;
420#[stable(feature = "nonnull", since = "1.25.0")]
421pub use non_null::NonNull;
422
423mod unique;
424#[unstable(feature = "ptr_internals", issue = "none")]
425pub use unique::Unique;
426
427mod const_ptr;
428mod mut_ptr;
429
430// Some functions are defined here because they accidentally got made
431// available in this module on stable. See <https://github.com/rust-lang/rust/issues/15702>.
432// (`transmute` also falls into this category, but it cannot be wrapped due to the
433// check that `T` and `U` have the same size.)
434
435/// Copies `count * size_of::<T>()` bytes from `src` to `dst`. The source
436/// and destination must *not* overlap.
437///
438/// For regions of memory which might overlap, use [`copy`] instead.
439///
440/// `copy_nonoverlapping` is semantically equivalent to C's [`memcpy`], but
441/// with the source and destination arguments swapped,
442/// and `count` counting the number of `T`s instead of bytes.
443///
444/// The copy is "untyped" in the sense that data may be uninitialized or otherwise violate the
445/// requirements of `T`. The initialization state is preserved exactly.
446///
447/// [`memcpy`]: https://en.cppreference.com/w/c/string/byte/memcpy
448///
449/// # Safety
450///
451/// Behavior is undefined if any of the following conditions are violated:
452///
453/// * `src` must be [valid] for reads of `count * size_of::<T>()` bytes.
454///
455/// * `dst` must be [valid] for writes of `count * size_of::<T>()` bytes.
456///
457/// * Both `src` and `dst` must be properly aligned.
458///
459/// * The region of memory beginning at `src` with a size of `count *
460///   size_of::<T>()` bytes must *not* overlap with the region of memory
461///   beginning at `dst` with the same size.
462///
463/// Like [`read`], `copy_nonoverlapping` creates a bitwise copy of `T`, regardless of
464/// whether `T` is [`Copy`]. If `T` is not [`Copy`], using *both* the values
465/// in the region beginning at `*src` and the region beginning at `*dst` can
466/// [violate memory safety][read-ownership].
467///
468/// Note that even if the effectively copied size (`count * size_of::<T>()`) is
469/// `0`, the pointers must be properly aligned.
470///
471/// [`read`]: crate::ptr::read
472/// [read-ownership]: crate::ptr::read#ownership-of-the-returned-value
473/// [valid]: crate::ptr#safety
474///
475/// # Examples
476///
477/// Manually implement [`Vec::append`]:
478///
479/// ```
480/// use std::ptr;
481///
482/// /// Moves all the elements of `src` into `dst`, leaving `src` empty.
483/// fn append<T>(dst: &mut Vec<T>, src: &mut Vec<T>) {
484///     let src_len = src.len();
485///     let dst_len = dst.len();
486///
487///     // Ensure that `dst` has enough capacity to hold all of `src`.
488///     dst.reserve(src_len);
489///
490///     unsafe {
491///         // The call to add is always safe because `Vec` will never
492///         // allocate more than `isize::MAX` bytes.
493///         let dst_ptr = dst.as_mut_ptr().add(dst_len);
494///         let src_ptr = src.as_ptr();
495///
496///         // Truncate `src` without dropping its contents. We do this first,
497///         // to avoid problems in case something further down panics.
498///         src.set_len(0);
499///
500///         // The two regions cannot overlap because mutable references do
501///         // not alias, and two different vectors cannot own the same
502///         // memory.
503///         ptr::copy_nonoverlapping(src_ptr, dst_ptr, src_len);
504///
505///         // Notify `dst` that it now holds the contents of `src`.
506///         dst.set_len(dst_len + src_len);
507///     }
508/// }
509///
510/// let mut a = vec!['r'];
511/// let mut b = vec!['u', 's', 't'];
512///
513/// append(&mut a, &mut b);
514///
515/// assert_eq!(a, &['r', 'u', 's', 't']);
516/// assert!(b.is_empty());
517/// ```
518///
519/// [`Vec::append`]: ../../std/vec/struct.Vec.html#method.append
520#[doc(alias = "memcpy")]
521#[stable(feature = "rust1", since = "1.0.0")]
522#[rustc_const_stable(feature = "const_intrinsic_copy", since = "1.83.0")]
523#[inline(always)]
524#[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
525#[rustc_diagnostic_item = "ptr_copy_nonoverlapping"]
526pub const unsafe fn copy_nonoverlapping<T>(src: *const T, dst: *mut T, count: usize) {
527    ub_checks::assert_unsafe_precondition!(
528        check_language_ub,
529        "ptr::copy_nonoverlapping requires that both pointer arguments are aligned and non-null \
530        and the specified memory ranges do not overlap",
531        (
532            src: *const () = src as *const (),
533            dst: *mut () = dst as *mut (),
534            size: usize = size_of::<T>(),
535            align: usize = align_of::<T>(),
536            count: usize = count,
537        ) => {
538            let zero_size = count == 0 || size == 0;
539            ub_checks::maybe_is_aligned_and_not_null(src, align, zero_size)
540                && ub_checks::maybe_is_aligned_and_not_null(dst, align, zero_size)
541                && ub_checks::maybe_is_nonoverlapping(src, dst, size, count)
542        }
543    );
544
545    // SAFETY: the safety contract for `copy_nonoverlapping` must be
546    // upheld by the caller.
547    unsafe { crate::intrinsics::copy_nonoverlapping(src, dst, count) }
548}
549
550/// Copies `count * size_of::<T>()` bytes from `src` to `dst`. The source
551/// and destination may overlap.
552///
553/// If the source and destination will *never* overlap,
554/// [`copy_nonoverlapping`] can be used instead.
555///
556/// `copy` is semantically equivalent to C's [`memmove`], but
557/// with the source and destination arguments swapped,
558/// and `count` counting the number of `T`s instead of bytes.
559/// Copying takes place as if the bytes were copied from `src`
560/// to a temporary array and then copied from the array to `dst`.
561///
562/// The copy is "untyped" in the sense that data may be uninitialized or otherwise violate the
563/// requirements of `T`. The initialization state is preserved exactly.
564///
565/// [`memmove`]: https://en.cppreference.com/w/c/string/byte/memmove
566///
567/// # Safety
568///
569/// Behavior is undefined if any of the following conditions are violated:
570///
571/// * `src` must be [valid] for reads of `count * size_of::<T>()` bytes.
572///
573/// * `dst` must be [valid] for writes of `count * size_of::<T>()` bytes, and must remain valid even
574///   when `src` is read for `count * size_of::<T>()` bytes. (This means if the memory ranges
575///   overlap, the `dst` pointer must not be invalidated by `src` reads.)
576///
577/// * Both `src` and `dst` must be properly aligned.
578///
579/// Like [`read`], `copy` creates a bitwise copy of `T`, regardless of
580/// whether `T` is [`Copy`]. If `T` is not [`Copy`], using both the values
581/// in the region beginning at `*src` and the region beginning at `*dst` can
582/// [violate memory safety][read-ownership].
583///
584/// Note that even if the effectively copied size (`count * size_of::<T>()`) is
585/// `0`, the pointers must be properly aligned.
586///
587/// [`read`]: crate::ptr::read
588/// [read-ownership]: crate::ptr::read#ownership-of-the-returned-value
589/// [valid]: crate::ptr#safety
590///
591/// # Examples
592///
593/// Efficiently create a Rust vector from an unsafe buffer:
594///
595/// ```
596/// use std::ptr;
597///
598/// /// # Safety
599/// ///
600/// /// * `ptr` must be correctly aligned for its type and non-zero.
601/// /// * `ptr` must be valid for reads of `elts` contiguous elements of type `T`.
602/// /// * Those elements must not be used after calling this function unless `T: Copy`.
603/// # #[allow(dead_code)]
604/// unsafe fn from_buf_raw<T>(ptr: *const T, elts: usize) -> Vec<T> {
605///     let mut dst = Vec::with_capacity(elts);
606///
607///     // SAFETY: Our precondition ensures the source is aligned and valid,
608///     // and `Vec::with_capacity` ensures that we have usable space to write them.
609///     unsafe { ptr::copy(ptr, dst.as_mut_ptr(), elts); }
610///
611///     // SAFETY: We created it with this much capacity earlier,
612///     // and the previous `copy` has initialized these elements.
613///     unsafe { dst.set_len(elts); }
614///     dst
615/// }
616/// ```
617#[doc(alias = "memmove")]
618#[stable(feature = "rust1", since = "1.0.0")]
619#[rustc_const_stable(feature = "const_intrinsic_copy", since = "1.83.0")]
620#[inline(always)]
621#[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
622#[rustc_diagnostic_item = "ptr_copy"]
623pub const unsafe fn copy<T>(src: *const T, dst: *mut T, count: usize) {
624    // SAFETY: the safety contract for `copy` must be upheld by the caller.
625    unsafe {
626        ub_checks::assert_unsafe_precondition!(
627            check_language_ub,
628            "ptr::copy requires that both pointer arguments are aligned and non-null",
629            (
630                src: *const () = src as *const (),
631                dst: *mut () = dst as *mut (),
632                align: usize = align_of::<T>(),
633                zero_size: bool = T::IS_ZST || count == 0,
634            ) =>
635            ub_checks::maybe_is_aligned_and_not_null(src, align, zero_size)
636                && ub_checks::maybe_is_aligned_and_not_null(dst, align, zero_size)
637        );
638        crate::intrinsics::copy(src, dst, count)
639    }
640}
641
642/// Sets `count * size_of::<T>()` bytes of memory starting at `dst` to
643/// `val`.
644///
645/// `write_bytes` is similar to C's [`memset`], but sets `count *
646/// size_of::<T>()` bytes to `val`.
647///
648/// [`memset`]: https://en.cppreference.com/w/c/string/byte/memset
649///
650/// # Safety
651///
652/// Behavior is undefined if any of the following conditions are violated:
653///
654/// * `dst` must be [valid] for writes of `count * size_of::<T>()` bytes.
655///
656/// * `dst` must be properly aligned.
657///
658/// Note that even if the effectively copied size (`count * size_of::<T>()`) is
659/// `0`, the pointer must be properly aligned.
660///
661/// Additionally, note that changing `*dst` in this way can easily lead to undefined behavior (UB)
662/// later if the written bytes are not a valid representation of some `T`. For instance, the
663/// following is an **incorrect** use of this function:
664///
665/// ```rust,no_run
666/// unsafe {
667///     let mut value: u8 = 0;
668///     let ptr: *mut bool = &mut value as *mut u8 as *mut bool;
669///     let _bool = ptr.read(); // This is fine, `ptr` points to a valid `bool`.
670///     ptr.write_bytes(42u8, 1); // This function itself does not cause UB...
671///     let _bool = ptr.read(); // ...but it makes this operation UB! ⚠️
672/// }
673/// ```
674///
675/// [valid]: crate::ptr#safety
676///
677/// # Examples
678///
679/// Basic usage:
680///
681/// ```
682/// use std::ptr;
683///
684/// let mut vec = vec![0u32; 4];
685/// unsafe {
686///     let vec_ptr = vec.as_mut_ptr();
687///     ptr::write_bytes(vec_ptr, 0xfe, 2);
688/// }
689/// assert_eq!(vec, [0xfefefefe, 0xfefefefe, 0, 0]);
690/// ```
691#[doc(alias = "memset")]
692#[stable(feature = "rust1", since = "1.0.0")]
693#[rustc_const_stable(feature = "const_ptr_write", since = "1.83.0")]
694#[inline(always)]
695#[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
696#[rustc_diagnostic_item = "ptr_write_bytes"]
697pub const unsafe fn write_bytes<T>(dst: *mut T, val: u8, count: usize) {
698    // SAFETY: the safety contract for `write_bytes` must be upheld by the caller.
699    unsafe {
700        ub_checks::assert_unsafe_precondition!(
701            check_language_ub,
702            "ptr::write_bytes requires that the destination pointer is aligned and non-null",
703            (
704                addr: *const () = dst as *const (),
705                align: usize = align_of::<T>(),
706                zero_size: bool = T::IS_ZST || count == 0,
707            ) => ub_checks::maybe_is_aligned_and_not_null(addr, align, zero_size)
708        );
709        crate::intrinsics::write_bytes(dst, val, count)
710    }
711}
712
713/// Executes the destructor (if any) of the pointed-to value.
714///
715/// This is almost the same as calling [`ptr::read`] and discarding
716/// the result, but has the following advantages:
717// FIXME: say something more useful than "almost the same"?
718// There are open questions here: `read` requires the value to be fully valid, e.g. if `T` is a
719// `bool` it must be 0 or 1, if it is a reference then it must be dereferenceable. `drop_in_place`
720// only requires that `*to_drop` be "valid for dropping" and we have not defined what that means. In
721// Miri it currently (May 2024) requires nothing at all for types without drop glue.
722///
723/// * It is *required* to use `drop_in_place` to drop unsized types like
724///   trait objects, because they can't be read out onto the stack and
725///   dropped normally.
726///
727/// * It is friendlier to the optimizer to do this over [`ptr::read`] when
728///   dropping manually allocated memory (e.g., in the implementations of
729///   `Box`/`Rc`/`Vec`), as the compiler doesn't need to prove that it's
730///   sound to elide the copy.
731///
732/// * It can be used to drop [pinned] data when `T` is not `repr(packed)`
733///   (pinned data must not be moved before it is dropped).
734///
735/// Unaligned values cannot be dropped in place, they must be copied to an aligned
736/// location first using [`ptr::read_unaligned`]. For packed structs, this move is
737/// done automatically by the compiler. This means the fields of packed structs
738/// are not dropped in-place.
739///
740/// [`ptr::read`]: self::read
741/// [`ptr::read_unaligned`]: self::read_unaligned
742/// [pinned]: crate::pin
743///
744/// # Safety
745///
746/// Behavior is undefined if any of the following conditions are violated:
747///
748/// * `to_drop` must be [valid] for both reads and writes.
749///
750/// * `to_drop` must be properly aligned, even if `T` has size 0.
751///
752/// * `to_drop` must be nonnull, even if `T` has size 0.
753///
754/// * The value `to_drop` points to must be valid for dropping, which may mean
755///   it must uphold additional invariants. These invariants depend on the type
756///   of the value being dropped. For instance, when dropping a Box, the box's
757///   pointer to the heap must be valid.
758///
759/// * While `drop_in_place` is executing, the only way to access parts of
760///   `to_drop` is through the `&mut self` references supplied to the
761///   `Drop::drop` methods that `drop_in_place` invokes.
762///
763/// Additionally, if `T` is not [`Copy`], using the pointed-to value after
764/// calling `drop_in_place` can cause undefined behavior. Note that `*to_drop =
765/// foo` counts as a use because it will cause the value to be dropped
766/// again. [`write()`] can be used to overwrite data without causing it to be
767/// dropped.
768///
769/// [valid]: self#safety
770///
771/// # Examples
772///
773/// Manually remove the last item from a vector:
774///
775/// ```
776/// use std::ptr;
777/// use std::rc::Rc;
778///
779/// let last = Rc::new(1);
780/// let weak = Rc::downgrade(&last);
781///
782/// let mut v = vec![Rc::new(0), last];
783///
784/// unsafe {
785///     // Get a raw pointer to the last element in `v`.
786///     let ptr = &mut v[1] as *mut _;
787///     // Shorten `v` to prevent the last item from being dropped. We do that first,
788///     // to prevent issues if the `drop_in_place` below panics.
789///     v.set_len(1);
790///     // Without a call `drop_in_place`, the last item would never be dropped,
791///     // and the memory it manages would be leaked.
792///     ptr::drop_in_place(ptr);
793/// }
794///
795/// assert_eq!(v, &[0.into()]);
796///
797/// // Ensure that the last item was dropped.
798/// assert!(weak.upgrade().is_none());
799/// ```
800#[stable(feature = "drop_in_place", since = "1.8.0")]
801#[lang = "drop_in_place"]
802#[allow(unconditional_recursion)]
803#[rustc_diagnostic_item = "ptr_drop_in_place"]
804pub unsafe fn drop_in_place<T: PointeeSized>(to_drop: *mut T) {
805    // Code here does not matter - this is replaced by the
806    // real drop glue by the compiler.
807
808    // SAFETY: see comment above
809    unsafe { drop_in_place(to_drop) }
810}
811
812/// Creates a null raw pointer.
813///
814/// This function is equivalent to zero-initializing the pointer:
815/// `MaybeUninit::<*const T>::zeroed().assume_init()`.
816/// The resulting pointer has the address 0.
817///
818/// # Examples
819///
820/// ```
821/// use std::ptr;
822///
823/// let p: *const i32 = ptr::null();
824/// assert!(p.is_null());
825/// assert_eq!(p as usize, 0); // this pointer has the address 0
826/// ```
827#[inline(always)]
828#[must_use]
829#[stable(feature = "rust1", since = "1.0.0")]
830#[rustc_promotable]
831#[rustc_const_stable(feature = "const_ptr_null", since = "1.24.0")]
832#[rustc_diagnostic_item = "ptr_null"]
833pub const fn null<T: PointeeSized + Thin>() -> *const T {
834    from_raw_parts(without_provenance::<()>(0), ())
835}
836
837/// Creates a null mutable raw pointer.
838///
839/// This function is equivalent to zero-initializing the pointer:
840/// `MaybeUninit::<*mut T>::zeroed().assume_init()`.
841/// The resulting pointer has the address 0.
842///
843/// # Examples
844///
845/// ```
846/// use std::ptr;
847///
848/// let p: *mut i32 = ptr::null_mut();
849/// assert!(p.is_null());
850/// assert_eq!(p as usize, 0); // this pointer has the address 0
851/// ```
852#[inline(always)]
853#[must_use]
854#[stable(feature = "rust1", since = "1.0.0")]
855#[rustc_promotable]
856#[rustc_const_stable(feature = "const_ptr_null", since = "1.24.0")]
857#[rustc_diagnostic_item = "ptr_null_mut"]
858pub const fn null_mut<T: PointeeSized + Thin>() -> *mut T {
859    from_raw_parts_mut(without_provenance_mut::<()>(0), ())
860}
861
862/// Creates a pointer with the given address and no [provenance][crate::ptr#provenance].
863///
864/// This is equivalent to `ptr::null().with_addr(addr)`.
865///
866/// Without provenance, this pointer is not associated with any actual allocation. Such a
867/// no-provenance pointer may be used for zero-sized memory accesses (if suitably aligned), but
868/// non-zero-sized memory accesses with a no-provenance pointer are UB. No-provenance pointers are
869/// little more than a `usize` address in disguise.
870///
871/// This is different from `addr as *const T`, which creates a pointer that picks up a previously
872/// exposed provenance. See [`with_exposed_provenance`] for more details on that operation.
873///
874/// This is a [Strict Provenance][crate::ptr#strict-provenance] API.
875#[inline(always)]
876#[must_use]
877#[stable(feature = "strict_provenance", since = "1.84.0")]
878#[rustc_const_stable(feature = "strict_provenance", since = "1.84.0")]
879pub const fn without_provenance<T>(addr: usize) -> *const T {
880    without_provenance_mut(addr)
881}
882
883/// Creates a new pointer that is dangling, but non-null and well-aligned.
884///
885/// This is useful for initializing types which lazily allocate, like
886/// `Vec::new` does.
887///
888/// Note that the pointer value may potentially represent a valid pointer to
889/// a `T`, which means this must not be used as a "not yet initialized"
890/// sentinel value. Types that lazily allocate must track initialization by
891/// some other means.
892#[inline(always)]
893#[must_use]
894#[stable(feature = "strict_provenance", since = "1.84.0")]
895#[rustc_const_stable(feature = "strict_provenance", since = "1.84.0")]
896pub const fn dangling<T>() -> *const T {
897    dangling_mut()
898}
899
900/// Creates a pointer with the given address and no [provenance][crate::ptr#provenance].
901///
902/// This is equivalent to `ptr::null_mut().with_addr(addr)`.
903///
904/// Without provenance, this pointer is not associated with any actual allocation. Such a
905/// no-provenance pointer may be used for zero-sized memory accesses (if suitably aligned), but
906/// non-zero-sized memory accesses with a no-provenance pointer are UB. No-provenance pointers are
907/// little more than a `usize` address in disguise.
908///
909/// This is different from `addr as *mut T`, which creates a pointer that picks up a previously
910/// exposed provenance. See [`with_exposed_provenance_mut`] for more details on that operation.
911///
912/// This is a [Strict Provenance][crate::ptr#strict-provenance] API.
913#[inline(always)]
914#[must_use]
915#[stable(feature = "strict_provenance", since = "1.84.0")]
916#[rustc_const_stable(feature = "strict_provenance", since = "1.84.0")]
917pub const fn without_provenance_mut<T>(addr: usize) -> *mut T {
918    // An int-to-pointer transmute currently has exactly the intended semantics: it creates a
919    // pointer without provenance. Note that this is *not* a stable guarantee about transmute
920    // semantics, it relies on sysroot crates having special status.
921    // SAFETY: every valid integer is also a valid pointer (as long as you don't dereference that
922    // pointer).
923    unsafe { mem::transmute(addr) }
924}
925
926/// Creates a new pointer that is dangling, but non-null and well-aligned.
927///
928/// This is useful for initializing types which lazily allocate, like
929/// `Vec::new` does.
930///
931/// Note that the pointer value may potentially represent a valid pointer to
932/// a `T`, which means this must not be used as a "not yet initialized"
933/// sentinel value. Types that lazily allocate must track initialization by
934/// some other means.
935#[inline(always)]
936#[must_use]
937#[stable(feature = "strict_provenance", since = "1.84.0")]
938#[rustc_const_stable(feature = "strict_provenance", since = "1.84.0")]
939pub const fn dangling_mut<T>() -> *mut T {
940    NonNull::dangling().as_ptr()
941}
942
943/// Converts an address back to a pointer, picking up some previously 'exposed'
944/// [provenance][crate::ptr#provenance].
945///
946/// This is fully equivalent to `addr as *const T`. The provenance of the returned pointer is that
947/// of *some* pointer that was previously exposed by passing it to
948/// [`expose_provenance`][pointer::expose_provenance], or a `ptr as usize` cast. In addition, memory
949/// which is outside the control of the Rust abstract machine (MMIO registers, for example) is
950/// always considered to be accessible with an exposed provenance, so long as this memory is disjoint
951/// from memory that will be used by the abstract machine such as the stack, heap, and statics.
952///
953/// The exact provenance that gets picked is not specified. The compiler will do its best to pick
954/// the "right" provenance for you (whatever that may be), but currently we cannot provide any
955/// guarantees about which provenance the resulting pointer will have -- and therefore there
956/// is no definite specification for which memory the resulting pointer may access.
957///
958/// If there is *no* previously 'exposed' provenance that justifies the way the returned pointer
959/// will be used, the program has undefined behavior. In particular, the aliasing rules still apply:
960/// pointers and references that have been invalidated due to aliasing accesses cannot be used
961/// anymore, even if they have been exposed!
962///
963/// Due to its inherent ambiguity, this operation may not be supported by tools that help you to
964/// stay conformant with the Rust memory model. It is recommended to use [Strict
965/// Provenance][self#strict-provenance] APIs such as [`with_addr`][pointer::with_addr] wherever
966/// possible.
967///
968/// On most platforms this will produce a value with the same bytes as the address. Platforms
969/// which need to store additional information in a pointer may not support this operation,
970/// since it is generally not possible to actually *compute* which provenance the returned
971/// pointer has to pick up.
972///
973/// This is an [Exposed Provenance][crate::ptr#exposed-provenance] API.
974#[must_use]
975#[inline(always)]
976#[stable(feature = "exposed_provenance", since = "1.84.0")]
977#[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
978#[allow(fuzzy_provenance_casts)] // this *is* the explicit provenance API one should use instead
979pub fn with_exposed_provenance<T>(addr: usize) -> *const T {
980    addr as *const T
981}
982
983/// Converts an address back to a mutable pointer, picking up some previously 'exposed'
984/// [provenance][crate::ptr#provenance].
985///
986/// This is fully equivalent to `addr as *mut T`. The provenance of the returned pointer is that
987/// of *some* pointer that was previously exposed by passing it to
988/// [`expose_provenance`][pointer::expose_provenance], or a `ptr as usize` cast. In addition, memory
989/// which is outside the control of the Rust abstract machine (MMIO registers, for example) is
990/// always considered to be accessible with an exposed provenance, so long as this memory is disjoint
991/// from memory that will be used by the abstract machine such as the stack, heap, and statics.
992///
993/// The exact provenance that gets picked is not specified. The compiler will do its best to pick
994/// the "right" provenance for you (whatever that may be), but currently we cannot provide any
995/// guarantees about which provenance the resulting pointer will have -- and therefore there
996/// is no definite specification for which memory the resulting pointer may access.
997///
998/// If there is *no* previously 'exposed' provenance that justifies the way the returned pointer
999/// will be used, the program has undefined behavior. In particular, the aliasing rules still apply:
1000/// pointers and references that have been invalidated due to aliasing accesses cannot be used
1001/// anymore, even if they have been exposed!
1002///
1003/// Due to its inherent ambiguity, this operation may not be supported by tools that help you to
1004/// stay conformant with the Rust memory model. It is recommended to use [Strict
1005/// Provenance][self#strict-provenance] APIs such as [`with_addr`][pointer::with_addr] wherever
1006/// possible.
1007///
1008/// On most platforms this will produce a value with the same bytes as the address. Platforms
1009/// which need to store additional information in a pointer may not support this operation,
1010/// since it is generally not possible to actually *compute* which provenance the returned
1011/// pointer has to pick up.
1012///
1013/// This is an [Exposed Provenance][crate::ptr#exposed-provenance] API.
1014#[must_use]
1015#[inline(always)]
1016#[stable(feature = "exposed_provenance", since = "1.84.0")]
1017#[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
1018#[allow(fuzzy_provenance_casts)] // this *is* the explicit provenance API one should use instead
1019pub fn with_exposed_provenance_mut<T>(addr: usize) -> *mut T {
1020    addr as *mut T
1021}
1022
1023/// Converts a reference to a raw pointer.
1024///
1025/// For `r: &T`, `from_ref(r)` is equivalent to `r as *const T` (except for the caveat noted below),
1026/// but is a bit safer since it will never silently change type or mutability, in particular if the
1027/// code is refactored.
1028///
1029/// The caller must ensure that the pointee outlives the pointer this function returns, or else it
1030/// will end up dangling.
1031///
1032/// The caller must also ensure that the memory the pointer (non-transitively) points to is never
1033/// written to (except inside an `UnsafeCell`) using this pointer or any pointer derived from it. If
1034/// you need to mutate the pointee, use [`from_mut`]. Specifically, to turn a mutable reference `m:
1035/// &mut T` into `*const T`, prefer `from_mut(m).cast_const()` to obtain a pointer that can later be
1036/// used for mutation.
1037///
1038/// ## Interaction with lifetime extension
1039///
1040/// Note that this has subtle interactions with the rules for lifetime extension of temporaries in
1041/// tail expressions. This code is valid, albeit in a non-obvious way:
1042/// ```rust
1043/// # type T = i32;
1044/// # fn foo() -> T { 42 }
1045/// // The temporary holding the return value of `foo` has its lifetime extended,
1046/// // because the surrounding expression involves no function call.
1047/// let p = &foo() as *const T;
1048/// unsafe { p.read() };
1049/// ```
1050/// Naively replacing the cast with `from_ref` is not valid:
1051/// ```rust,no_run
1052/// # use std::ptr;
1053/// # type T = i32;
1054/// # fn foo() -> T { 42 }
1055/// // The temporary holding the return value of `foo` does *not* have its lifetime extended,
1056/// // because the surrounding expression involves a function call.
1057/// let p = ptr::from_ref(&foo());
1058/// unsafe { p.read() }; // UB! Reading from a dangling pointer ⚠️
1059/// ```
1060/// The recommended way to write this code is to avoid relying on lifetime extension
1061/// when raw pointers are involved:
1062/// ```rust
1063/// # use std::ptr;
1064/// # type T = i32;
1065/// # fn foo() -> T { 42 }
1066/// let x = foo();
1067/// let p = ptr::from_ref(&x);
1068/// unsafe { p.read() };
1069/// ```
1070#[inline(always)]
1071#[must_use]
1072#[stable(feature = "ptr_from_ref", since = "1.76.0")]
1073#[rustc_const_stable(feature = "ptr_from_ref", since = "1.76.0")]
1074#[rustc_never_returns_null_ptr]
1075#[rustc_diagnostic_item = "ptr_from_ref"]
1076pub const fn from_ref<T: PointeeSized>(r: &T) -> *const T {
1077    r
1078}
1079
1080/// Converts a mutable reference to a raw pointer.
1081///
1082/// For `r: &mut T`, `from_mut(r)` is equivalent to `r as *mut T` (except for the caveat noted
1083/// below), but is a bit safer since it will never silently change type or mutability, in particular
1084/// if the code is refactored.
1085///
1086/// The caller must ensure that the pointee outlives the pointer this function returns, or else it
1087/// will end up dangling.
1088///
1089/// ## Interaction with lifetime extension
1090///
1091/// Note that this has subtle interactions with the rules for lifetime extension of temporaries in
1092/// tail expressions. This code is valid, albeit in a non-obvious way:
1093/// ```rust
1094/// # type T = i32;
1095/// # fn foo() -> T { 42 }
1096/// // The temporary holding the return value of `foo` has its lifetime extended,
1097/// // because the surrounding expression involves no function call.
1098/// let p = &mut foo() as *mut T;
1099/// unsafe { p.write(T::default()) };
1100/// ```
1101/// Naively replacing the cast with `from_mut` is not valid:
1102/// ```rust,no_run
1103/// # use std::ptr;
1104/// # type T = i32;
1105/// # fn foo() -> T { 42 }
1106/// // The temporary holding the return value of `foo` does *not* have its lifetime extended,
1107/// // because the surrounding expression involves a function call.
1108/// let p = ptr::from_mut(&mut foo());
1109/// unsafe { p.write(T::default()) }; // UB! Writing to a dangling pointer ⚠️
1110/// ```
1111/// The recommended way to write this code is to avoid relying on lifetime extension
1112/// when raw pointers are involved:
1113/// ```rust
1114/// # use std::ptr;
1115/// # type T = i32;
1116/// # fn foo() -> T { 42 }
1117/// let mut x = foo();
1118/// let p = ptr::from_mut(&mut x);
1119/// unsafe { p.write(T::default()) };
1120/// ```
1121#[inline(always)]
1122#[must_use]
1123#[stable(feature = "ptr_from_ref", since = "1.76.0")]
1124#[rustc_const_stable(feature = "ptr_from_ref", since = "1.76.0")]
1125#[rustc_never_returns_null_ptr]
1126pub const fn from_mut<T: PointeeSized>(r: &mut T) -> *mut T {
1127    r
1128}
1129
1130/// Forms a raw slice from a pointer and a length.
1131///
1132/// The `len` argument is the number of **elements**, not the number of bytes.
1133///
1134/// This function is safe, but actually using the return value is unsafe.
1135/// See the documentation of [`slice::from_raw_parts`] for slice safety requirements.
1136///
1137/// [`slice::from_raw_parts`]: crate::slice::from_raw_parts
1138///
1139/// # Examples
1140///
1141/// ```rust
1142/// use std::ptr;
1143///
1144/// // create a slice pointer when starting out with a pointer to the first element
1145/// let x = [5, 6, 7];
1146/// let raw_pointer = x.as_ptr();
1147/// let slice = ptr::slice_from_raw_parts(raw_pointer, 3);
1148/// assert_eq!(unsafe { &*slice }[2], 7);
1149/// ```
1150///
1151/// You must ensure that the pointer is valid and not null before dereferencing
1152/// the raw slice. A slice reference must never have a null pointer, even if it's empty.
1153///
1154/// ```rust,should_panic
1155/// use std::ptr;
1156/// let danger: *const [u8] = ptr::slice_from_raw_parts(ptr::null(), 0);
1157/// unsafe {
1158///     danger.as_ref().expect("references must not be null");
1159/// }
1160/// ```
1161#[inline]
1162#[stable(feature = "slice_from_raw_parts", since = "1.42.0")]
1163#[rustc_const_stable(feature = "const_slice_from_raw_parts", since = "1.64.0")]
1164#[rustc_diagnostic_item = "ptr_slice_from_raw_parts"]
1165pub const fn slice_from_raw_parts<T>(data: *const T, len: usize) -> *const [T] {
1166    from_raw_parts(data, len)
1167}
1168
1169/// Forms a raw mutable slice from a pointer and a length.
1170///
1171/// The `len` argument is the number of **elements**, not the number of bytes.
1172///
1173/// Performs the same functionality as [`slice_from_raw_parts`], except that a
1174/// raw mutable slice is returned, as opposed to a raw immutable slice.
1175///
1176/// This function is safe, but actually using the return value is unsafe.
1177/// See the documentation of [`slice::from_raw_parts_mut`] for slice safety requirements.
1178///
1179/// [`slice::from_raw_parts_mut`]: crate::slice::from_raw_parts_mut
1180///
1181/// # Examples
1182///
1183/// ```rust
1184/// use std::ptr;
1185///
1186/// let x = &mut [5, 6, 7];
1187/// let raw_pointer = x.as_mut_ptr();
1188/// let slice = ptr::slice_from_raw_parts_mut(raw_pointer, 3);
1189///
1190/// unsafe {
1191///     (*slice)[2] = 99; // assign a value at an index in the slice
1192/// };
1193///
1194/// assert_eq!(unsafe { &*slice }[2], 99);
1195/// ```
1196///
1197/// You must ensure that the pointer is valid and not null before dereferencing
1198/// the raw slice. A slice reference must never have a null pointer, even if it's empty.
1199///
1200/// ```rust,should_panic
1201/// use std::ptr;
1202/// let danger: *mut [u8] = ptr::slice_from_raw_parts_mut(ptr::null_mut(), 0);
1203/// unsafe {
1204///     danger.as_mut().expect("references must not be null");
1205/// }
1206/// ```
1207#[inline]
1208#[stable(feature = "slice_from_raw_parts", since = "1.42.0")]
1209#[rustc_const_stable(feature = "const_slice_from_raw_parts_mut", since = "1.83.0")]
1210#[rustc_diagnostic_item = "ptr_slice_from_raw_parts_mut"]
1211pub const fn slice_from_raw_parts_mut<T>(data: *mut T, len: usize) -> *mut [T] {
1212    from_raw_parts_mut(data, len)
1213}
1214
1215/// Swaps the values at two mutable locations of the same type, without
1216/// deinitializing either.
1217///
1218/// But for the following exceptions, this function is semantically
1219/// equivalent to [`mem::swap`]:
1220///
1221/// * It operates on raw pointers instead of references. When references are
1222///   available, [`mem::swap`] should be preferred.
1223///
1224/// * The two pointed-to values may overlap. If the values do overlap, then the
1225///   overlapping region of memory from `x` will be used. This is demonstrated
1226///   in the second example below.
1227///
1228/// * The operation is "untyped" in the sense that data may be uninitialized or otherwise violate
1229///   the requirements of `T`. The initialization state is preserved exactly.
1230///
1231/// # Safety
1232///
1233/// Behavior is undefined if any of the following conditions are violated:
1234///
1235/// * Both `x` and `y` must be [valid] for both reads and writes. They must remain valid even when the
1236///   other pointer is written. (This means if the memory ranges overlap, the two pointers must not
1237///   be subject to aliasing restrictions relative to each other.)
1238///
1239/// * Both `x` and `y` must be properly aligned.
1240///
1241/// Note that even if `T` has size `0`, the pointers must be properly aligned.
1242///
1243/// [valid]: self#safety
1244///
1245/// # Examples
1246///
1247/// Swapping two non-overlapping regions:
1248///
1249/// ```
1250/// use std::ptr;
1251///
1252/// let mut array = [0, 1, 2, 3];
1253///
1254/// let (x, y) = array.split_at_mut(2);
1255/// let x = x.as_mut_ptr().cast::<[u32; 2]>(); // this is `array[0..2]`
1256/// let y = y.as_mut_ptr().cast::<[u32; 2]>(); // this is `array[2..4]`
1257///
1258/// unsafe {
1259///     ptr::swap(x, y);
1260///     assert_eq!([2, 3, 0, 1], array);
1261/// }
1262/// ```
1263///
1264/// Swapping two overlapping regions:
1265///
1266/// ```
1267/// use std::ptr;
1268///
1269/// let mut array: [i32; 4] = [0, 1, 2, 3];
1270///
1271/// let array_ptr: *mut i32 = array.as_mut_ptr();
1272///
1273/// let x = array_ptr as *mut [i32; 3]; // this is `array[0..3]`
1274/// let y = unsafe { array_ptr.add(1) } as *mut [i32; 3]; // this is `array[1..4]`
1275///
1276/// unsafe {
1277///     ptr::swap(x, y);
1278///     // The indices `1..3` of the slice overlap between `x` and `y`.
1279///     // Reasonable results would be for to them be `[2, 3]`, so that indices `0..3` are
1280///     // `[1, 2, 3]` (matching `y` before the `swap`); or for them to be `[0, 1]`
1281///     // so that indices `1..4` are `[0, 1, 2]` (matching `x` before the `swap`).
1282///     // This implementation is defined to make the latter choice.
1283///     assert_eq!([1, 0, 1, 2], array);
1284/// }
1285/// ```
1286#[inline]
1287#[stable(feature = "rust1", since = "1.0.0")]
1288#[rustc_const_stable(feature = "const_swap", since = "1.85.0")]
1289#[rustc_diagnostic_item = "ptr_swap"]
1290pub const unsafe fn swap<T>(x: *mut T, y: *mut T) {
1291    // Give ourselves some scratch space to work with.
1292    // We do not have to worry about drops: `MaybeUninit` does nothing when dropped.
1293    let mut tmp = MaybeUninit::<T>::uninit();
1294
1295    // Perform the swap
1296    // SAFETY: the caller must guarantee that `x` and `y` are
1297    // valid for writes and properly aligned. `tmp` cannot be
1298    // overlapping either `x` or `y` because `tmp` was just allocated
1299    // on the stack as a separate allocation.
1300    unsafe {
1301        copy_nonoverlapping(x, tmp.as_mut_ptr(), 1);
1302        copy(y, x, 1); // `x` and `y` may overlap
1303        copy_nonoverlapping(tmp.as_ptr(), y, 1);
1304    }
1305}
1306
1307/// Swaps `count * size_of::<T>()` bytes between the two regions of memory
1308/// beginning at `x` and `y`. The two regions must *not* overlap.
1309///
1310/// The operation is "untyped" in the sense that data may be uninitialized or otherwise violate the
1311/// requirements of `T`. The initialization state is preserved exactly.
1312///
1313/// # Safety
1314///
1315/// Behavior is undefined if any of the following conditions are violated:
1316///
1317/// * Both `x` and `y` must be [valid] for both reads and writes of `count *
1318///   size_of::<T>()` bytes.
1319///
1320/// * Both `x` and `y` must be properly aligned.
1321///
1322/// * The region of memory beginning at `x` with a size of `count *
1323///   size_of::<T>()` bytes must *not* overlap with the region of memory
1324///   beginning at `y` with the same size.
1325///
1326/// Note that even if the effectively copied size (`count * size_of::<T>()`) is `0`,
1327/// the pointers must be properly aligned.
1328///
1329/// [valid]: self#safety
1330///
1331/// # Examples
1332///
1333/// Basic usage:
1334///
1335/// ```
1336/// use std::ptr;
1337///
1338/// let mut x = [1, 2, 3, 4];
1339/// let mut y = [7, 8, 9];
1340///
1341/// unsafe {
1342///     ptr::swap_nonoverlapping(x.as_mut_ptr(), y.as_mut_ptr(), 2);
1343/// }
1344///
1345/// assert_eq!(x, [7, 8, 3, 4]);
1346/// assert_eq!(y, [1, 2, 9]);
1347/// ```
1348///
1349/// # Const evaluation limitations
1350///
1351/// If this function is invoked during const-evaluation, the current implementation has a small (and
1352/// rarely relevant) limitation: if `count` is at least 2 and the data pointed to by `x` or `y`
1353/// contains a pointer that crosses the boundary of two `T`-sized chunks of memory, the function may
1354/// fail to evaluate (similar to a panic during const-evaluation). This behavior may change in the
1355/// future.
1356///
1357/// The limitation is illustrated by the following example:
1358///
1359/// ```
1360/// use std::mem::size_of;
1361/// use std::ptr;
1362///
1363/// const { unsafe {
1364///     const PTR_SIZE: usize = size_of::<*const i32>();
1365///     let mut data1 = [0u8; PTR_SIZE];
1366///     let mut data2 = [0u8; PTR_SIZE];
1367///     // Store a pointer in `data1`.
1368///     data1.as_mut_ptr().cast::<*const i32>().write_unaligned(&42);
1369///     // Swap the contents of `data1` and `data2` by swapping `PTR_SIZE` many `u8`-sized chunks.
1370///     // This call will fail, because the pointer in `data1` crosses the boundary
1371///     // between several of the 1-byte chunks that are being swapped here.
1372///     //ptr::swap_nonoverlapping(data1.as_mut_ptr(), data2.as_mut_ptr(), PTR_SIZE);
1373///     // Swap the contents of `data1` and `data2` by swapping a single chunk of size
1374///     // `[u8; PTR_SIZE]`. That works, as there is no pointer crossing the boundary between
1375///     // two chunks.
1376///     ptr::swap_nonoverlapping(&mut data1, &mut data2, 1);
1377///     // Read the pointer from `data2` and dereference it.
1378///     let ptr = data2.as_ptr().cast::<*const i32>().read_unaligned();
1379///     assert!(*ptr == 42);
1380/// } }
1381/// ```
1382#[inline]
1383#[stable(feature = "swap_nonoverlapping", since = "1.27.0")]
1384#[rustc_const_stable(feature = "const_swap_nonoverlapping", since = "1.88.0")]
1385#[rustc_diagnostic_item = "ptr_swap_nonoverlapping"]
1386#[rustc_allow_const_fn_unstable(const_eval_select)] // both implementations behave the same
1387#[track_caller]
1388pub const unsafe fn swap_nonoverlapping<T>(x: *mut T, y: *mut T, count: usize) {
1389    ub_checks::assert_unsafe_precondition!(
1390        check_library_ub,
1391        "ptr::swap_nonoverlapping requires that both pointer arguments are aligned and non-null \
1392        and the specified memory ranges do not overlap",
1393        (
1394            x: *mut () = x as *mut (),
1395            y: *mut () = y as *mut (),
1396            size: usize = size_of::<T>(),
1397            align: usize = align_of::<T>(),
1398            count: usize = count,
1399        ) => {
1400            let zero_size = size == 0 || count == 0;
1401            ub_checks::maybe_is_aligned_and_not_null(x, align, zero_size)
1402                && ub_checks::maybe_is_aligned_and_not_null(y, align, zero_size)
1403                && ub_checks::maybe_is_nonoverlapping(x, y, size, count)
1404        }
1405    );
1406
1407    const_eval_select!(
1408        @capture[T] { x: *mut T, y: *mut T, count: usize }:
1409        if const {
1410            // At compile-time we want to always copy this in chunks of `T`, to ensure that if there
1411            // are pointers inside `T` we will copy them in one go rather than trying to copy a part
1412            // of a pointer (which would not work).
1413            // SAFETY: Same preconditions as this function
1414            unsafe { swap_nonoverlapping_const(x, y, count) }
1415        } else {
1416            // Going though a slice here helps codegen know the size fits in `isize`
1417            let slice = slice_from_raw_parts_mut(x, count);
1418            // SAFETY: This is all readable from the pointer, meaning it's one
1419            // allocation, and thus cannot be more than isize::MAX bytes.
1420            let bytes = unsafe { mem::size_of_val_raw::<[T]>(slice) };
1421            if let Some(bytes) = NonZero::new(bytes) {
1422                // SAFETY: These are the same ranges, just expressed in a different
1423                // type, so they're still non-overlapping.
1424                unsafe { swap_nonoverlapping_bytes(x.cast(), y.cast(), bytes) };
1425            }
1426        }
1427    )
1428}
1429
1430/// Same behavior and safety conditions as [`swap_nonoverlapping`]
1431#[inline]
1432const unsafe fn swap_nonoverlapping_const<T>(x: *mut T, y: *mut T, count: usize) {
1433    let mut i = 0;
1434    while i < count {
1435        // SAFETY: By precondition, `i` is in-bounds because it's below `n`
1436        let x = unsafe { x.add(i) };
1437        // SAFETY: By precondition, `i` is in-bounds because it's below `n`
1438        // and it's distinct from `x` since the ranges are non-overlapping
1439        let y = unsafe { y.add(i) };
1440
1441        // SAFETY: we're only ever given pointers that are valid to read/write,
1442        // including being aligned, and nothing here panics so it's drop-safe.
1443        unsafe {
1444            // Note that it's critical that these use `copy_nonoverlapping`,
1445            // rather than `read`/`write`, to avoid #134713 if T has padding.
1446            let mut temp = MaybeUninit::<T>::uninit();
1447            copy_nonoverlapping(x, temp.as_mut_ptr(), 1);
1448            copy_nonoverlapping(y, x, 1);
1449            copy_nonoverlapping(temp.as_ptr(), y, 1);
1450        }
1451
1452        i += 1;
1453    }
1454}
1455
1456// Don't let MIR inline this, because we really want it to keep its noalias metadata
1457#[rustc_no_mir_inline]
1458#[inline]
1459fn swap_chunk<const N: usize>(x: &mut MaybeUninit<[u8; N]>, y: &mut MaybeUninit<[u8; N]>) {
1460    let a = *x;
1461    let b = *y;
1462    *x = b;
1463    *y = a;
1464}
1465
1466#[inline]
1467unsafe fn swap_nonoverlapping_bytes(x: *mut u8, y: *mut u8, bytes: NonZero<usize>) {
1468    // Same as `swap_nonoverlapping::<[u8; N]>`.
1469    unsafe fn swap_nonoverlapping_chunks<const N: usize>(
1470        x: *mut MaybeUninit<[u8; N]>,
1471        y: *mut MaybeUninit<[u8; N]>,
1472        chunks: NonZero<usize>,
1473    ) {
1474        let chunks = chunks.get();
1475        for i in 0..chunks {
1476            // SAFETY: i is in [0, chunks) so the adds and dereferences are in-bounds.
1477            unsafe { swap_chunk(&mut *x.add(i), &mut *y.add(i)) };
1478        }
1479    }
1480
1481    // Same as `swap_nonoverlapping_bytes`, but accepts at most 1+2+4=7 bytes
1482    #[inline]
1483    unsafe fn swap_nonoverlapping_short(x: *mut u8, y: *mut u8, bytes: NonZero<usize>) {
1484        // Tail handling for auto-vectorized code sometimes has element-at-a-time behaviour,
1485        // see <https://github.com/rust-lang/rust/issues/134946>.
1486        // By swapping as different sizes, rather than as a loop over bytes,
1487        // we make sure not to end up with, say, seven byte-at-a-time copies.
1488
1489        let bytes = bytes.get();
1490        let mut i = 0;
1491        macro_rules! swap_prefix {
1492            ($($n:literal)+) => {$(
1493                if (bytes & $n) != 0 {
1494                    // SAFETY: `i` can only have the same bits set as those in bytes,
1495                    // so these `add`s are in-bounds of `bytes`.  But the bit for
1496                    // `$n` hasn't been set yet, so the `$n` bytes that `swap_chunk`
1497                    // will read and write are within the usable range.
1498                    unsafe { swap_chunk::<$n>(&mut*x.add(i).cast(), &mut*y.add(i).cast()) };
1499                    i |= $n;
1500                }
1501            )+};
1502        }
1503        swap_prefix!(4 2 1);
1504        debug_assert_eq!(i, bytes);
1505    }
1506
1507    const CHUNK_SIZE: usize = size_of::<*const ()>();
1508    let bytes = bytes.get();
1509
1510    let chunks = bytes / CHUNK_SIZE;
1511    let tail = bytes % CHUNK_SIZE;
1512    if let Some(chunks) = NonZero::new(chunks) {
1513        // SAFETY: this is bytes/CHUNK_SIZE*CHUNK_SIZE bytes, which is <= bytes,
1514        // so it's within the range of our non-overlapping bytes.
1515        unsafe { swap_nonoverlapping_chunks::<CHUNK_SIZE>(x.cast(), y.cast(), chunks) };
1516    }
1517    if let Some(tail) = NonZero::new(tail) {
1518        const { assert!(CHUNK_SIZE <= 8) };
1519        let delta = chunks * CHUNK_SIZE;
1520        // SAFETY: the tail length is below CHUNK SIZE because of the remainder,
1521        // and CHUNK_SIZE is at most 8 by the const assert, so tail <= 7
1522        unsafe { swap_nonoverlapping_short(x.add(delta), y.add(delta), tail) };
1523    }
1524}
1525
1526/// Moves `src` into the pointed `dst`, returning the previous `dst` value.
1527///
1528/// Neither value is dropped.
1529///
1530/// This function is semantically equivalent to [`mem::replace`] except that it
1531/// operates on raw pointers instead of references. When references are
1532/// available, [`mem::replace`] should be preferred.
1533///
1534/// # Safety
1535///
1536/// Behavior is undefined if any of the following conditions are violated:
1537///
1538/// * `dst` must be [valid] for both reads and writes.
1539///
1540/// * `dst` must be properly aligned.
1541///
1542/// * `dst` must point to a properly initialized value of type `T`.
1543///
1544/// Note that even if `T` has size `0`, the pointer must be properly aligned.
1545///
1546/// [valid]: self#safety
1547///
1548/// # Examples
1549///
1550/// ```
1551/// use std::ptr;
1552///
1553/// let mut rust = vec!['b', 'u', 's', 't'];
1554///
1555/// // `mem::replace` would have the same effect without requiring the unsafe
1556/// // block.
1557/// let b = unsafe {
1558///     ptr::replace(&mut rust[0], 'r')
1559/// };
1560///
1561/// assert_eq!(b, 'b');
1562/// assert_eq!(rust, &['r', 'u', 's', 't']);
1563/// ```
1564#[inline]
1565#[stable(feature = "rust1", since = "1.0.0")]
1566#[rustc_const_stable(feature = "const_replace", since = "1.83.0")]
1567#[rustc_diagnostic_item = "ptr_replace"]
1568#[track_caller]
1569pub const unsafe fn replace<T>(dst: *mut T, src: T) -> T {
1570    // SAFETY: the caller must guarantee that `dst` is valid to be
1571    // cast to a mutable reference (valid for writes, aligned, initialized),
1572    // and cannot overlap `src` since `dst` must point to a distinct
1573    // allocation.
1574    unsafe {
1575        ub_checks::assert_unsafe_precondition!(
1576            check_language_ub,
1577            "ptr::replace requires that the pointer argument is aligned and non-null",
1578            (
1579                addr: *const () = dst as *const (),
1580                align: usize = align_of::<T>(),
1581                is_zst: bool = T::IS_ZST,
1582            ) => ub_checks::maybe_is_aligned_and_not_null(addr, align, is_zst)
1583        );
1584        mem::replace(&mut *dst, src)
1585    }
1586}
1587
1588/// Reads the value from `src` without moving it. This leaves the
1589/// memory in `src` unchanged.
1590///
1591/// # Safety
1592///
1593/// Behavior is undefined if any of the following conditions are violated:
1594///
1595/// * `src` must be [valid] for reads.
1596///
1597/// * `src` must be properly aligned. Use [`read_unaligned`] if this is not the
1598///   case.
1599///
1600/// * `src` must point to a properly initialized value of type `T`.
1601///
1602/// Note that even if `T` has size `0`, the pointer must be properly aligned.
1603///
1604/// # Examples
1605///
1606/// Basic usage:
1607///
1608/// ```
1609/// let x = 12;
1610/// let y = &x as *const i32;
1611///
1612/// unsafe {
1613///     assert_eq!(std::ptr::read(y), 12);
1614/// }
1615/// ```
1616///
1617/// Manually implement [`mem::swap`]:
1618///
1619/// ```
1620/// use std::ptr;
1621///
1622/// fn swap<T>(a: &mut T, b: &mut T) {
1623///     unsafe {
1624///         // Create a bitwise copy of the value at `a` in `tmp`.
1625///         let tmp = ptr::read(a);
1626///
1627///         // Exiting at this point (either by explicitly returning or by
1628///         // calling a function which panics) would cause the value in `tmp` to
1629///         // be dropped while the same value is still referenced by `a`. This
1630///         // could trigger undefined behavior if `T` is not `Copy`.
1631///
1632///         // Create a bitwise copy of the value at `b` in `a`.
1633///         // This is safe because mutable references cannot alias.
1634///         ptr::copy_nonoverlapping(b, a, 1);
1635///
1636///         // As above, exiting here could trigger undefined behavior because
1637///         // the same value is referenced by `a` and `b`.
1638///
1639///         // Move `tmp` into `b`.
1640///         ptr::write(b, tmp);
1641///
1642///         // `tmp` has been moved (`write` takes ownership of its second argument),
1643///         // so nothing is dropped implicitly here.
1644///     }
1645/// }
1646///
1647/// let mut foo = "foo".to_owned();
1648/// let mut bar = "bar".to_owned();
1649///
1650/// swap(&mut foo, &mut bar);
1651///
1652/// assert_eq!(foo, "bar");
1653/// assert_eq!(bar, "foo");
1654/// ```
1655///
1656/// ## Ownership of the Returned Value
1657///
1658/// `read` creates a bitwise copy of `T`, regardless of whether `T` is [`Copy`].
1659/// If `T` is not [`Copy`], using both the returned value and the value at
1660/// `*src` can violate memory safety. Note that assigning to `*src` counts as a
1661/// use because it will attempt to drop the value at `*src`.
1662///
1663/// [`write()`] can be used to overwrite data without causing it to be dropped.
1664///
1665/// ```
1666/// use std::ptr;
1667///
1668/// let mut s = String::from("foo");
1669/// unsafe {
1670///     // `s2` now points to the same underlying memory as `s`.
1671///     let mut s2: String = ptr::read(&s);
1672///
1673///     assert_eq!(s2, "foo");
1674///
1675///     // Assigning to `s2` causes its original value to be dropped. Beyond
1676///     // this point, `s` must no longer be used, as the underlying memory has
1677///     // been freed.
1678///     s2 = String::default();
1679///     assert_eq!(s2, "");
1680///
1681///     // Assigning to `s` would cause the old value to be dropped again,
1682///     // resulting in undefined behavior.
1683///     // s = String::from("bar"); // ERROR
1684///
1685///     // `ptr::write` can be used to overwrite a value without dropping it.
1686///     ptr::write(&mut s, String::from("bar"));
1687/// }
1688///
1689/// assert_eq!(s, "bar");
1690/// ```
1691///
1692/// [valid]: self#safety
1693#[inline]
1694#[stable(feature = "rust1", since = "1.0.0")]
1695#[rustc_const_stable(feature = "const_ptr_read", since = "1.71.0")]
1696#[track_caller]
1697#[rustc_diagnostic_item = "ptr_read"]
1698pub const unsafe fn read<T>(src: *const T) -> T {
1699    // It would be semantically correct to implement this via `copy_nonoverlapping`
1700    // and `MaybeUninit`, as was done before PR #109035. Calling `assume_init`
1701    // provides enough information to know that this is a typed operation.
1702
1703    // However, as of March 2023 the compiler was not capable of taking advantage
1704    // of that information. Thus, the implementation here switched to an intrinsic,
1705    // which lowers to `_0 = *src` in MIR, to address a few issues:
1706    //
1707    // - Using `MaybeUninit::assume_init` after a `copy_nonoverlapping` was not
1708    //   turning the untyped copy into a typed load. As such, the generated
1709    //   `load` in LLVM didn't get various metadata, such as `!range` (#73258),
1710    //   `!nonnull`, and `!noundef`, resulting in poorer optimization.
1711    // - Going through the extra local resulted in multiple extra copies, even
1712    //   in optimized MIR.  (Ignoring StorageLive/Dead, the intrinsic is one
1713    //   MIR statement, while the previous implementation was eight.)  LLVM
1714    //   could sometimes optimize them away, but because `read` is at the core
1715    //   of so many things, not having them in the first place improves what we
1716    //   hand off to the backend.  For example, `mem::replace::<Big>` previously
1717    //   emitted 4 `alloca` and 6 `memcpy`s, but is now 1 `alloc` and 3 `memcpy`s.
1718    // - In general, this approach keeps us from getting any more bugs (like
1719    //   #106369) that boil down to "`read(p)` is worse than `*p`", as this
1720    //   makes them look identical to the backend (or other MIR consumers).
1721    //
1722    // Future enhancements to MIR optimizations might well allow this to return
1723    // to the previous implementation, rather than using an intrinsic.
1724
1725    // SAFETY: the caller must guarantee that `src` is valid for reads.
1726    unsafe {
1727        #[cfg(debug_assertions)] // Too expensive to always enable (for now?)
1728        ub_checks::assert_unsafe_precondition!(
1729            check_language_ub,
1730            "ptr::read requires that the pointer argument is aligned and non-null",
1731            (
1732                addr: *const () = src as *const (),
1733                align: usize = align_of::<T>(),
1734                is_zst: bool = T::IS_ZST,
1735            ) => ub_checks::maybe_is_aligned_and_not_null(addr, align, is_zst)
1736        );
1737        crate::intrinsics::read_via_copy(src)
1738    }
1739}
1740
1741/// Reads the value from `src` without moving it. This leaves the
1742/// memory in `src` unchanged.
1743///
1744/// Unlike [`read`], `read_unaligned` works with unaligned pointers.
1745///
1746/// # Safety
1747///
1748/// Behavior is undefined if any of the following conditions are violated:
1749///
1750/// * `src` must be [valid] for reads.
1751///
1752/// * `src` must point to a properly initialized value of type `T`.
1753///
1754/// Like [`read`], `read_unaligned` creates a bitwise copy of `T`, regardless of
1755/// whether `T` is [`Copy`]. If `T` is not [`Copy`], using both the returned
1756/// value and the value at `*src` can [violate memory safety][read-ownership].
1757///
1758/// [read-ownership]: read#ownership-of-the-returned-value
1759/// [valid]: self#safety
1760///
1761/// ## On `packed` structs
1762///
1763/// Attempting to create a raw pointer to an `unaligned` struct field with
1764/// an expression such as `&packed.unaligned as *const FieldType` creates an
1765/// intermediate unaligned reference before converting that to a raw pointer.
1766/// That this reference is temporary and immediately cast is inconsequential
1767/// as the compiler always expects references to be properly aligned.
1768/// As a result, using `&packed.unaligned as *const FieldType` causes immediate
1769/// *undefined behavior* in your program.
1770///
1771/// Instead you must use the `&raw const` syntax to create the pointer.
1772/// You may use that constructed pointer together with this function.
1773///
1774/// An example of what not to do and how this relates to `read_unaligned` is:
1775///
1776/// ```
1777/// #[repr(packed, C)]
1778/// struct Packed {
1779///     _padding: u8,
1780///     unaligned: u32,
1781/// }
1782///
1783/// let packed = Packed {
1784///     _padding: 0x00,
1785///     unaligned: 0x01020304,
1786/// };
1787///
1788/// // Take the address of a 32-bit integer which is not aligned.
1789/// // In contrast to `&packed.unaligned as *const _`, this has no undefined behavior.
1790/// let unaligned = &raw const packed.unaligned;
1791///
1792/// let v = unsafe { std::ptr::read_unaligned(unaligned) };
1793/// assert_eq!(v, 0x01020304);
1794/// ```
1795///
1796/// Accessing unaligned fields directly with e.g. `packed.unaligned` is safe however.
1797///
1798/// # Examples
1799///
1800/// Read a `usize` value from a byte buffer:
1801///
1802/// ```
1803/// fn read_usize(x: &[u8]) -> usize {
1804///     assert!(x.len() >= size_of::<usize>());
1805///
1806///     let ptr = x.as_ptr() as *const usize;
1807///
1808///     unsafe { ptr.read_unaligned() }
1809/// }
1810/// ```
1811#[inline]
1812#[stable(feature = "ptr_unaligned", since = "1.17.0")]
1813#[rustc_const_stable(feature = "const_ptr_read", since = "1.71.0")]
1814#[track_caller]
1815#[rustc_diagnostic_item = "ptr_read_unaligned"]
1816pub const unsafe fn read_unaligned<T>(src: *const T) -> T {
1817    let mut tmp = MaybeUninit::<T>::uninit();
1818    // SAFETY: the caller must guarantee that `src` is valid for reads.
1819    // `src` cannot overlap `tmp` because `tmp` was just allocated on
1820    // the stack as a separate allocation.
1821    //
1822    // Also, since we just wrote a valid value into `tmp`, it is guaranteed
1823    // to be properly initialized.
1824    unsafe {
1825        copy_nonoverlapping(src as *const u8, tmp.as_mut_ptr() as *mut u8, size_of::<T>());
1826        tmp.assume_init()
1827    }
1828}
1829
1830/// Overwrites a memory location with the given value without reading or
1831/// dropping the old value.
1832///
1833/// `write` does not drop the contents of `dst`. This is safe, but it could leak
1834/// allocations or resources, so care should be taken not to overwrite an object
1835/// that should be dropped.
1836///
1837/// Additionally, it does not drop `src`. Semantically, `src` is moved into the
1838/// location pointed to by `dst`.
1839///
1840/// This is appropriate for initializing uninitialized memory, or overwriting
1841/// memory that has previously been [`read`] from.
1842///
1843/// # Safety
1844///
1845/// Behavior is undefined if any of the following conditions are violated:
1846///
1847/// * `dst` must be [valid] for writes.
1848///
1849/// * `dst` must be properly aligned. Use [`write_unaligned`] if this is not the
1850///   case.
1851///
1852/// Note that even if `T` has size `0`, the pointer must be properly aligned.
1853///
1854/// [valid]: self#safety
1855///
1856/// # Examples
1857///
1858/// Basic usage:
1859///
1860/// ```
1861/// let mut x = 0;
1862/// let y = &mut x as *mut i32;
1863/// let z = 12;
1864///
1865/// unsafe {
1866///     std::ptr::write(y, z);
1867///     assert_eq!(std::ptr::read(y), 12);
1868/// }
1869/// ```
1870///
1871/// Manually implement [`mem::swap`]:
1872///
1873/// ```
1874/// use std::ptr;
1875///
1876/// fn swap<T>(a: &mut T, b: &mut T) {
1877///     unsafe {
1878///         // Create a bitwise copy of the value at `a` in `tmp`.
1879///         let tmp = ptr::read(a);
1880///
1881///         // Exiting at this point (either by explicitly returning or by
1882///         // calling a function which panics) would cause the value in `tmp` to
1883///         // be dropped while the same value is still referenced by `a`. This
1884///         // could trigger undefined behavior if `T` is not `Copy`.
1885///
1886///         // Create a bitwise copy of the value at `b` in `a`.
1887///         // This is safe because mutable references cannot alias.
1888///         ptr::copy_nonoverlapping(b, a, 1);
1889///
1890///         // As above, exiting here could trigger undefined behavior because
1891///         // the same value is referenced by `a` and `b`.
1892///
1893///         // Move `tmp` into `b`.
1894///         ptr::write(b, tmp);
1895///
1896///         // `tmp` has been moved (`write` takes ownership of its second argument),
1897///         // so nothing is dropped implicitly here.
1898///     }
1899/// }
1900///
1901/// let mut foo = "foo".to_owned();
1902/// let mut bar = "bar".to_owned();
1903///
1904/// swap(&mut foo, &mut bar);
1905///
1906/// assert_eq!(foo, "bar");
1907/// assert_eq!(bar, "foo");
1908/// ```
1909#[inline]
1910#[stable(feature = "rust1", since = "1.0.0")]
1911#[rustc_const_stable(feature = "const_ptr_write", since = "1.83.0")]
1912#[rustc_diagnostic_item = "ptr_write"]
1913#[track_caller]
1914pub const unsafe fn write<T>(dst: *mut T, src: T) {
1915    // Semantically, it would be fine for this to be implemented as a
1916    // `copy_nonoverlapping` and appropriate drop suppression of `src`.
1917
1918    // However, implementing via that currently produces more MIR than is ideal.
1919    // Using an intrinsic keeps it down to just the simple `*dst = move src` in
1920    // MIR (11 statements shorter, at the time of writing), and also allows
1921    // `src` to stay an SSA value in codegen_ssa, rather than a memory one.
1922
1923    // SAFETY: the caller must guarantee that `dst` is valid for writes.
1924    // `dst` cannot overlap `src` because the caller has mutable access
1925    // to `dst` while `src` is owned by this function.
1926    unsafe {
1927        #[cfg(debug_assertions)] // Too expensive to always enable (for now?)
1928        ub_checks::assert_unsafe_precondition!(
1929            check_language_ub,
1930            "ptr::write requires that the pointer argument is aligned and non-null",
1931            (
1932                addr: *mut () = dst as *mut (),
1933                align: usize = align_of::<T>(),
1934                is_zst: bool = T::IS_ZST,
1935            ) => ub_checks::maybe_is_aligned_and_not_null(addr, align, is_zst)
1936        );
1937        intrinsics::write_via_move(dst, src)
1938    }
1939}
1940
1941/// Overwrites a memory location with the given value without reading or
1942/// dropping the old value.
1943///
1944/// Unlike [`write()`], the pointer may be unaligned.
1945///
1946/// `write_unaligned` does not drop the contents of `dst`. This is safe, but it
1947/// could leak allocations or resources, so care should be taken not to overwrite
1948/// an object that should be dropped.
1949///
1950/// Additionally, it does not drop `src`. Semantically, `src` is moved into the
1951/// location pointed to by `dst`.
1952///
1953/// This is appropriate for initializing uninitialized memory, or overwriting
1954/// memory that has previously been read with [`read_unaligned`].
1955///
1956/// # Safety
1957///
1958/// Behavior is undefined if any of the following conditions are violated:
1959///
1960/// * `dst` must be [valid] for writes.
1961///
1962/// [valid]: self#safety
1963///
1964/// ## On `packed` structs
1965///
1966/// Attempting to create a raw pointer to an `unaligned` struct field with
1967/// an expression such as `&packed.unaligned as *const FieldType` creates an
1968/// intermediate unaligned reference before converting that to a raw pointer.
1969/// That this reference is temporary and immediately cast is inconsequential
1970/// as the compiler always expects references to be properly aligned.
1971/// As a result, using `&packed.unaligned as *const FieldType` causes immediate
1972/// *undefined behavior* in your program.
1973///
1974/// Instead, you must use the `&raw mut` syntax to create the pointer.
1975/// You may use that constructed pointer together with this function.
1976///
1977/// An example of how to do it and how this relates to `write_unaligned` is:
1978///
1979/// ```
1980/// #[repr(packed, C)]
1981/// struct Packed {
1982///     _padding: u8,
1983///     unaligned: u32,
1984/// }
1985///
1986/// let mut packed: Packed = unsafe { std::mem::zeroed() };
1987///
1988/// // Take the address of a 32-bit integer which is not aligned.
1989/// // In contrast to `&packed.unaligned as *mut _`, this has no undefined behavior.
1990/// let unaligned = &raw mut packed.unaligned;
1991///
1992/// unsafe { std::ptr::write_unaligned(unaligned, 42) };
1993///
1994/// assert_eq!({packed.unaligned}, 42); // `{...}` forces copying the field instead of creating a reference.
1995/// ```
1996///
1997/// Accessing unaligned fields directly with e.g. `packed.unaligned` is safe however
1998/// (as can be seen in the `assert_eq!` above).
1999///
2000/// # Examples
2001///
2002/// Write a `usize` value to a byte buffer:
2003///
2004/// ```
2005/// fn write_usize(x: &mut [u8], val: usize) {
2006///     assert!(x.len() >= size_of::<usize>());
2007///
2008///     let ptr = x.as_mut_ptr() as *mut usize;
2009///
2010///     unsafe { ptr.write_unaligned(val) }
2011/// }
2012/// ```
2013#[inline]
2014#[stable(feature = "ptr_unaligned", since = "1.17.0")]
2015#[rustc_const_stable(feature = "const_ptr_write", since = "1.83.0")]
2016#[rustc_diagnostic_item = "ptr_write_unaligned"]
2017#[track_caller]
2018pub const unsafe fn write_unaligned<T>(dst: *mut T, src: T) {
2019    // SAFETY: the caller must guarantee that `dst` is valid for writes.
2020    // `dst` cannot overlap `src` because the caller has mutable access
2021    // to `dst` while `src` is owned by this function.
2022    unsafe {
2023        copy_nonoverlapping((&raw const src) as *const u8, dst as *mut u8, size_of::<T>());
2024        // We are calling the intrinsic directly to avoid function calls in the generated code.
2025        intrinsics::forget(src);
2026    }
2027}
2028
2029/// Performs a volatile read of the value from `src` without moving it.
2030///
2031/// Volatile operations are intended to act on I/O memory. As such, they are considered externally
2032/// observable events (just like syscalls, but less opaque), and are guaranteed to not be elided or
2033/// reordered by the compiler across other externally observable events. With this in mind, there
2034/// are two cases of usage that need to be distinguished:
2035///
2036/// - When a volatile operation is used for memory inside an [allocation], it behaves exactly like
2037///   [`read`], except for the additional guarantee that it won't be elided or reordered (see
2038///   above). This implies that the operation will actually access memory and not e.g. be lowered to
2039///   reusing data from a previous read. Other than that, all the usual rules for memory accesses
2040///   apply (including provenance).  In particular, just like in C, whether an operation is volatile
2041///   has no bearing whatsoever on questions involving concurrent accesses from multiple threads.
2042///   Volatile accesses behave exactly like non-atomic accesses in that regard.
2043///
2044/// - Volatile operations, however, may also be used to access memory that is _outside_ of any Rust
2045///   allocation. In this use-case, the pointer does *not* have to be [valid] for reads. This is
2046///   typically used for CPU and peripheral registers that must be accessed via an I/O memory
2047///   mapping, most commonly at fixed addresses reserved by the hardware. These often have special
2048///   semantics associated to their manipulation, and cannot be used as general purpose memory.
2049///   Here, any address value is possible, including 0 and [`usize::MAX`], so long as the semantics
2050///   of such a read are well-defined by the target hardware. The provenance of the pointer is
2051///   irrelevant, and it can be created with [`without_provenance`]. The access must not trap. It
2052///   can cause side-effects, but those must not affect Rust-allocated memory in any way. This
2053///   access is still not considered [atomic], and as such it cannot be used for inter-thread
2054///   synchronization.
2055///
2056/// Note that volatile memory operations where T is a zero-sized type are noops and may be ignored.
2057///
2058/// [allocation]: crate::ptr#allocated-object
2059/// [atomic]: crate::sync::atomic#memory-model-for-atomic-accesses
2060///
2061/// # Safety
2062///
2063/// Like [`read`], `read_volatile` creates a bitwise copy of `T`, regardless of whether `T` is
2064/// [`Copy`]. If `T` is not [`Copy`], using both the returned value and the value at `*src` can
2065/// [violate memory safety][read-ownership]. However, storing non-[`Copy`] types in volatile memory
2066/// is almost certainly incorrect.
2067///
2068/// Behavior is undefined if any of the following conditions are violated:
2069///
2070/// * `src` must be either [valid] for reads, or it must point to memory outside of all Rust
2071///   allocations and reading from that memory must:
2072///   - not trap, and
2073///   - not cause any memory inside a Rust allocation to be modified.
2074///
2075/// * `src` must be properly aligned.
2076///
2077/// * Reading from `src` must produce a properly initialized value of type `T`.
2078///
2079/// Note that even if `T` has size `0`, the pointer must be properly aligned.
2080///
2081/// [valid]: self#safety
2082/// [read-ownership]: read#ownership-of-the-returned-value
2083///
2084/// # Examples
2085///
2086/// Basic usage:
2087///
2088/// ```
2089/// let x = 12;
2090/// let y = &x as *const i32;
2091///
2092/// unsafe {
2093///     assert_eq!(std::ptr::read_volatile(y), 12);
2094/// }
2095/// ```
2096#[inline]
2097#[stable(feature = "volatile", since = "1.9.0")]
2098#[track_caller]
2099#[rustc_diagnostic_item = "ptr_read_volatile"]
2100pub unsafe fn read_volatile<T>(src: *const T) -> T {
2101    // SAFETY: the caller must uphold the safety contract for `volatile_load`.
2102    unsafe {
2103        ub_checks::assert_unsafe_precondition!(
2104            check_language_ub,
2105            "ptr::read_volatile requires that the pointer argument is aligned",
2106            (
2107                addr: *const () = src as *const (),
2108                align: usize = align_of::<T>(),
2109            ) => ub_checks::maybe_is_aligned(addr, align)
2110        );
2111        intrinsics::volatile_load(src)
2112    }
2113}
2114
2115/// Performs a volatile write of a memory location with the given value without reading or dropping
2116/// the old value.
2117///
2118/// Volatile operations are intended to act on I/O memory. As such, they are considered externally
2119/// observable events (just like syscalls), and are guaranteed to not be elided or reordered by the
2120/// compiler across other externally observable events. With this in mind, there are two cases of
2121/// usage that need to be distinguished:
2122///
2123/// - When a volatile operation is used for memory inside an [allocation], it behaves exactly like
2124///   [`write`][write()], except for the additional guarantee that it won't be elided or reordered
2125///   (see above). This implies that the operation will actually access memory and not e.g. be
2126///   lowered to a register access. Other than that, all the usual rules for memory accesses apply
2127///   (including provenance). In particular, just like in C, whether an operation is volatile has no
2128///   bearing whatsoever on questions involving concurrent access from multiple threads. Volatile
2129///   accesses behave exactly like non-atomic accesses in that regard.
2130///
2131/// - Volatile operations, however, may also be used to access memory that is _outside_ of any Rust
2132///   allocation. In this use-case, the pointer does *not* have to be [valid] for writes. This is
2133///   typically used for CPU and peripheral registers that must be accessed via an I/O memory
2134///   mapping, most commonly at fixed addresses reserved by the hardware. These often have special
2135///   semantics associated to their manipulation, and cannot be used as general purpose memory.
2136///   Here, any address value is possible, including 0 and [`usize::MAX`], so long as the semantics
2137///   of such a write are well-defined by the target hardware. The provenance of the pointer is
2138///   irrelevant, and it can be created with [`without_provenance`]. The access must not trap. It
2139///   can cause side-effects, but those must not affect Rust-allocated memory in any way. This
2140///   access is still not considered [atomic], and as such it cannot be used for inter-thread
2141///   synchronization.
2142///
2143/// Note that volatile memory operations on zero-sized types (e.g., if a zero-sized type is passed
2144/// to `write_volatile`) are noops and may be ignored.
2145///
2146/// `write_volatile` does not drop the contents of `dst`. This is safe, but it could leak
2147/// allocations or resources, so care should be taken not to overwrite an object that should be
2148/// dropped when operating on Rust memory. Additionally, it does not drop `src`. Semantically, `src`
2149/// is moved into the location pointed to by `dst`.
2150///
2151/// [allocation]: crate::ptr#allocated-object
2152/// [atomic]: crate::sync::atomic#memory-model-for-atomic-accesses
2153///
2154/// # Safety
2155///
2156/// Behavior is undefined if any of the following conditions are violated:
2157///
2158/// * `dst` must be either [valid] for writes, or it must point to memory outside of all Rust
2159///   allocations and writing to that memory must:
2160///   - not trap, and
2161///   - not cause any memory inside a Rust allocation to be modified.
2162///
2163/// * `dst` must be properly aligned.
2164///
2165/// Note that even if `T` has size `0`, the pointer must be properly aligned.
2166///
2167/// [valid]: self#safety
2168///
2169/// # Examples
2170///
2171/// Basic usage:
2172///
2173/// ```
2174/// let mut x = 0;
2175/// let y = &mut x as *mut i32;
2176/// let z = 12;
2177///
2178/// unsafe {
2179///     std::ptr::write_volatile(y, z);
2180///     assert_eq!(std::ptr::read_volatile(y), 12);
2181/// }
2182/// ```
2183#[inline]
2184#[stable(feature = "volatile", since = "1.9.0")]
2185#[rustc_diagnostic_item = "ptr_write_volatile"]
2186#[track_caller]
2187pub unsafe fn write_volatile<T>(dst: *mut T, src: T) {
2188    // SAFETY: the caller must uphold the safety contract for `volatile_store`.
2189    unsafe {
2190        ub_checks::assert_unsafe_precondition!(
2191            check_language_ub,
2192            "ptr::write_volatile requires that the pointer argument is aligned",
2193            (
2194                addr: *mut () = dst as *mut (),
2195                align: usize = align_of::<T>(),
2196            ) => ub_checks::maybe_is_aligned(addr, align)
2197        );
2198        intrinsics::volatile_store(dst, src);
2199    }
2200}
2201
2202/// Align pointer `p`.
2203///
2204/// Calculate offset (in terms of elements of `size_of::<T>()` stride) that has to be applied
2205/// to pointer `p` so that pointer `p` would get aligned to `a`.
2206///
2207/// # Safety
2208/// `a` must be a power of two.
2209///
2210/// # Notes
2211/// This implementation has been carefully tailored to not panic. It is UB for this to panic.
2212/// The only real change that can be made here is change of `INV_TABLE_MOD_16` and associated
2213/// constants.
2214///
2215/// If we ever decide to make it possible to call the intrinsic with `a` that is not a
2216/// power-of-two, it will probably be more prudent to just change to a naive implementation rather
2217/// than trying to adapt this to accommodate that change.
2218///
2219/// Any questions go to @nagisa.
2220#[allow(ptr_to_integer_transmute_in_consts)]
2221pub(crate) unsafe fn align_offset<T: Sized>(p: *const T, a: usize) -> usize {
2222    // FIXME(#75598): Direct use of these intrinsics improves codegen significantly at opt-level <=
2223    // 1, where the method versions of these operations are not inlined.
2224    use intrinsics::{
2225        assume, cttz_nonzero, exact_div, mul_with_overflow, unchecked_rem, unchecked_shl,
2226        unchecked_shr, unchecked_sub, wrapping_add, wrapping_mul, wrapping_sub,
2227    };
2228
2229    /// Calculate multiplicative modular inverse of `x` modulo `m`.
2230    ///
2231    /// This implementation is tailored for `align_offset` and has following preconditions:
2232    ///
2233    /// * `m` is a power-of-two;
2234    /// * `x < m`; (if `x ≥ m`, pass in `x % m` instead)
2235    ///
2236    /// Implementation of this function shall not panic. Ever.
2237    #[inline]
2238    const unsafe fn mod_inv(x: usize, m: usize) -> usize {
2239        /// Multiplicative modular inverse table modulo 2⁴ = 16.
2240        ///
2241        /// Note, that this table does not contain values where inverse does not exist (i.e., for
2242        /// `0⁻¹ mod 16`, `2⁻¹ mod 16`, etc.)
2243        const INV_TABLE_MOD_16: [u8; 8] = [1, 11, 13, 7, 9, 3, 5, 15];
2244        /// Modulo for which the `INV_TABLE_MOD_16` is intended.
2245        const INV_TABLE_MOD: usize = 16;
2246
2247        // SAFETY: `m` is required to be a power-of-two, hence non-zero.
2248        let m_minus_one = unsafe { unchecked_sub(m, 1) };
2249        let mut inverse = INV_TABLE_MOD_16[(x & (INV_TABLE_MOD - 1)) >> 1] as usize;
2250        let mut mod_gate = INV_TABLE_MOD;
2251        // We iterate "up" using the following formula:
2252        //
2253        // $$ xy ≡ 1 (mod 2ⁿ) → xy (2 - xy) ≡ 1 (mod 2²ⁿ) $$
2254        //
2255        // This application needs to be applied at least until `2²ⁿ ≥ m`, at which point we can
2256        // finally reduce the computation to our desired `m` by taking `inverse mod m`.
2257        //
2258        // This computation is `O(log log m)`, which is to say, that on 64-bit machines this loop
2259        // will always finish in at most 4 iterations.
2260        loop {
2261            // y = y * (2 - xy) mod n
2262            //
2263            // Note, that we use wrapping operations here intentionally – the original formula
2264            // uses e.g., subtraction `mod n`. It is entirely fine to do them `mod
2265            // usize::MAX` instead, because we take the result `mod n` at the end
2266            // anyway.
2267            if mod_gate >= m {
2268                break;
2269            }
2270            inverse = wrapping_mul(inverse, wrapping_sub(2usize, wrapping_mul(x, inverse)));
2271            let (new_gate, overflow) = mul_with_overflow(mod_gate, mod_gate);
2272            if overflow {
2273                break;
2274            }
2275            mod_gate = new_gate;
2276        }
2277        inverse & m_minus_one
2278    }
2279
2280    let stride = size_of::<T>();
2281
2282    let addr: usize = p.addr();
2283
2284    // SAFETY: `a` is a power-of-two, therefore non-zero.
2285    let a_minus_one = unsafe { unchecked_sub(a, 1) };
2286
2287    if stride == 0 {
2288        // SPECIAL_CASE: handle 0-sized types. No matter how many times we step, the address will
2289        // stay the same, so no offset will be able to align the pointer unless it is already
2290        // aligned. This branch _will_ be optimized out as `stride` is known at compile-time.
2291        let p_mod_a = addr & a_minus_one;
2292        return if p_mod_a == 0 { 0 } else { usize::MAX };
2293    }
2294
2295    // SAFETY: `stride == 0` case has been handled by the special case above.
2296    let a_mod_stride = unsafe { unchecked_rem(a, stride) };
2297    if a_mod_stride == 0 {
2298        // SPECIAL_CASE: In cases where the `a` is divisible by `stride`, byte offset to align a
2299        // pointer can be computed more simply through `-p (mod a)`. In the off-chance the byte
2300        // offset is not a multiple of `stride`, the input pointer was misaligned and no pointer
2301        // offset will be able to produce a `p` aligned to the specified `a`.
2302        //
2303        // The naive `-p (mod a)` equation inhibits LLVM's ability to select instructions
2304        // like `lea`. We compute `(round_up_to_next_alignment(p, a) - p)` instead. This
2305        // redistributes operations around the load-bearing, but pessimizing `and` instruction
2306        // sufficiently for LLVM to be able to utilize the various optimizations it knows about.
2307        //
2308        // LLVM handles the branch here particularly nicely. If this branch needs to be evaluated
2309        // at runtime, it will produce a mask `if addr_mod_stride == 0 { 0 } else { usize::MAX }`
2310        // in a branch-free way and then bitwise-OR it with whatever result the `-p mod a`
2311        // computation produces.
2312
2313        let aligned_address = wrapping_add(addr, a_minus_one) & wrapping_sub(0, a);
2314        let byte_offset = wrapping_sub(aligned_address, addr);
2315        // FIXME: Remove the assume after <https://github.com/llvm/llvm-project/issues/62502>
2316        // SAFETY: Masking by `-a` can only affect the low bits, and thus cannot have reduced
2317        // the value by more than `a-1`, so even though the intermediate values might have
2318        // wrapped, the byte_offset is always in `[0, a)`.
2319        unsafe { assume(byte_offset < a) };
2320
2321        // SAFETY: `stride == 0` case has been handled by the special case above.
2322        let addr_mod_stride = unsafe { unchecked_rem(addr, stride) };
2323
2324        return if addr_mod_stride == 0 {
2325            // SAFETY: `stride` is non-zero. This is guaranteed to divide exactly as well, because
2326            // addr has been verified to be aligned to the original type’s alignment requirements.
2327            unsafe { exact_div(byte_offset, stride) }
2328        } else {
2329            usize::MAX
2330        };
2331    }
2332
2333    // GENERAL_CASE: From here on we’re handling the very general case where `addr` may be
2334    // misaligned, there isn’t an obvious relationship between `stride` and `a` that we can take an
2335    // advantage of, etc. This case produces machine code that isn’t particularly high quality,
2336    // compared to the special cases above. The code produced here is still within the realm of
2337    // miracles, given the situations this case has to deal with.
2338
2339    // SAFETY: a is power-of-two hence non-zero. stride == 0 case is handled above.
2340    // FIXME(const-hack) replace with min
2341    let gcdpow = unsafe {
2342        let x = cttz_nonzero(stride);
2343        let y = cttz_nonzero(a);
2344        if x < y { x } else { y }
2345    };
2346    // SAFETY: gcdpow has an upper-bound that’s at most the number of bits in a `usize`.
2347    let gcd = unsafe { unchecked_shl(1usize, gcdpow) };
2348    // SAFETY: gcd is always greater or equal to 1.
2349    if addr & unsafe { unchecked_sub(gcd, 1) } == 0 {
2350        // This branch solves for the following linear congruence equation:
2351        //
2352        // ` p + so = 0 mod a `
2353        //
2354        // `p` here is the pointer value, `s` - stride of `T`, `o` offset in `T`s, and `a` - the
2355        // requested alignment.
2356        //
2357        // With `g = gcd(a, s)`, and the above condition asserting that `p` is also divisible by
2358        // `g`, we can denote `a' = a/g`, `s' = s/g`, `p' = p/g`, then this becomes equivalent to:
2359        //
2360        // ` p' + s'o = 0 mod a' `
2361        // ` o = (a' - (p' mod a')) * (s'^-1 mod a') `
2362        //
2363        // The first term is "the relative alignment of `p` to `a`" (divided by the `g`), the
2364        // second term is "how does incrementing `p` by `s` bytes change the relative alignment of
2365        // `p`" (again divided by `g`). Division by `g` is necessary to make the inverse well
2366        // formed if `a` and `s` are not co-prime.
2367        //
2368        // Furthermore, the result produced by this solution is not "minimal", so it is necessary
2369        // to take the result `o mod lcm(s, a)`. This `lcm(s, a)` is the same as `a'`.
2370
2371        // SAFETY: `gcdpow` has an upper-bound not greater than the number of trailing 0-bits in
2372        // `a`.
2373        let a2 = unsafe { unchecked_shr(a, gcdpow) };
2374        // SAFETY: `a2` is non-zero. Shifting `a` by `gcdpow` cannot shift out any of the set bits
2375        // in `a` (of which it has exactly one).
2376        let a2minus1 = unsafe { unchecked_sub(a2, 1) };
2377        // SAFETY: `gcdpow` has an upper-bound not greater than the number of trailing 0-bits in
2378        // `a`.
2379        let s2 = unsafe { unchecked_shr(stride & a_minus_one, gcdpow) };
2380        // SAFETY: `gcdpow` has an upper-bound not greater than the number of trailing 0-bits in
2381        // `a`. Furthermore, the subtraction cannot overflow, because `a2 = a >> gcdpow` will
2382        // always be strictly greater than `(p % a) >> gcdpow`.
2383        let minusp2 = unsafe { unchecked_sub(a2, unchecked_shr(addr & a_minus_one, gcdpow)) };
2384        // SAFETY: `a2` is a power-of-two, as proven above. `s2` is strictly less than `a2`
2385        // because `(s % a) >> gcdpow` is strictly less than `a >> gcdpow`.
2386        return wrapping_mul(minusp2, unsafe { mod_inv(s2, a2) }) & a2minus1;
2387    }
2388
2389    // Cannot be aligned at all.
2390    usize::MAX
2391}
2392
2393/// Compares raw pointers for equality.
2394///
2395/// This is the same as using the `==` operator, but less generic:
2396/// the arguments have to be `*const T` raw pointers,
2397/// not anything that implements `PartialEq`.
2398///
2399/// This can be used to compare `&T` references (which coerce to `*const T` implicitly)
2400/// by their address rather than comparing the values they point to
2401/// (which is what the `PartialEq for &T` implementation does).
2402///
2403/// When comparing wide pointers, both the address and the metadata are tested for equality.
2404/// However, note that comparing trait object pointers (`*const dyn Trait`) is unreliable: pointers
2405/// to values of the same underlying type can compare inequal (because vtables are duplicated in
2406/// multiple codegen units), and pointers to values of *different* underlying type can compare equal
2407/// (since identical vtables can be deduplicated within a codegen unit).
2408///
2409/// # Examples
2410///
2411/// ```
2412/// use std::ptr;
2413///
2414/// let five = 5;
2415/// let other_five = 5;
2416/// let five_ref = &five;
2417/// let same_five_ref = &five;
2418/// let other_five_ref = &other_five;
2419///
2420/// assert!(five_ref == same_five_ref);
2421/// assert!(ptr::eq(five_ref, same_five_ref));
2422///
2423/// assert!(five_ref == other_five_ref);
2424/// assert!(!ptr::eq(five_ref, other_five_ref));
2425/// ```
2426///
2427/// Slices are also compared by their length (fat pointers):
2428///
2429/// ```
2430/// let a = [1, 2, 3];
2431/// assert!(std::ptr::eq(&a[..3], &a[..3]));
2432/// assert!(!std::ptr::eq(&a[..2], &a[..3]));
2433/// assert!(!std::ptr::eq(&a[0..2], &a[1..3]));
2434/// ```
2435#[stable(feature = "ptr_eq", since = "1.17.0")]
2436#[inline(always)]
2437#[must_use = "pointer comparison produces a value"]
2438#[rustc_diagnostic_item = "ptr_eq"]
2439#[allow(ambiguous_wide_pointer_comparisons)] // it's actually clear here
2440pub fn eq<T: PointeeSized>(a: *const T, b: *const T) -> bool {
2441    a == b
2442}
2443
2444/// Compares the *addresses* of the two pointers for equality,
2445/// ignoring any metadata in fat pointers.
2446///
2447/// If the arguments are thin pointers of the same type,
2448/// then this is the same as [`eq`].
2449///
2450/// # Examples
2451///
2452/// ```
2453/// use std::ptr;
2454///
2455/// let whole: &[i32; 3] = &[1, 2, 3];
2456/// let first: &i32 = &whole[0];
2457///
2458/// assert!(ptr::addr_eq(whole, first));
2459/// assert!(!ptr::eq::<dyn std::fmt::Debug>(whole, first));
2460/// ```
2461#[stable(feature = "ptr_addr_eq", since = "1.76.0")]
2462#[inline(always)]
2463#[must_use = "pointer comparison produces a value"]
2464pub fn addr_eq<T: PointeeSized, U: PointeeSized>(p: *const T, q: *const U) -> bool {
2465    (p as *const ()) == (q as *const ())
2466}
2467
2468/// Compares the *addresses* of the two function pointers for equality.
2469///
2470/// This is the same as `f == g`, but using this function makes clear that the potentially
2471/// surprising semantics of function pointer comparison are involved.
2472///
2473/// There are **very few guarantees** about how functions are compiled and they have no intrinsic
2474/// “identity”; in particular, this comparison:
2475///
2476/// * May return `true` unexpectedly, in cases where functions are equivalent.
2477///
2478///   For example, the following program is likely (but not guaranteed) to print `(true, true)`
2479///   when compiled with optimization:
2480///
2481///   ```
2482///   let f: fn(i32) -> i32 = |x| x;
2483///   let g: fn(i32) -> i32 = |x| x + 0;  // different closure, different body
2484///   let h: fn(u32) -> u32 = |x| x + 0;  // different signature too
2485///   dbg!(std::ptr::fn_addr_eq(f, g), std::ptr::fn_addr_eq(f, h)); // not guaranteed to be equal
2486///   ```
2487///
2488/// * May return `false` in any case.
2489///
2490///   This is particularly likely with generic functions but may happen with any function.
2491///   (From an implementation perspective, this is possible because functions may sometimes be
2492///   processed more than once by the compiler, resulting in duplicate machine code.)
2493///
2494/// Despite these false positives and false negatives, this comparison can still be useful.
2495/// Specifically, if
2496///
2497/// * `T` is the same type as `U`, `T` is a [subtype] of `U`, or `U` is a [subtype] of `T`, and
2498/// * `ptr::fn_addr_eq(f, g)` returns true,
2499///
2500/// then calling `f` and calling `g` will be equivalent.
2501///
2502///
2503/// # Examples
2504///
2505/// ```
2506/// use std::ptr;
2507///
2508/// fn a() { println!("a"); }
2509/// fn b() { println!("b"); }
2510/// assert!(!ptr::fn_addr_eq(a as fn(), b as fn()));
2511/// ```
2512///
2513/// [subtype]: https://doc.rust-lang.org/reference/subtyping.html
2514#[stable(feature = "ptr_fn_addr_eq", since = "1.85.0")]
2515#[inline(always)]
2516#[must_use = "function pointer comparison produces a value"]
2517pub fn fn_addr_eq<T: FnPtr, U: FnPtr>(f: T, g: U) -> bool {
2518    f.addr() == g.addr()
2519}
2520
2521/// Hash a raw pointer.
2522///
2523/// This can be used to hash a `&T` reference (which coerces to `*const T` implicitly)
2524/// by its address rather than the value it points to
2525/// (which is what the `Hash for &T` implementation does).
2526///
2527/// # Examples
2528///
2529/// ```
2530/// use std::hash::{DefaultHasher, Hash, Hasher};
2531/// use std::ptr;
2532///
2533/// let five = 5;
2534/// let five_ref = &five;
2535///
2536/// let mut hasher = DefaultHasher::new();
2537/// ptr::hash(five_ref, &mut hasher);
2538/// let actual = hasher.finish();
2539///
2540/// let mut hasher = DefaultHasher::new();
2541/// (five_ref as *const i32).hash(&mut hasher);
2542/// let expected = hasher.finish();
2543///
2544/// assert_eq!(actual, expected);
2545/// ```
2546#[stable(feature = "ptr_hash", since = "1.35.0")]
2547pub fn hash<T: PointeeSized, S: hash::Hasher>(hashee: *const T, into: &mut S) {
2548    use crate::hash::Hash;
2549    hashee.hash(into);
2550}
2551
2552#[stable(feature = "fnptr_impls", since = "1.4.0")]
2553impl<F: FnPtr> PartialEq for F {
2554    #[inline]
2555    fn eq(&self, other: &Self) -> bool {
2556        self.addr() == other.addr()
2557    }
2558}
2559#[stable(feature = "fnptr_impls", since = "1.4.0")]
2560impl<F: FnPtr> Eq for F {}
2561
2562#[stable(feature = "fnptr_impls", since = "1.4.0")]
2563impl<F: FnPtr> PartialOrd for F {
2564    #[inline]
2565    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
2566        self.addr().partial_cmp(&other.addr())
2567    }
2568}
2569#[stable(feature = "fnptr_impls", since = "1.4.0")]
2570impl<F: FnPtr> Ord for F {
2571    #[inline]
2572    fn cmp(&self, other: &Self) -> Ordering {
2573        self.addr().cmp(&other.addr())
2574    }
2575}
2576
2577#[stable(feature = "fnptr_impls", since = "1.4.0")]
2578impl<F: FnPtr> hash::Hash for F {
2579    fn hash<HH: hash::Hasher>(&self, state: &mut HH) {
2580        state.write_usize(self.addr() as _)
2581    }
2582}
2583
2584#[stable(feature = "fnptr_impls", since = "1.4.0")]
2585impl<F: FnPtr> fmt::Pointer for F {
2586    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2587        fmt::pointer_fmt_inner(self.addr() as _, f)
2588    }
2589}
2590
2591#[stable(feature = "fnptr_impls", since = "1.4.0")]
2592impl<F: FnPtr> fmt::Debug for F {
2593    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2594        fmt::pointer_fmt_inner(self.addr() as _, f)
2595    }
2596}
2597
2598/// Creates a `const` raw pointer to a place, without creating an intermediate reference.
2599///
2600/// `addr_of!(expr)` is equivalent to `&raw const expr`. The macro is *soft-deprecated*;
2601/// use `&raw const` instead.
2602///
2603/// It is still an open question under which conditions writing through an `addr_of!`-created
2604/// pointer is permitted. If the place `expr` evaluates to is based on a raw pointer, then the
2605/// result of `addr_of!` inherits all permissions from that raw pointer. However, if the place is
2606/// based on a reference, local variable, or `static`, then until all details are decided, the same
2607/// rules as for shared references apply: it is UB to write through a pointer created with this
2608/// operation, except for bytes located inside an `UnsafeCell`. Use `&raw mut` (or [`addr_of_mut`])
2609/// to create a raw pointer that definitely permits mutation.
2610///
2611/// Creating a reference with `&`/`&mut` is only allowed if the pointer is properly aligned
2612/// and points to initialized data. For cases where those requirements do not hold,
2613/// raw pointers should be used instead. However, `&expr as *const _` creates a reference
2614/// before casting it to a raw pointer, and that reference is subject to the same rules
2615/// as all other references. This macro can create a raw pointer *without* creating
2616/// a reference first.
2617///
2618/// See [`addr_of_mut`] for how to create a pointer to uninitialized data.
2619/// Doing that with `addr_of` would not make much sense since one could only
2620/// read the data, and that would be Undefined Behavior.
2621///
2622/// # Safety
2623///
2624/// The `expr` in `addr_of!(expr)` is evaluated as a place expression, but never loads from the
2625/// place or requires the place to be dereferenceable. This means that `addr_of!((*ptr).field)`
2626/// still requires the projection to `field` to be in-bounds, using the same rules as [`offset`].
2627/// However, `addr_of!(*ptr)` is defined behavior even if `ptr` is null, dangling, or misaligned.
2628///
2629/// Note that `Deref`/`Index` coercions (and their mutable counterparts) are applied inside
2630/// `addr_of!` like everywhere else, in which case a reference is created to call `Deref::deref` or
2631/// `Index::index`, respectively. The statements above only apply when no such coercions are
2632/// applied.
2633///
2634/// [`offset`]: pointer::offset
2635///
2636/// # Example
2637///
2638/// **Correct usage: Creating a pointer to unaligned data**
2639///
2640/// ```
2641/// use std::ptr;
2642///
2643/// #[repr(packed)]
2644/// struct Packed {
2645///     f1: u8,
2646///     f2: u16,
2647/// }
2648///
2649/// let packed = Packed { f1: 1, f2: 2 };
2650/// // `&packed.f2` would create an unaligned reference, and thus be Undefined Behavior!
2651/// let raw_f2 = ptr::addr_of!(packed.f2);
2652/// assert_eq!(unsafe { raw_f2.read_unaligned() }, 2);
2653/// ```
2654///
2655/// **Incorrect usage: Out-of-bounds fields projection**
2656///
2657/// ```rust,no_run
2658/// use std::ptr;
2659///
2660/// #[repr(C)]
2661/// struct MyStruct {
2662///     field1: i32,
2663///     field2: i32,
2664/// }
2665///
2666/// let ptr: *const MyStruct = ptr::null();
2667/// let fieldptr = unsafe { ptr::addr_of!((*ptr).field2) }; // Undefined Behavior ⚠️
2668/// ```
2669///
2670/// The field projection `.field2` would offset the pointer by 4 bytes,
2671/// but the pointer is not in-bounds of an allocation for 4 bytes,
2672/// so this offset is Undefined Behavior.
2673/// See the [`offset`] docs for a full list of requirements for inbounds pointer arithmetic; the
2674/// same requirements apply to field projections, even inside `addr_of!`. (In particular, it makes
2675/// no difference whether the pointer is null or dangling.)
2676#[stable(feature = "raw_ref_macros", since = "1.51.0")]
2677#[rustc_macro_transparency = "semitransparent"]
2678pub macro addr_of($place:expr) {
2679    &raw const $place
2680}
2681
2682/// Creates a `mut` raw pointer to a place, without creating an intermediate reference.
2683///
2684/// `addr_of_mut!(expr)` is equivalent to `&raw mut expr`. The macro is *soft-deprecated*;
2685/// use `&raw mut` instead.
2686///
2687/// Creating a reference with `&`/`&mut` is only allowed if the pointer is properly aligned
2688/// and points to initialized data. For cases where those requirements do not hold,
2689/// raw pointers should be used instead. However, `&mut expr as *mut _` creates a reference
2690/// before casting it to a raw pointer, and that reference is subject to the same rules
2691/// as all other references. This macro can create a raw pointer *without* creating
2692/// a reference first.
2693///
2694/// # Safety
2695///
2696/// The `expr` in `addr_of_mut!(expr)` is evaluated as a place expression, but never loads from the
2697/// place or requires the place to be dereferenceable. This means that `addr_of_mut!((*ptr).field)`
2698/// still requires the projection to `field` to be in-bounds, using the same rules as [`offset`].
2699/// However, `addr_of_mut!(*ptr)` is defined behavior even if `ptr` is null, dangling, or misaligned.
2700///
2701/// Note that `Deref`/`Index` coercions (and their mutable counterparts) are applied inside
2702/// `addr_of_mut!` like everywhere else, in which case a reference is created to call `Deref::deref`
2703/// or `Index::index`, respectively. The statements above only apply when no such coercions are
2704/// applied.
2705///
2706/// [`offset`]: pointer::offset
2707///
2708/// # Examples
2709///
2710/// **Correct usage: Creating a pointer to unaligned data**
2711///
2712/// ```
2713/// use std::ptr;
2714///
2715/// #[repr(packed)]
2716/// struct Packed {
2717///     f1: u8,
2718///     f2: u16,
2719/// }
2720///
2721/// let mut packed = Packed { f1: 1, f2: 2 };
2722/// // `&mut packed.f2` would create an unaligned reference, and thus be Undefined Behavior!
2723/// let raw_f2 = ptr::addr_of_mut!(packed.f2);
2724/// unsafe { raw_f2.write_unaligned(42); }
2725/// assert_eq!({packed.f2}, 42); // `{...}` forces copying the field instead of creating a reference.
2726/// ```
2727///
2728/// **Correct usage: Creating a pointer to uninitialized data**
2729///
2730/// ```rust
2731/// use std::{ptr, mem::MaybeUninit};
2732///
2733/// struct Demo {
2734///     field: bool,
2735/// }
2736///
2737/// let mut uninit = MaybeUninit::<Demo>::uninit();
2738/// // `&uninit.as_mut().field` would create a reference to an uninitialized `bool`,
2739/// // and thus be Undefined Behavior!
2740/// let f1_ptr = unsafe { ptr::addr_of_mut!((*uninit.as_mut_ptr()).field) };
2741/// unsafe { f1_ptr.write(true); }
2742/// let init = unsafe { uninit.assume_init() };
2743/// ```
2744///
2745/// **Incorrect usage: Out-of-bounds fields projection**
2746///
2747/// ```rust,no_run
2748/// use std::ptr;
2749///
2750/// #[repr(C)]
2751/// struct MyStruct {
2752///     field1: i32,
2753///     field2: i32,
2754/// }
2755///
2756/// let ptr: *mut MyStruct = ptr::null_mut();
2757/// let fieldptr = unsafe { ptr::addr_of_mut!((*ptr).field2) }; // Undefined Behavior ⚠️
2758/// ```
2759///
2760/// The field projection `.field2` would offset the pointer by 4 bytes,
2761/// but the pointer is not in-bounds of an allocation for 4 bytes,
2762/// so this offset is Undefined Behavior.
2763/// See the [`offset`] docs for a full list of requirements for inbounds pointer arithmetic; the
2764/// same requirements apply to field projections, even inside `addr_of_mut!`. (In particular, it
2765/// makes no difference whether the pointer is null or dangling.)
2766#[stable(feature = "raw_ref_macros", since = "1.51.0")]
2767#[rustc_macro_transparency = "semitransparent"]
2768pub macro addr_of_mut($place:expr) {
2769    &raw mut $place
2770}