library/core/src/ptr/const_ptr.rs

   1 use super::*;
   2 use crate::cmp::Ordering::{self, Equal, Greater, Less};
   3 use crate::intrinsics;
   4 use crate::mem;
   5 use crate::slice::{self, SliceIndex};
   6
   7 impl<T: ?Sized> *const T {
   8     /// Returns `true` if the pointer is null.
   9     ///
  10     /// Note that unsized types have many possible null pointers, as only the
  11     /// raw data pointer is considered, not their length, vtable, etc.
  12     /// Therefore, two pointers that are null may still not compare equal to
  13     /// each other.
  14     ///
  15     /// ## Behavior during const evaluation
  16     ///
  17     /// When this function is used during const evaluation, it may return `false` for pointers
  18     /// that turn out to be null at runtime. Specifically, when a pointer to some memory
  19     /// is offset beyond its bounds in such a way that the resulting pointer is null,
  20     /// the function will still return `false`. There is no way for CTFE to know
  21     /// the absolute position of that memory, so we cannot tell if the pointer is
  22     /// null or not.
  23     ///
  24     /// # Examples
  25     ///
  26     /// Basic usage:
  27     ///
  28     /// ```
  29     /// let s: &str = "Follow the rabbit";
  30     /// let ptr: *const u8 = s.as_ptr();
  31     /// assert!(!ptr.is_null());
  32     /// ```
  33     #[stable(feature = "rust1", since = "1.0.0")]
  34     #[rustc_const_unstable(feature = "const_ptr_is_null", issue = "74939")]
  35     #[inline]
  36     pub const fn is_null(self) -> bool {
  37         // Compare via a cast to a thin pointer, so fat pointers are only
  38         // considering their "data" part for null-ness.
  39         (self as *const u8).guaranteed_eq(null())
  40     }
  41
  42     /// Casts to a pointer of another type.
  43     #[stable(feature = "ptr_cast", since = "1.38.0")]
  44     #[rustc_const_stable(feature = "const_ptr_cast", since = "1.38.0")]
  45     #[inline]
  46     pub const fn cast<U>(self) -> *const U {
  47         self as _
  48     }
  49
  50     /// Use the pointer value in a new pointer of another type.
  51     ///
  52     /// In case `val` is a (fat) pointer to an unsized type, this operation
  53     /// will ignore the pointer part, whereas for (thin) pointers to sized
  54     /// types, this has the same effect as a simple cast.
  55     ///
  56     /// The resulting pointer will have provenance of `self`, i.e., for a fat
  57     /// pointer, this operation is semantically the same as creating a new
  58     /// fat pointer with the data pointer value of `self` but the metadata of
  59     /// `val`.
  60     ///
  61     /// # Examples
  62     ///
  63     /// This function is primarily useful for allowing byte-wise pointer
  64     /// arithmetic on potentially fat pointers:
  65     ///
  66     /// ```
  67     /// #![feature(set_ptr_value)]
  68     /// # use core::fmt::Debug;
  69     /// let arr: [i32; 3] = [1, 2, 3];
  70     /// let mut ptr = arr.as_ptr() as *const dyn Debug;
  71     /// let thin = ptr as *const u8;
  72     /// unsafe {
  73     ///     ptr = thin.add(8).with_metadata_of(ptr);
  74     ///     # assert_eq!(*(ptr as *const i32), 3);
  75     ///     println!("{:?}", &*ptr); // will print "3"
  76     /// }
  77     /// ```
  78     #[unstable(feature = "set_ptr_value", issue = "75091")]
  79     #[must_use = "returns a new pointer rather than modifying its argument"]
  80     #[inline]
  81     pub fn with_metadata_of<U>(self, mut val: *const U) -> *const U
  82     where
  83         U: ?Sized,
  84     {
  85         let target = &mut val as *mut *const U as *mut *const u8;
  86         // SAFETY: In case of a thin pointer, this operations is identical
  87         // to a simple assignment. In case of a fat pointer, with the current
  88         // fat pointer layout implementation, the first field of such a
  89         // pointer is always the data pointer, which is likewise assigned.
  90         unsafe { *target = self as *const u8 };
  91         val
  92     }
  93
  94     /// Changes constness without changing the type.
  95     ///
  96     /// This is a bit safer than `as` because it wouldn't silently change the type if the code is
  97     /// refactored.
  98     #[unstable(feature = "ptr_const_cast", issue = "92675")]
  99     #[rustc_const_unstable(feature = "ptr_const_cast", issue = "92675")]
 100     pub const fn cast_mut(self) -> *mut T {
 101         self as _
 102     }
 103
 104     /// Casts a pointer to its raw bits.
 105     ///
 106     /// This is equivalent to `as usize`, but is more specific to enhance readability.
 107     /// The inverse method is [`from_bits`](#method.from_bits).
 108     ///
 109     /// In particular, `*p as usize` and `p as usize` will both compile for
 110     /// pointers to numeric types but do very different things, so using this
 111     /// helps emphasize that reading the bits was intentional.
 112     ///
 113     /// # Examples
 114     ///
 115     /// ```
 116     /// #![feature(ptr_to_from_bits)]
 117     /// let array = [13, 42];
 118     /// let p0: *const i32 = &array[0];
 119     /// assert_eq!(<*const _>::from_bits(p0.to_bits()), p0);
 120     /// let p1: *const i32 = &array[1];
 121     /// assert_eq!(p1.to_bits() - p0.to_bits(), 4);
 122     /// ```
 123     #[unstable(feature = "ptr_to_from_bits", issue = "91126")]
 124     pub fn to_bits(self) -> usize
 125     where
 126         T: Sized,
 127     {
 128         self as usize
 129     }
 130
 131     /// Creates a pointer from its raw bits.
 132     ///
 133     /// This is equivalent to `as *const T`, but is more specific to enhance readability.
 134     /// The inverse method is [`to_bits`](#method.to_bits).
 135     ///
 136     /// # Examples
 137     ///
 138     /// ```
 139     /// #![feature(ptr_to_from_bits)]
 140     /// use std::ptr::NonNull;
 141     /// let dangling: *const u8 = NonNull::dangling().as_ptr();
 142     /// assert_eq!(<*const u8>::from_bits(1), dangling);
 143     /// ```
 144     #[unstable(feature = "ptr_to_from_bits", issue = "91126")]
 145     pub fn from_bits(bits: usize) -> Self
 146     where
 147         T: Sized,
 148     {
 149         bits as Self
 150     }
 151
 152     /// Gets the "address" portion of the pointer.
 153     ///
 154     /// This is similar to `self as usize`, which semantically discards *provenance* and
 155     /// *address-space* information. However, unlike `self as usize`, casting the returned address
 156     /// back to a pointer yields [`invalid`][], which is undefined behavior to dereference. To
 157     /// properly restore the lost information and obtain a dereferencable pointer, use
 158     /// [`with_addr`][pointer::with_addr] or [`map_addr`][pointer::map_addr].
 159     ///
 160     /// If using those APIs is not possible because there is no way to preserve a pointer with the
 161     /// required provenance, use [`expose_addr`][pointer::expose_addr] and
 162     /// [`from_exposed_addr`][from_exposed_addr] instead. However, note that this makes
 163     /// your code less portable and less amenable to tools that check for compliance with the Rust
 164     /// memory model.
 165     ///
 166     /// On most platforms this will produce a value with the same bytes as the original
 167     /// pointer, because all the bytes are dedicated to describing the address.
 168     /// Platforms which need to store additional information in the pointer may
 169     /// perform a change of representation to produce a value containing only the address
 170     /// portion of the pointer. What that means is up to the platform to define.
 171     ///
 172     /// This API and its claimed semantics are part of the Strict Provenance experiment, and as such
 173     /// might change in the future (including possibly weakening this so it becomes wholly
 174     /// equivalent to `self as usize`). See the [module documentation][crate::ptr] for details.
 175     #[must_use]
 176     #[inline]
 177     #[unstable(feature = "strict_provenance", issue = "95228")]
 178     pub fn addr(self) -> usize
 179     where
 180         T: Sized,
 181     {
 182         // FIXME(strict_provenance_magic): I am magic and should be a compiler intrinsic.
 183         // SAFETY: Pointer-to-integer transmutes are valid (if you are okay with losing the
 184         // provenance).
 185         unsafe { mem::transmute(self) }
 186     }
 187
 188     /// Gets the "address" portion of the pointer, and 'exposes' the "provenance" part for future
 189     /// use in [`from_exposed_addr`][].
 190     ///
 191     /// This is equivalent to `self as usize`, which semantically discards *provenance* and
 192     /// *address-space* information. Furthermore, this (like the `as` cast) has the implicit
 193     /// side-effect of marking the provenance as 'exposed', so on platforms that support it you can
 194     /// later call [`from_exposed_addr`][] to reconstitute the original pointer including its
 195     /// provenance. (Reconstructing address space information, if required, is your responsibility.)
 196     ///
 197     /// Using this method means that code is *not* following Strict Provenance rules. Supporting
 198     /// [`from_exposed_addr`][] complicates specification and reasoning and may not be supported by
 199     /// tools that help you to stay conformant with the Rust memory model, so it is recommended to
 200     /// use [`addr`][pointer::addr] wherever possible.
 201     ///
 202     /// On most platforms this will produce a value with the same bytes as the original pointer,
 203     /// because all the bytes are dedicated to describing the address. Platforms which need to store
 204     /// additional information in the pointer may not support this operation, since the 'expose'
 205     /// side-effect which is required for [`from_exposed_addr`][] to work is typically not
 206     /// available.
 207     ///
 208     /// This API and its claimed semantics are part of the Strict Provenance experiment, see the
 209     /// [module documentation][crate::ptr] for details.
 210     ///
 211     /// [`from_exposed_addr`]: from_exposed_addr
 212     #[must_use]
 213     #[inline]
 214     #[unstable(feature = "strict_provenance", issue = "95228")]
 215     pub fn expose_addr(self) -> usize
 216     where
 217         T: Sized,
 218     {
 219         // FIXME(strict_provenance_magic): I am magic and should be a compiler intrinsic.
 220         self as usize
 221     }
 222
 223     /// Creates a new pointer with the given address.
 224     ///
 225     /// This performs the same operation as an `addr as ptr` cast, but copies
 226     /// the *address-space* and *provenance* of `self` to the new pointer.
 227     /// This allows us to dynamically preserve and propagate this important
 228     /// information in a way that is otherwise impossible with a unary cast.
 229     ///
 230     /// This is equivalent to using [`wrapping_offset`][pointer::wrapping_offset] to offset
 231     /// `self` to the given address, and therefore has all the same capabilities and restrictions.
 232     ///
 233     /// This API and its claimed semantics are part of the Strict Provenance experiment,
 234     /// see the [module documentation][crate::ptr] for details.
 235     #[must_use]
 236     #[inline]
 237     #[unstable(feature = "strict_provenance", issue = "95228")]
 238     pub fn with_addr(self, addr: usize) -> Self
 239     where
 240         T: Sized,
 241     {
 242         // FIXME(strict_provenance_magic): I am magic and should be a compiler intrinsic.
 243         //
 244         // In the mean-time, this operation is defined to be "as if" it was
 245         // a wrapping_offset, so we can emulate it as such. This should properly
 246         // restore pointer provenance even under today's compiler.
 247         let self_addr = self.addr() as isize;
 248         let dest_addr = addr as isize;
 249         let offset = dest_addr.wrapping_sub(self_addr);
 250
 251         // This is the canonical desugarring of this operation
 252         self.cast::<u8>().wrapping_offset(offset).cast::<T>()
 253     }
 254
 255     /// Creates a new pointer by mapping `self`'s address to a new one.
 256     ///
 257     /// This is a convenience for [`with_addr`][pointer::with_addr], see that method for details.
 258     ///
 259     /// This API and its claimed semantics are part of the Strict Provenance experiment,
 260     /// see the [module documentation][crate::ptr] for details.
 261     #[must_use]
 262     #[inline]
 263     #[unstable(feature = "strict_provenance", issue = "95228")]
 264     pub fn map_addr(self, f: impl FnOnce(usize) -> usize) -> Self
 265     where
 266         T: Sized,
 267     {
 268         self.with_addr(f(self.addr()))
 269     }
 270
 271     /// Decompose a (possibly wide) pointer into its address and metadata components.
 272     ///
 273     /// The pointer can be later reconstructed with [`from_raw_parts`].
 274     #[unstable(feature = "ptr_metadata", issue = "81513")]
 275     #[rustc_const_unstable(feature = "ptr_metadata", issue = "81513")]
 276     #[inline]
 277     pub const fn to_raw_parts(self) -> (*const (), <T as super::Pointee>::Metadata) {
 278         (self.cast(), metadata(self))
 279     }
 280
 281     /// Returns `None` if the pointer is null, or else returns a shared reference to
 282     /// the value wrapped in `Some`. If the value may be uninitialized, [`as_uninit_ref`]
 283     /// must be used instead.
 284     ///
 285     /// [`as_uninit_ref`]: #method.as_uninit_ref
 286     ///
 287     /// # Safety
 288     ///
 289     /// When calling this method, you have to ensure that *either* the pointer is null *or*
 290     /// all of the following is true:
 291     ///
 292     /// * The pointer must be properly aligned.
 293     ///
 294     /// * It must be "dereferenceable" in the sense defined in [the module documentation].
 295     ///
 296     /// * The pointer must point to an initialized instance of `T`.
 297     ///
 298     /// * You must enforce Rust's aliasing rules, since the returned lifetime `'a` is
 299     ///   arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
 300     ///   In particular, while this reference exists, the memory the pointer points to must
 301     ///   not get mutated (except inside `UnsafeCell`).
 302     ///
 303     /// This applies even if the result of this method is unused!
 304     /// (The part about being initialized is not yet fully decided, but until
 305     /// it is, the only safe approach is to ensure that they are indeed initialized.)
 306     ///
 307     /// [the module documentation]: crate::ptr#safety
 308     ///
 309     /// # Examples
 310     ///
 311     /// Basic usage:
 312     ///
 313     /// ```
 314     /// let ptr: *const u8 = &10u8 as *const u8;
 315     ///
 316     /// unsafe {
 317     ///     if let Some(val_back) = ptr.as_ref() {
 318     ///         println!("We got back the value: {val_back}!");
 319     ///     }
 320     /// }
 321     /// ```
 322     ///
 323     /// # Null-unchecked version
 324     ///
 325     /// If you are sure the pointer can never be null and are looking for some kind of
 326     /// `as_ref_unchecked` that returns the `&T` instead of `Option<&T>`, know that you can
 327     /// dereference the pointer directly.
 328     ///
 329     /// ```
 330     /// let ptr: *const u8 = &10u8 as *const u8;
 331     ///
 332     /// unsafe {
 333     ///     let val_back = &*ptr;
 334     ///     println!("We got back the value: {val_back}!");
 335     /// }
 336     /// ```
 337     #[stable(feature = "ptr_as_ref", since = "1.9.0")]
 338     #[rustc_const_unstable(feature = "const_ptr_as_ref", issue = "91822")]
 339     #[inline]
 340     pub const unsafe fn as_ref<'a>(self) -> Option<&'a T> {
 341         // SAFETY: the caller must guarantee that `self` is valid
 342         // for a reference if it isn't null.
 343         if self.is_null() { None } else { unsafe { Some(&*self) } }
 344     }
 345
 346     /// Returns `None` if the pointer is null, or else returns a shared reference to
 347     /// the value wrapped in `Some`. In contrast to [`as_ref`], this does not require
 348     /// that the value has to be initialized.
 349     ///
 350     /// [`as_ref`]: #method.as_ref
 351     ///
 352     /// # Safety
 353     ///
 354     /// When calling this method, you have to ensure that *either* the pointer is null *or*
 355     /// all of the following is true:
 356     ///
 357     /// * The pointer must be properly aligned.
 358     ///
 359     /// * It must be "dereferenceable" in the sense defined in [the module documentation].
 360     ///
 361     /// * You must enforce Rust's aliasing rules, since the returned lifetime `'a` is
 362     ///   arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
 363     ///   In particular, while this reference exists, the memory the pointer points to must
 364     ///   not get mutated (except inside `UnsafeCell`).
 365     ///
 366     /// This applies even if the result of this method is unused!
 367     ///
 368     /// [the module documentation]: crate::ptr#safety
 369     ///
 370     /// # Examples
 371     ///
 372     /// Basic usage:
 373     ///
 374     /// ```
 375     /// #![feature(ptr_as_uninit)]
 376     ///
 377     /// let ptr: *const u8 = &10u8 as *const u8;
 378     ///
 379     /// unsafe {
 380     ///     if let Some(val_back) = ptr.as_uninit_ref() {
 381     ///         println!("We got back the value: {}!", val_back.assume_init());
 382     ///     }
 383     /// }
 384     /// ```
 385     #[inline]
 386     #[unstable(feature = "ptr_as_uninit", issue = "75402")]
 387     #[rustc_const_unstable(feature = "const_ptr_as_ref", issue = "91822")]
 388     pub const unsafe fn as_uninit_ref<'a>(self) -> Option<&'a MaybeUninit<T>>
 389     where
 390         T: Sized,
 391     {
 392         // SAFETY: the caller must guarantee that `self` meets all the
 393         // requirements for a reference.
 394         if self.is_null() { None } else { Some(unsafe { &*(self as *const MaybeUninit<T>) }) }
 395     }
 396
 397     /// Calculates the offset from a pointer.
 398     ///
 399     /// `count` is in units of T; e.g., a `count` of 3 represents a pointer
 400     /// offset of `3 * size_of::<T>()` bytes.
 401     ///
 402     /// # Safety
 403     ///
 404     /// If any of the following conditions are violated, the result is Undefined
 405     /// Behavior:
 406     ///
 407     /// * Both the starting and resulting pointer must be either in bounds or one
 408     ///   byte past the end of the same [allocated object].
 409     ///
 410     /// * The computed offset, **in bytes**, cannot overflow an `isize`.
 411     ///
 412     /// * The offset being in bounds cannot rely on "wrapping around" the address
 413     ///   space. That is, the infinite-precision sum, **in bytes** must fit in a usize.
 414     ///
 415     /// The compiler and standard library generally tries to ensure allocations
 416     /// never reach a size where an offset is a concern. For instance, `Vec`
 417     /// and `Box` ensure they never allocate more than `isize::MAX` bytes, so
 418     /// `vec.as_ptr().add(vec.len())` is always safe.
 419     ///
 420     /// Most platforms fundamentally can't even construct such an allocation.
 421     /// For instance, no known 64-bit platform can ever serve a request
 422     /// for 2<sup>63</sup> bytes due to page-table limitations or splitting the address space.
 423     /// However, some 32-bit and 16-bit platforms may successfully serve a request for
 424     /// more than `isize::MAX` bytes with things like Physical Address
 425     /// Extension. As such, memory acquired directly from allocators or memory
 426     /// mapped files *may* be too large to handle with this function.
 427     ///
 428     /// Consider using [`wrapping_offset`] instead if these constraints are
 429     /// difficult to satisfy. The only advantage of this method is that it
 430     /// enables more aggressive compiler optimizations.
 431     ///
 432     /// [`wrapping_offset`]: #method.wrapping_offset
 433     /// [allocated object]: crate::ptr#allocated-object
 434     ///
 435     /// # Examples
 436     ///
 437     /// Basic usage:
 438     ///
 439     /// ```
 440     /// let s: &str = "123";
 441     /// let ptr: *const u8 = s.as_ptr();
 442     ///
 443     /// unsafe {
 444     ///     println!("{}", *ptr.offset(1) as char);
 445     ///     println!("{}", *ptr.offset(2) as char);
 446     /// }
 447     /// ```
 448     #[stable(feature = "rust1", since = "1.0.0")]
 449     #[must_use = "returns a new pointer rather than modifying its argument"]
 450     #[rustc_const_stable(feature = "const_ptr_offset", since = "1.61.0")]
 451     #[inline(always)]
 452     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
 453     pub const unsafe fn offset(self, count: isize) -> *const T
 454     where
 455         T: Sized,
 456     {
 457         // SAFETY: the caller must uphold the safety contract for `offset`.
 458         unsafe { intrinsics::offset(self, count) }
 459     }
 460
 461     /// Calculates the offset from a pointer in bytes.
 462     ///
 463     /// `count` is in units of **bytes**.
 464     ///
 465     /// This is purely a convenience for casting to a `u8` pointer and
 466     /// using [offset][pointer::offset] on it. See that method for documentation
 467     /// and safety requirements.
 468     ///
 469     /// For non-`Sized` pointees this operation changes only the data pointer,
 470     /// leaving the metadata untouched.
 471     #[must_use]
 472     #[inline(always)]
 473     #[unstable(feature = "pointer_byte_offsets", issue = "96283")]
 474     #[rustc_const_unstable(feature = "const_pointer_byte_offsets", issue = "96283")]
 475     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
 476     pub const unsafe fn byte_offset(self, count: isize) -> Self {
 477         // SAFETY: the caller must uphold the safety contract for `offset`.
 478         let this = unsafe { self.cast::<u8>().offset(count).cast::<()>() };
 479         from_raw_parts::<T>(this, metadata(self))
 480     }
 481
 482     /// Calculates the offset from a pointer using wrapping arithmetic.
 483     ///
 484     /// `count` is in units of T; e.g., a `count` of 3 represents a pointer
 485     /// offset of `3 * size_of::<T>()` bytes.
 486     ///
 487     /// # Safety
 488     ///
 489     /// This operation itself is always safe, but using the resulting pointer is not.
 490     ///
 491     /// The resulting pointer "remembers" the [allocated object] that `self` points to; it must not
 492     /// be used to read or write other allocated objects.
 493     ///
 494     /// In other words, `let z = x.wrapping_offset((y as isize) - (x as isize))` does *not* make `z`
 495     /// the same as `y` even if we assume `T` has size `1` and there is no overflow: `z` is still
 496     /// attached to the object `x` is attached to, and dereferencing it is Undefined Behavior unless
 497     /// `x` and `y` point into the same allocated object.
 498     ///
 499     /// Compared to [`offset`], this method basically delays the requirement of staying within the
 500     /// same allocated object: [`offset`] is immediate Undefined Behavior when crossing object
 501     /// boundaries; `wrapping_offset` produces a pointer but still leads to Undefined Behavior if a
 502     /// pointer is dereferenced when it is out-of-bounds of the object it is attached to. [`offset`]
 503     /// can be optimized better and is thus preferable in performance-sensitive code.
 504     ///
 505     /// The delayed check only considers the value of the pointer that was dereferenced, not the
 506     /// intermediate values used during the computation of the final result. For example,
 507     /// `x.wrapping_offset(o).wrapping_offset(o.wrapping_neg())` is always the same as `x`. In other
 508     /// words, leaving the allocated object and then re-entering it later is permitted.
 509     ///
 510     /// [`offset`]: #method.offset
 511     /// [allocated object]: crate::ptr#allocated-object
 512     ///
 513     /// # Examples
 514     ///
 515     /// Basic usage:
 516     ///
 517     /// ```
 518     /// // Iterate using a raw pointer in increments of two elements
 519     /// let data = [1u8, 2, 3, 4, 5];
 520     /// let mut ptr: *const u8 = data.as_ptr();
 521     /// let step = 2;
 522     /// let end_rounded_up = ptr.wrapping_offset(6);
 523     ///
 524     /// // This loop prints "1, 3, 5, "
 525     /// while ptr != end_rounded_up {
 526     ///     unsafe {
 527     ///         print!("{}, ", *ptr);
 528     ///     }
 529     ///     ptr = ptr.wrapping_offset(step);
 530     /// }
 531     /// ```
 532     #[stable(feature = "ptr_wrapping_offset", since = "1.16.0")]
 533     #[must_use = "returns a new pointer rather than modifying its argument"]
 534     #[rustc_const_stable(feature = "const_ptr_offset", since = "1.61.0")]
 535     #[inline(always)]
 536     pub const fn wrapping_offset(self, count: isize) -> *const T
 537     where
 538         T: Sized,
 539     {
 540         // SAFETY: the `arith_offset` intrinsic has no prerequisites to be called.
 541         unsafe { intrinsics::arith_offset(self, count) }
 542     }
 543
 544     /// Calculates the offset from a pointer in bytes using wrapping arithmetic.
 545     ///
 546     /// `count` is in units of **bytes**.
 547     ///
 548     /// This is purely a convenience for casting to a `u8` pointer and
 549     /// using [wrapping_offset][pointer::wrapping_offset] on it. See that method
 550     /// for documentation.
 551     ///
 552     /// For non-`Sized` pointees this operation changes only the data pointer,
 553     /// leaving the metadata untouched.
 554     #[must_use]
 555     #[inline(always)]
 556     #[unstable(feature = "pointer_byte_offsets", issue = "96283")]
 557     #[rustc_const_unstable(feature = "const_pointer_byte_offsets", issue = "96283")]
 558     pub const fn wrapping_byte_offset(self, count: isize) -> Self {
 559         from_raw_parts::<T>(self.cast::<u8>().wrapping_offset(count).cast::<()>(), metadata(self))
 560     }
 561
 562     /// Calculates the distance between two pointers. The returned value is in
 563     /// units of T: the distance in bytes divided by `mem::size_of::<T>()`.
 564     ///
 565     /// This function is the inverse of [`offset`].
 566     ///
 567     /// [`offset`]: #method.offset
 568     ///
 569     /// # Safety
 570     ///
 571     /// If any of the following conditions are violated, the result is Undefined
 572     /// Behavior:
 573     ///
 574     /// * Both the starting and other pointer must be either in bounds or one
 575     ///   byte past the end of the same [allocated object].
 576     ///
 577     /// * Both pointers must be *derived from* a pointer to the same object.
 578     ///   (See below for an example.)
 579     ///
 580     /// * The distance between the pointers, in bytes, must be an exact multiple
 581     ///   of the size of `T`.
 582     ///
 583     /// * The distance between the pointers, **in bytes**, cannot overflow an `isize`.
 584     ///
 585     /// * The distance being in bounds cannot rely on "wrapping around" the address space.
 586     ///
 587     /// Rust types are never larger than `isize::MAX` and Rust allocations never wrap around the
 588     /// address space, so two pointers within some value of any Rust type `T` will always satisfy
 589     /// the last two conditions. The standard library also generally ensures that allocations
 590     /// never reach a size where an offset is a concern. For instance, `Vec` and `Box` ensure they
 591     /// never allocate more than `isize::MAX` bytes, so `ptr_into_vec.offset_from(vec.as_ptr())`
 592     /// always satisfies the last two conditions.
 593     ///
 594     /// Most platforms fundamentally can't even construct such a large allocation.
 595     /// For instance, no known 64-bit platform can ever serve a request
 596     /// for 2<sup>63</sup> bytes due to page-table limitations or splitting the address space.
 597     /// However, some 32-bit and 16-bit platforms may successfully serve a request for
 598     /// more than `isize::MAX` bytes with things like Physical Address
 599     /// Extension. As such, memory acquired directly from allocators or memory
 600     /// mapped files *may* be too large to handle with this function.
 601     /// (Note that [`offset`] and [`add`] also have a similar limitation and hence cannot be used on
 602     /// such large allocations either.)
 603     ///
 604     /// [`add`]: #method.add
 605     /// [allocated object]: crate::ptr#allocated-object
 606     ///
 607     /// # Panics
 608     ///
 609     /// This function panics if `T` is a Zero-Sized Type ("ZST").
 610     ///
 611     /// # Examples
 612     ///
 613     /// Basic usage:
 614     ///
 615     /// ```
 616     /// let a = [0; 5];
 617     /// let ptr1: *const i32 = &a[1];
 618     /// let ptr2: *const i32 = &a[3];
 619     /// unsafe {
 620     ///     assert_eq!(ptr2.offset_from(ptr1), 2);
 621     ///     assert_eq!(ptr1.offset_from(ptr2), -2);
 622     ///     assert_eq!(ptr1.offset(2), ptr2);
 623     ///     assert_eq!(ptr2.offset(-2), ptr1);
 624     /// }
 625     /// ```
 626     ///
 627     /// *Incorrect* usage:
 628     ///
 629     /// ```rust,no_run
 630     /// let ptr1 = Box::into_raw(Box::new(0u8)) as *const u8;
 631     /// let ptr2 = Box::into_raw(Box::new(1u8)) as *const u8;
 632     /// let diff = (ptr2 as isize).wrapping_sub(ptr1 as isize);
 633     /// // Make ptr2_other an "alias" of ptr2, but derived from ptr1.
 634     /// let ptr2_other = (ptr1 as *const u8).wrapping_offset(diff);
 635     /// assert_eq!(ptr2 as usize, ptr2_other as usize);
 636     /// // Since ptr2_other and ptr2 are derived from pointers to different objects,
 637     /// // computing their offset is undefined behavior, even though
 638     /// // they point to the same address!
 639     /// unsafe {
 640     ///     let zero = ptr2_other.offset_from(ptr2); // Undefined Behavior
 641     /// }
 642     /// ```
 643     #[stable(feature = "ptr_offset_from", since = "1.47.0")]
 644     #[rustc_const_unstable(feature = "const_ptr_offset_from", issue = "92980")]
 645     #[inline]
 646     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
 647     pub const unsafe fn offset_from(self, origin: *const T) -> isize
 648     where
 649         T: Sized,
 650     {
 651         let pointee_size = mem::size_of::<T>();
 652         assert!(0 < pointee_size && pointee_size <= isize::MAX as usize);
 653         // SAFETY: the caller must uphold the safety contract for `ptr_offset_from`.
 654         unsafe { intrinsics::ptr_offset_from(self, origin) }
 655     }
 656
 657     /// Calculates the distance between two pointers. The returned value is in
 658     /// units of **bytes**.
 659     ///
 660     /// This is purely a convenience for casting to a `u8` pointer and
 661     /// using [offset_from][pointer::offset_from] on it. See that method for
 662     /// documentation and safety requirements.
 663     ///
 664     /// For non-`Sized` pointees this operation considers only the data pointers,
 665     /// ignoring the metadata.
 666     #[inline(always)]
 667     #[unstable(feature = "pointer_byte_offsets", issue = "96283")]
 668     #[rustc_const_unstable(feature = "const_pointer_byte_offsets", issue = "96283")]
 669     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
 670     pub const unsafe fn byte_offset_from(self, origin: *const T) -> isize {
 671         // SAFETY: the caller must uphold the safety contract for `offset_from`.
 672         unsafe { self.cast::<u8>().offset_from(origin.cast::<u8>()) }
 673     }
 674
 675     /// Calculates the distance between two pointers, *where it's known that
 676     /// `self` is equal to or greater than `origin`*. The returned value is in
 677     /// units of T: the distance in bytes is divided by `mem::size_of::<T>()`.
 678     ///
 679     /// This computes the same value that [`offset_from`](#method.offset_from)
 680     /// would compute, but with the added precondition that that the offset is
 681     /// guaranteed to be non-negative.  This method is equivalent to
 682     /// `usize::from(self.offset_from(origin)).unwrap_unchecked()`,
 683     /// but it provides slightly more information to the optimizer, which can
 684     /// sometimes allow it to optimize slightly better with some backends.
 685     ///
 686     /// This method can be though of as recovering the `count` that was passed
 687     /// to [`add`](#method.add) (or, with the parameters in the other order,
 688     /// to [`sub`](#method.sub)).  The following are all equivalent, assuming
 689     /// that their safety preconditions are met:
 690     /// ```rust
 691     /// # #![feature(ptr_sub_ptr)]
 692     /// # unsafe fn blah(ptr: *const i32, origin: *const i32, count: usize) -> bool {
 693     /// ptr.sub_ptr(origin) == count
 694     /// # &&
 695     /// origin.add(count) == ptr
 696     /// # &&
 697     /// ptr.sub(count) == origin
 698     /// # }
 699     /// ```
 700     ///
 701     /// # Safety
 702     ///
 703     /// - The distance between the pointers must be non-negative (`self >= origin`)
 704     ///
 705     /// - *All* the safety conditions of [`offset_from`](#method.offset_from)
 706     ///   apply to this method as well; see it for the full details.
 707     ///
 708     /// Importantly, despite the return type of this method being able to represent
 709     /// a larger offset, it's still *not permitted* to pass pointers which differ
 710     /// by more than `isize::MAX` *bytes*.  As such, the result of this method will
 711     /// always be less than or equal to `isize::MAX as usize`.
 712     ///
 713     /// # Panics
 714     ///
 715     /// This function panics if `T` is a Zero-Sized Type ("ZST").
 716     ///
 717     /// # Examples
 718     ///
 719     /// ```
 720     /// #![feature(ptr_sub_ptr)]
 721     ///
 722     /// let a = [0; 5];
 723     /// let ptr1: *const i32 = &a[1];
 724     /// let ptr2: *const i32 = &a[3];
 725     /// unsafe {
 726     ///     assert_eq!(ptr2.sub_ptr(ptr1), 2);
 727     ///     assert_eq!(ptr1.add(2), ptr2);
 728     ///     assert_eq!(ptr2.sub(2), ptr1);
 729     ///     assert_eq!(ptr2.sub_ptr(ptr2), 0);
 730     /// }
 731     ///
 732     /// // This would be incorrect, as the pointers are not correctly ordered:
 733     /// // ptr1.sub_ptr(ptr2)
 734     /// ```
 735     #[unstable(feature = "ptr_sub_ptr", issue = "95892")]
 736     #[rustc_const_unstable(feature = "const_ptr_sub_ptr", issue = "95892")]
 737     #[inline]
 738     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
 739     pub const unsafe fn sub_ptr(self, origin: *const T) -> usize
 740     where
 741         T: Sized,
 742     {
 743         // SAFETY: The comparison has no side-effects, and the intrinsic
 744         // does this check internally in the CTFE implementation.
 745         unsafe { assert_unsafe_precondition!(self >= origin) };
 746
 747         let pointee_size = mem::size_of::<T>();
 748         assert!(0 < pointee_size && pointee_size <= isize::MAX as usize);
 749         // SAFETY: the caller must uphold the safety contract for `ptr_offset_from_unsigned`.
 750         unsafe { intrinsics::ptr_offset_from_unsigned(self, origin) }
 751     }
 752
 753     /// Returns whether two pointers are guaranteed to be equal.
 754     ///
 755     /// At runtime this function behaves like `self == other`.
 756     /// However, in some contexts (e.g., compile-time evaluation),
 757     /// it is not always possible to determine equality of two pointers, so this function may
 758     /// spuriously return `false` for pointers that later actually turn out to be equal.
 759     /// But when it returns `true`, the pointers are guaranteed to be equal.
 760     ///
 761     /// This function is the mirror of [`guaranteed_ne`], but not its inverse. There are pointer
 762     /// comparisons for which both functions return `false`.
 763     ///
 764     /// [`guaranteed_ne`]: #method.guaranteed_ne
 765     ///
 766     /// The return value may change depending on the compiler version and unsafe code must not
 767     /// rely on the result of this function for soundness. It is suggested to only use this function
 768     /// for performance optimizations where spurious `false` return values by this function do not
 769     /// affect the outcome, but just the performance.
 770     /// The consequences of using this method to make runtime and compile-time code behave
 771     /// differently have not been explored. This method should not be used to introduce such
 772     /// differences, and it should also not be stabilized before we have a better understanding
 773     /// of this issue.
 774     #[unstable(feature = "const_raw_ptr_comparison", issue = "53020")]
 775     #[rustc_const_unstable(feature = "const_raw_ptr_comparison", issue = "53020")]
 776     #[inline]
 777     pub const fn guaranteed_eq(self, other: *const T) -> bool
 778     where
 779         T: Sized,
 780     {
 781         intrinsics::ptr_guaranteed_eq(self, other)
 782     }
 783
 784     /// Returns whether two pointers are guaranteed to be unequal.
 785     ///
 786     /// At runtime this function behaves like `self != other`.
 787     /// However, in some contexts (e.g., compile-time evaluation),
 788     /// it is not always possible to determine the inequality of two pointers, so this function may
 789     /// spuriously return `false` for pointers that later actually turn out to be unequal.
 790     /// But when it returns `true`, the pointers are guaranteed to be unequal.
 791     ///
 792     /// This function is the mirror of [`guaranteed_eq`], but not its inverse. There are pointer
 793     /// comparisons for which both functions return `false`.
 794     ///
 795     /// [`guaranteed_eq`]: #method.guaranteed_eq
 796     ///
 797     /// The return value may change depending on the compiler version and unsafe code must not
 798     /// rely on the result of this function for soundness. It is suggested to only use this function
 799     /// for performance optimizations where spurious `false` return values by this function do not
 800     /// affect the outcome, but just the performance.
 801     /// The consequences of using this method to make runtime and compile-time code behave
 802     /// differently have not been explored. This method should not be used to introduce such
 803     /// differences, and it should also not be stabilized before we have a better understanding
 804     /// of this issue.
 805     #[unstable(feature = "const_raw_ptr_comparison", issue = "53020")]
 806     #[rustc_const_unstable(feature = "const_raw_ptr_comparison", issue = "53020")]
 807     #[inline]
 808     pub const fn guaranteed_ne(self, other: *const T) -> bool
 809     where
 810         T: Sized,
 811     {
 812         intrinsics::ptr_guaranteed_ne(self, other)
 813     }
 814
 815     /// Calculates the offset from a pointer (convenience for `.offset(count as isize)`).
 816     ///
 817     /// `count` is in units of T; e.g., a `count` of 3 represents a pointer
 818     /// offset of `3 * size_of::<T>()` bytes.
 819     ///
 820     /// # Safety
 821     ///
 822     /// If any of the following conditions are violated, the result is Undefined
 823     /// Behavior:
 824     ///
 825     /// * Both the starting and resulting pointer must be either in bounds or one
 826     ///   byte past the end of the same [allocated object].
 827     ///
 828     /// * The computed offset, **in bytes**, cannot overflow an `isize`.
 829     ///
 830     /// * The offset being in bounds cannot rely on "wrapping around" the address
 831     ///   space. That is, the infinite-precision sum must fit in a `usize`.
 832     ///
 833     /// The compiler and standard library generally tries to ensure allocations
 834     /// never reach a size where an offset is a concern. For instance, `Vec`
 835     /// and `Box` ensure they never allocate more than `isize::MAX` bytes, so
 836     /// `vec.as_ptr().add(vec.len())` is always safe.
 837     ///
 838     /// Most platforms fundamentally can't even construct such an allocation.
 839     /// For instance, no known 64-bit platform can ever serve a request
 840     /// for 2<sup>63</sup> bytes due to page-table limitations or splitting the address space.
 841     /// However, some 32-bit and 16-bit platforms may successfully serve a request for
 842     /// more than `isize::MAX` bytes with things like Physical Address
 843     /// Extension. As such, memory acquired directly from allocators or memory
 844     /// mapped files *may* be too large to handle with this function.
 845     ///
 846     /// Consider using [`wrapping_add`] instead if these constraints are
 847     /// difficult to satisfy. The only advantage of this method is that it
 848     /// enables more aggressive compiler optimizations.
 849     ///
 850     /// [`wrapping_add`]: #method.wrapping_add
 851     /// [allocated object]: crate::ptr#allocated-object
 852     ///
 853     /// # Examples
 854     ///
 855     /// Basic usage:
 856     ///
 857     /// ```
 858     /// let s: &str = "123";
 859     /// let ptr: *const u8 = s.as_ptr();
 860     ///
 861     /// unsafe {
 862     ///     println!("{}", *ptr.add(1) as char);
 863     ///     println!("{}", *ptr.add(2) as char);
 864     /// }
 865     /// ```
 866     #[stable(feature = "pointer_methods", since = "1.26.0")]
 867     #[must_use = "returns a new pointer rather than modifying its argument"]
 868     #[rustc_const_stable(feature = "const_ptr_offset", since = "1.61.0")]
 869     #[inline(always)]
 870     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
 871     pub const unsafe fn add(self, count: usize) -> Self
 872     where
 873         T: Sized,
 874     {
 875         // SAFETY: the caller must uphold the safety contract for `offset`.
 876         unsafe { self.offset(count as isize) }
 877     }
 878
 879     /// Calculates the offset from a pointer in bytes (convenience for `.byte_offset(count as isize)`).
 880     ///
 881     /// `count` is in units of bytes.
 882     ///
 883     /// This is purely a convenience for casting to a `u8` pointer and
 884     /// using [add][pointer::add] on it. See that method for documentation
 885     /// and safety requirements.
 886     ///
 887     /// For non-`Sized` pointees this operation changes only the data pointer,
 888     /// leaving the metadata untouched.
 889     #[must_use]
 890     #[inline(always)]
 891     #[unstable(feature = "pointer_byte_offsets", issue = "96283")]
 892     #[rustc_const_unstable(feature = "const_pointer_byte_offsets", issue = "96283")]
 893     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
 894     pub const unsafe fn byte_add(self, count: usize) -> Self {
 895         // SAFETY: the caller must uphold the safety contract for `add`.
 896         let this = unsafe { self.cast::<u8>().add(count).cast::<()>() };
 897         from_raw_parts::<T>(this, metadata(self))
 898     }
 899
 900     /// Calculates the offset from a pointer (convenience for
 901     /// `.offset((count as isize).wrapping_neg())`).
 902     ///
 903     /// `count` is in units of T; e.g., a `count` of 3 represents a pointer
 904     /// offset of `3 * size_of::<T>()` bytes.
 905     ///
 906     /// # Safety
 907     ///
 908     /// If any of the following conditions are violated, the result is Undefined
 909     /// Behavior:
 910     ///
 911     /// * Both the starting and resulting pointer must be either in bounds or one
 912     ///   byte past the end of the same [allocated object].
 913     ///
 914     /// * The computed offset cannot exceed `isize::MAX` **bytes**.
 915     ///
 916     /// * The offset being in bounds cannot rely on "wrapping around" the address
 917     ///   space. That is, the infinite-precision sum must fit in a usize.
 918     ///
 919     /// The compiler and standard library generally tries to ensure allocations
 920     /// never reach a size where an offset is a concern. For instance, `Vec`
 921     /// and `Box` ensure they never allocate more than `isize::MAX` bytes, so
 922     /// `vec.as_ptr().add(vec.len()).sub(vec.len())` is always safe.
 923     ///
 924     /// Most platforms fundamentally can't even construct such an allocation.
 925     /// For instance, no known 64-bit platform can ever serve a request
 926     /// for 2<sup>63</sup> bytes due to page-table limitations or splitting the address space.
 927     /// However, some 32-bit and 16-bit platforms may successfully serve a request for
 928     /// more than `isize::MAX` bytes with things like Physical Address
 929     /// Extension. As such, memory acquired directly from allocators or memory
 930     /// mapped files *may* be too large to handle with this function.
 931     ///
 932     /// Consider using [`wrapping_sub`] instead if these constraints are
 933     /// difficult to satisfy. The only advantage of this method is that it
 934     /// enables more aggressive compiler optimizations.
 935     ///
 936     /// [`wrapping_sub`]: #method.wrapping_sub
 937     /// [allocated object]: crate::ptr#allocated-object
 938     ///
 939     /// # Examples
 940     ///
 941     /// Basic usage:
 942     ///
 943     /// ```
 944     /// let s: &str = "123";
 945     ///
 946     /// unsafe {
 947     ///     let end: *const u8 = s.as_ptr().add(3);
 948     ///     println!("{}", *end.sub(1) as char);
 949     ///     println!("{}", *end.sub(2) as char);
 950     /// }
 951     /// ```
 952     #[stable(feature = "pointer_methods", since = "1.26.0")]
 953     #[must_use = "returns a new pointer rather than modifying its argument"]
 954     #[rustc_const_stable(feature = "const_ptr_offset", since = "1.61.0")]
 955     #[inline]
 956     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
 957     pub const unsafe fn sub(self, count: usize) -> Self
 958     where
 959         T: Sized,
 960     {
 961         // SAFETY: the caller must uphold the safety contract for `offset`.
 962         unsafe { self.offset((count as isize).wrapping_neg()) }
 963     }
 964
 965     /// Calculates the offset from a pointer in bytes (convenience for
 966     /// `.byte_offset((count as isize).wrapping_neg())`).
 967     ///
 968     /// `count` is in units of bytes.
 969     ///
 970     /// This is purely a convenience for casting to a `u8` pointer and
 971     /// using [sub][pointer::sub] on it. See that method for documentation
 972     /// and safety requirements.
 973     ///
 974     /// For non-`Sized` pointees this operation changes only the data pointer,
 975     /// leaving the metadata untouched.
 976     #[must_use]
 977     #[inline(always)]
 978     #[unstable(feature = "pointer_byte_offsets", issue = "96283")]
 979     #[rustc_const_unstable(feature = "const_pointer_byte_offsets", issue = "96283")]
 980     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
 981     pub const unsafe fn byte_sub(self, count: usize) -> Self {
 982         // SAFETY: the caller must uphold the safety contract for `sub`.
 983         let this = unsafe { self.cast::<u8>().sub(count).cast::<()>() };
 984         from_raw_parts::<T>(this, metadata(self))
 985     }
 986
 987     /// Calculates the offset from a pointer using wrapping arithmetic.
 988     /// (convenience for `.wrapping_offset(count as isize)`)
 989     ///
 990     /// `count` is in units of T; e.g., a `count` of 3 represents a pointer
 991     /// offset of `3 * size_of::<T>()` bytes.
 992     ///
 993     /// # Safety
 994     ///
 995     /// This operation itself is always safe, but using the resulting pointer is not.
 996     ///
 997     /// The resulting pointer "remembers" the [allocated object] that `self` points to; it must not
 998     /// be used to read or write other allocated objects.
 999     ///
1000     /// In other words, `let z = x.wrapping_add((y as usize) - (x as usize))` does *not* make `z`
1001     /// the same as `y` even if we assume `T` has size `1` and there is no overflow: `z` is still
1002     /// attached to the object `x` is attached to, and dereferencing it is Undefined Behavior unless
1003     /// `x` and `y` point into the same allocated object.
1004     ///
1005     /// Compared to [`add`], this method basically delays the requirement of staying within the
1006     /// same allocated object: [`add`] is immediate Undefined Behavior when crossing object
1007     /// boundaries; `wrapping_add` produces a pointer but still leads to Undefined Behavior if a
1008     /// pointer is dereferenced when it is out-of-bounds of the object it is attached to. [`add`]
1009     /// can be optimized better and is thus preferable in performance-sensitive code.
1010     ///
1011     /// The delayed check only considers the value of the pointer that was dereferenced, not the
1012     /// intermediate values used during the computation of the final result. For example,
1013     /// `x.wrapping_add(o).wrapping_sub(o)` is always the same as `x`. In other words, leaving the
1014     /// allocated object and then re-entering it later is permitted.
1015     ///
1016     /// [`add`]: #method.add
1017     /// [allocated object]: crate::ptr#allocated-object
1018     ///
1019     /// # Examples
1020     ///
1021     /// Basic usage:
1022     ///
1023     /// ```
1024     /// // Iterate using a raw pointer in increments of two elements
1025     /// let data = [1u8, 2, 3, 4, 5];
1026     /// let mut ptr: *const u8 = data.as_ptr();
1027     /// let step = 2;
1028     /// let end_rounded_up = ptr.wrapping_add(6);
1029     ///
1030     /// // This loop prints "1, 3, 5, "
1031     /// while ptr != end_rounded_up {
1032     ///     unsafe {
1033     ///         print!("{}, ", *ptr);
1034     ///     }
1035     ///     ptr = ptr.wrapping_add(step);
1036     /// }
1037     /// ```
1038     #[stable(feature = "pointer_methods", since = "1.26.0")]
1039     #[must_use = "returns a new pointer rather than modifying its argument"]
1040     #[rustc_const_stable(feature = "const_ptr_offset", since = "1.61.0")]
1041     #[inline(always)]
1042     pub const fn wrapping_add(self, count: usize) -> Self
1043     where
1044         T: Sized,
1045     {
1046         self.wrapping_offset(count as isize)
1047     }
1048
1049     /// Calculates the offset from a pointer in bytes using wrapping arithmetic.
1050     /// (convenience for `.wrapping_byte_offset(count as isize)`)
1051     ///
1052     /// `count` is in units of bytes.
1053     ///
1054     /// This is purely a convenience for casting to a `u8` pointer and
1055     /// using [wrapping_add][pointer::wrapping_add] on it. See that method for documentation.
1056     ///
1057     /// For non-`Sized` pointees this operation changes only the data pointer,
1058     /// leaving the metadata untouched.
1059     #[must_use]
1060     #[inline(always)]
1061     #[unstable(feature = "pointer_byte_offsets", issue = "96283")]
1062     #[rustc_const_unstable(feature = "const_pointer_byte_offsets", issue = "96283")]
1063     pub const fn wrapping_byte_add(self, count: usize) -> Self {
1064         from_raw_parts::<T>(self.cast::<u8>().wrapping_add(count).cast::<()>(), metadata(self))
1065     }
1066
1067     /// Calculates the offset from a pointer using wrapping arithmetic.
1068     /// (convenience for `.wrapping_offset((count as isize).wrapping_neg())`)
1069     ///
1070     /// `count` is in units of T; e.g., a `count` of 3 represents a pointer
1071     /// offset of `3 * size_of::<T>()` bytes.
1072     ///
1073     /// # Safety
1074     ///
1075     /// This operation itself is always safe, but using the resulting pointer is not.
1076     ///
1077     /// The resulting pointer "remembers" the [allocated object] that `self` points to; it must not
1078     /// be used to read or write other allocated objects.
1079     ///
1080     /// In other words, `let z = x.wrapping_sub((x as usize) - (y as usize))` does *not* make `z`
1081     /// the same as `y` even if we assume `T` has size `1` and there is no overflow: `z` is still
1082     /// attached to the object `x` is attached to, and dereferencing it is Undefined Behavior unless
1083     /// `x` and `y` point into the same allocated object.
1084     ///
1085     /// Compared to [`sub`], this method basically delays the requirement of staying within the
1086     /// same allocated object: [`sub`] is immediate Undefined Behavior when crossing object
1087     /// boundaries; `wrapping_sub` produces a pointer but still leads to Undefined Behavior if a
1088     /// pointer is dereferenced when it is out-of-bounds of the object it is attached to. [`sub`]
1089     /// can be optimized better and is thus preferable in performance-sensitive code.
1090     ///
1091     /// The delayed check only considers the value of the pointer that was dereferenced, not the
1092     /// intermediate values used during the computation of the final result. For example,
1093     /// `x.wrapping_add(o).wrapping_sub(o)` is always the same as `x`. In other words, leaving the
1094     /// allocated object and then re-entering it later is permitted.
1095     ///
1096     /// [`sub`]: #method.sub
1097     /// [allocated object]: crate::ptr#allocated-object
1098     ///
1099     /// # Examples
1100     ///
1101     /// Basic usage:
1102     ///
1103     /// ```
1104     /// // Iterate using a raw pointer in increments of two elements (backwards)
1105     /// let data = [1u8, 2, 3, 4, 5];
1106     /// let mut ptr: *const u8 = data.as_ptr();
1107     /// let start_rounded_down = ptr.wrapping_sub(2);
1108     /// ptr = ptr.wrapping_add(4);
1109     /// let step = 2;
1110     /// // This loop prints "5, 3, 1, "
1111     /// while ptr != start_rounded_down {
1112     ///     unsafe {
1113     ///         print!("{}, ", *ptr);
1114     ///     }
1115     ///     ptr = ptr.wrapping_sub(step);
1116     /// }
1117     /// ```
1118     #[stable(feature = "pointer_methods", since = "1.26.0")]
1119     #[must_use = "returns a new pointer rather than modifying its argument"]
1120     #[rustc_const_stable(feature = "const_ptr_offset", since = "1.61.0")]
1121     #[inline]
1122     pub const fn wrapping_sub(self, count: usize) -> Self
1123     where
1124         T: Sized,
1125     {
1126         self.wrapping_offset((count as isize).wrapping_neg())
1127     }
1128
1129     /// Calculates the offset from a pointer in bytes using wrapping arithmetic.
1130     /// (convenience for `.wrapping_offset((count as isize).wrapping_neg())`)
1131     ///
1132     /// `count` is in units of bytes.
1133     ///
1134     /// This is purely a convenience for casting to a `u8` pointer and
1135     /// using [wrapping_sub][pointer::wrapping_sub] on it. See that method for documentation.
1136     ///
1137     /// For non-`Sized` pointees this operation changes only the data pointer,
1138     /// leaving the metadata untouched.
1139     #[must_use]
1140     #[inline(always)]
1141     #[unstable(feature = "pointer_byte_offsets", issue = "96283")]
1142     #[rustc_const_unstable(feature = "const_pointer_byte_offsets", issue = "96283")]
1143     pub const fn wrapping_byte_sub(self, count: usize) -> Self {
1144         from_raw_parts::<T>(self.cast::<u8>().wrapping_sub(count).cast::<()>(), metadata(self))
1145     }
1146
1147     /// Reads the value from `self` without moving it. This leaves the
1148     /// memory in `self` unchanged.
1149     ///
1150     /// See [`ptr::read`] for safety concerns and examples.
1151     ///
1152     /// [`ptr::read`]: crate::ptr::read()
1153     #[stable(feature = "pointer_methods", since = "1.26.0")]
1154     #[rustc_const_unstable(feature = "const_ptr_read", issue = "80377")]
1155     #[inline]
1156     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
1157     pub const unsafe fn read(self) -> T
1158     where
1159         T: Sized,
1160     {
1161         // SAFETY: the caller must uphold the safety contract for `read`.
1162         unsafe { read(self) }
1163     }
1164
1165     /// Performs a volatile read of the value from `self` without moving it. This
1166     /// leaves the memory in `self` unchanged.
1167     ///
1168     /// Volatile operations are intended to act on I/O memory, and are guaranteed
1169     /// to not be elided or reordered by the compiler across other volatile
1170     /// operations.
1171     ///
1172     /// See [`ptr::read_volatile`] for safety concerns and examples.
1173     ///
1174     /// [`ptr::read_volatile`]: crate::ptr::read_volatile()
1175     #[stable(feature = "pointer_methods", since = "1.26.0")]
1176     #[inline]
1177     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
1178     pub unsafe fn read_volatile(self) -> T
1179     where
1180         T: Sized,
1181     {
1182         // SAFETY: the caller must uphold the safety contract for `read_volatile`.
1183         unsafe { read_volatile(self) }
1184     }
1185
1186     /// Reads the value from `self` without moving it. This leaves the
1187     /// memory in `self` unchanged.
1188     ///
1189     /// Unlike `read`, the pointer may be unaligned.
1190     ///
1191     /// See [`ptr::read_unaligned`] for safety concerns and examples.
1192     ///
1193     /// [`ptr::read_unaligned`]: crate::ptr::read_unaligned()
1194     #[stable(feature = "pointer_methods", since = "1.26.0")]
1195     #[rustc_const_unstable(feature = "const_ptr_read", issue = "80377")]
1196     #[inline]
1197     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
1198     pub const unsafe fn read_unaligned(self) -> T
1199     where
1200         T: Sized,
1201     {
1202         // SAFETY: the caller must uphold the safety contract for `read_unaligned`.
1203         unsafe { read_unaligned(self) }
1204     }
1205
1206     /// Copies `count * size_of<T>` bytes from `self` to `dest`. The source
1207     /// and destination may overlap.
1208     ///
1209     /// NOTE: this has the *same* argument order as [`ptr::copy`].
1210     ///
1211     /// See [`ptr::copy`] for safety concerns and examples.
1212     ///
1213     /// [`ptr::copy`]: crate::ptr::copy()
1214     #[rustc_const_stable(feature = "const_intrinsic_copy", since = "1.63.0")]
1215     #[stable(feature = "pointer_methods", since = "1.26.0")]
1216     #[inline]
1217     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
1218     pub const unsafe fn copy_to(self, dest: *mut T, count: usize)
1219     where
1220         T: Sized,
1221     {
1222         // SAFETY: the caller must uphold the safety contract for `copy`.
1223         unsafe { copy(self, dest, count) }
1224     }
1225
1226     /// Copies `count * size_of<T>` bytes from `self` to `dest`. The source
1227     /// and destination may *not* overlap.
1228     ///
1229     /// NOTE: this has the *same* argument order as [`ptr::copy_nonoverlapping`].
1230     ///
1231     /// See [`ptr::copy_nonoverlapping`] for safety concerns and examples.
1232     ///
1233     /// [`ptr::copy_nonoverlapping`]: crate::ptr::copy_nonoverlapping()
1234     #[rustc_const_stable(feature = "const_intrinsic_copy", since = "1.63.0")]
1235     #[stable(feature = "pointer_methods", since = "1.26.0")]
1236     #[inline]
1237     #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
1238     pub const unsafe fn copy_to_nonoverlapping(self, dest: *mut T, count: usize)
1239     where
1240         T: Sized,
1241     {
1242         // SAFETY: the caller must uphold the safety contract for `copy_nonoverlapping`.
1243         unsafe { copy_nonoverlapping(self, dest, count) }
1244     }
1245
1246     /// Computes the offset that needs to be applied to the pointer in order to make it aligned to
1247     /// `align`.
1248     ///
1249     /// If it is not possible to align the pointer, the implementation returns
1250     /// `usize::MAX`. It is permissible for the implementation to *always*
1251     /// return `usize::MAX`. Only your algorithm's performance can depend
1252     /// on getting a usable offset here, not its correctness.
1253     ///
1254     /// The offset is expressed in number of `T` elements, and not bytes. The value returned can be
1255     /// used with the `wrapping_add` method.
1256     ///
1257     /// There are no guarantees whatsoever that offsetting the pointer will not overflow or go
1258     /// beyond the allocation that the pointer points into. It is up to the caller to ensure that
1259     /// the returned offset is correct in all terms other than alignment.
1260     ///
1261     /// # Panics
1262     ///
1263     /// The function panics if `align` is not a power-of-two.
1264     ///
1265     /// # Examples
1266     ///
1267     /// Accessing adjacent `u8` as `u16`
1268     ///
1269     /// ```
1270     /// # fn foo(n: usize) {
1271     /// # use std::mem::align_of;
1272     /// # unsafe {
1273     /// let x = [5u8, 6u8, 7u8, 8u8, 9u8];
1274     /// let ptr = x.as_ptr().add(n) as *const u8;
1275     /// let offset = ptr.align_offset(align_of::<u16>());
1276     /// if offset < x.len() - n - 1 {
1277     ///     let u16_ptr = ptr.add(offset) as *const u16;
1278     ///     assert_ne!(*u16_ptr, 500);
1279     /// } else {
1280     ///     // while the pointer can be aligned via `offset`, it would point
1281     ///     // outside the allocation
1282     /// }
1283     /// # } }
1284     /// ```
1285     #[stable(feature = "align_offset", since = "1.36.0")]
1286     #[rustc_const_unstable(feature = "const_align_offset", issue = "90962")]
1287     pub const fn align_offset(self, align: usize) -> usize
1288     where
1289         T: Sized,
1290     {
1291         if !align.is_power_of_two() {
1292             panic!("align_offset: align is not a power-of-two");
1293         }
1294
1295         fn rt_impl<T>(p: *const T, align: usize) -> usize {
1296             // SAFETY: `align` has been checked to be a power of 2 above
1297             unsafe { align_offset(p, align) }
1298         }
1299
1300         const fn ctfe_impl<T>(_: *const T, _: usize) -> usize {
1301             usize::MAX
1302         }
1303
1304         // SAFETY:
1305         // It is permissible for `align_offset` to always return `usize::MAX`,
1306         // algorithm correctness can not depend on `align_offset` returning non-max values.
1307         //
1308         // As such the behaviour can't change after replacing `align_offset` with `usize::MAX`, only performance can.
1309         unsafe { intrinsics::const_eval_select((self, align), ctfe_impl, rt_impl) }
1310     }
1311
1312     /// Returns whether the pointer is properly aligned for `T`.
1313     #[must_use]
1314     #[inline]
1315     #[unstable(feature = "pointer_is_aligned", issue = "96284")]
1316     pub fn is_aligned(self) -> bool
1317     where
1318         T: Sized,
1319     {
1320         self.is_aligned_to(core::mem::align_of::<T>())
1321     }
1322
1323     /// Returns whether the pointer is aligned to `align`.
1324     ///
1325     /// For non-`Sized` pointees this operation considers only the data pointer,
1326     /// ignoring the metadata.
1327     ///
1328     /// # Panics
1329     ///
1330     /// The function panics if `align` is not a power-of-two (this includes 0).
1331     #[must_use]
1332     #[inline]
1333     #[unstable(feature = "pointer_is_aligned", issue = "96284")]
1334     pub fn is_aligned_to(self, align: usize) -> bool {
1335         if !align.is_power_of_two() {
1336             panic!("is_aligned_to: align is not a power-of-two");
1337         }
1338
1339         // SAFETY: `is_power_of_two()` will return `false` for zero.
1340         unsafe { core::intrinsics::assume(align != 0) };
1341
1342         // Cast is needed for `T: !Sized`
1343         self.cast::<u8>().addr() % align == 0
1344     }
1345 }
1346
1347 impl<T> *const [T] {
1348     /// Returns the length of a raw slice.
1349     ///
1350     /// The returned value is the number of **elements**, not the number of bytes.
1351     ///
1352     /// This function is safe, even when the raw slice cannot be cast to a slice
1353     /// reference because the pointer is null or unaligned.
1354     ///
1355     /// # Examples
1356     ///
1357     /// ```rust
1358     /// #![feature(slice_ptr_len)]
1359     ///
1360     /// use std::ptr;
1361     ///
1362     /// let slice: *const [i8] = ptr::slice_from_raw_parts(ptr::null(), 3);
1363     /// assert_eq!(slice.len(), 3);
1364     /// ```
1365     #[inline]
1366     #[unstable(feature = "slice_ptr_len", issue = "71146")]
1367     #[rustc_const_unstable(feature = "const_slice_ptr_len", issue = "71146")]
1368     pub const fn len(self) -> usize {
1369         metadata(self)
1370     }
1371
1372     /// Returns a raw pointer to the slice's buffer.
1373     ///
1374     /// This is equivalent to casting `self` to `*const T`, but more type-safe.
1375     ///
1376     /// # Examples
1377     ///
1378     /// ```rust
1379     /// #![feature(slice_ptr_get)]
1380     /// use std::ptr;
1381     ///
1382     /// let slice: *const [i8] = ptr::slice_from_raw_parts(ptr::null(), 3);
1383     /// assert_eq!(slice.as_ptr(), ptr::null());
1384     /// ```
1385     #[inline]
1386     #[unstable(feature = "slice_ptr_get", issue = "74265")]
1387     #[rustc_const_unstable(feature = "slice_ptr_get", issue = "74265")]
1388     pub const fn as_ptr(self) -> *const T {
1389         self as *const T
1390     }
1391
1392     /// Returns a raw pointer to an element or subslice, without doing bounds
1393     /// checking.
1394     ///
1395     /// Calling this method with an out-of-bounds index or when `self` is not dereferenceable
1396     /// is *[undefined behavior]* even if the resulting pointer is not used.
1397     ///
1398     /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
1399     ///
1400     /// # Examples
1401     ///
1402     /// ```
1403     /// #![feature(slice_ptr_get)]
1404     ///
1405     /// let x = &[1, 2, 4] as *const [i32];
1406     ///
1407     /// unsafe {
1408     ///     assert_eq!(x.get_unchecked(1), x.as_ptr().add(1));
1409     /// }
1410     /// ```
1411     #[unstable(feature = "slice_ptr_get", issue = "74265")]
1412     #[rustc_const_unstable(feature = "const_slice_index", issue = "none")]
1413     #[inline]
1414     pub const unsafe fn get_unchecked<I>(self, index: I) -> *const I::Output
1415     where
1416         I: ~const SliceIndex<[T]>,
1417     {
1418         // SAFETY: the caller ensures that `self` is dereferenceable and `index` in-bounds.
1419         unsafe { index.get_unchecked(self) }
1420     }
1421
1422     /// Returns `None` if the pointer is null, or else returns a shared slice to
1423     /// the value wrapped in `Some`. In contrast to [`as_ref`], this does not require
1424     /// that the value has to be initialized.
1425     ///
1426     /// [`as_ref`]: #method.as_ref
1427     ///
1428     /// # Safety
1429     ///
1430     /// When calling this method, you have to ensure that *either* the pointer is null *or*
1431     /// all of the following is true:
1432     ///
1433     /// * The pointer must be [valid] for reads for `ptr.len() * mem::size_of::<T>()` many bytes,
1434     ///   and it must be properly aligned. This means in particular:
1435     ///
1436     ///     * The entire memory range of this slice must be contained within a single [allocated object]!
1437     ///       Slices can never span across multiple allocated objects.
1438     ///
1439     ///     * The pointer must be aligned even for zero-length slices. One
1440     ///       reason for this is that enum layout optimizations may rely on references
1441     ///       (including slices of any length) being aligned and non-null to distinguish
1442     ///       them from other data. You can obtain a pointer that is usable as `data`
1443     ///       for zero-length slices using [`NonNull::dangling()`].
1444     ///
1445     /// * The total size `ptr.len() * mem::size_of::<T>()` of the slice must be no larger than `isize::MAX`.
1446     ///   See the safety documentation of [`pointer::offset`].
1447     ///
1448     /// * You must enforce Rust's aliasing rules, since the returned lifetime `'a` is
1449     ///   arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
1450     ///   In particular, while this reference exists, the memory the pointer points to must
1451     ///   not get mutated (except inside `UnsafeCell`).
1452     ///
1453     /// This applies even if the result of this method is unused!
1454     ///
1455     /// See also [`slice::from_raw_parts`][].
1456     ///
1457     /// [valid]: crate::ptr#safety
1458     /// [allocated object]: crate::ptr#allocated-object
1459     #[inline]
1460     #[unstable(feature = "ptr_as_uninit", issue = "75402")]
1461     #[rustc_const_unstable(feature = "const_ptr_as_ref", issue = "91822")]
1462     pub const unsafe fn as_uninit_slice<'a>(self) -> Option<&'a [MaybeUninit<T>]> {
1463         if self.is_null() {
1464             None
1465         } else {
1466             // SAFETY: the caller must uphold the safety contract for `as_uninit_slice`.
1467             Some(unsafe { slice::from_raw_parts(self as *const MaybeUninit<T>, self.len()) })
1468         }
1469     }
1470 }
1471
1472 // Equality for pointers
1473 #[stable(feature = "rust1", since = "1.0.0")]
1474 impl<T: ?Sized> PartialEq for *const T {
1475     #[inline]
1476     fn eq(&self, other: &*const T) -> bool {
1477         *self == *other
1478     }
1479 }
1480
1481 #[stable(feature = "rust1", since = "1.0.0")]
1482 impl<T: ?Sized> Eq for *const T {}
1483
1484 // Comparison for pointers
1485 #[stable(feature = "rust1", since = "1.0.0")]
1486 impl<T: ?Sized> Ord for *const T {
1487     #[inline]
1488     fn cmp(&self, other: &*const T) -> Ordering {
1489         if self < other {
1490             Less
1491         } else if self == other {
1492             Equal
1493         } else {
1494             Greater
1495         }
1496     }
1497 }
1498
1499 #[stable(feature = "rust1", since = "1.0.0")]
1500 impl<T: ?Sized> PartialOrd for *const T {
1501     #[inline]
1502     fn partial_cmp(&self, other: &*const T) -> Option<Ordering> {
1503         Some(self.cmp(other))
1504     }
1505
1506     #[inline]
1507     fn lt(&self, other: &*const T) -> bool {
1508         *self < *other
1509     }
1510
1511     #[inline]
1512     fn le(&self, other: &*const T) -> bool {
1513         *self <= *other
1514     }
1515
1516     #[inline]
1517     fn gt(&self, other: &*const T) -> bool {
1518         *self > *other
1519     }
1520
1521     #[inline]
1522     fn ge(&self, other: &*const T) -> bool {
1523         *self >= *other
1524     }
1525 }