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 #[lang = "const_ptr"]
   8 impl<T: ?Sized> *const T {
   9     /// Returns `true` if the pointer is null.
  10     ///
  11     /// Note that unsized types have many possible null pointers, as only the
  12     /// raw data pointer is considered, not their length, vtable, etc.
  13     /// Therefore, two pointers that are null may still not compare equal to
  14     /// each other.
  15     ///
  16     /// ## Behavior during const evaluation
  17     ///
  18     /// When this function is used during const evaluation, it may return `false` for pointers
  19     /// that turn out to be null at runtime. Specifically, when a pointer to some memory
  20     /// is offset beyond its bounds in such a way that the resulting pointer is null,
  21     /// the function will still return `false`. There is no way for CTFE to know
  22     /// the absolute position of that memory, so we cannot tell if the pointer is
  23     /// null or not.
  24     ///
  25     /// # Examples
  26     ///
  27     /// Basic usage:
  28     ///
  29     /// ```
  30     /// let s: &str = "Follow the rabbit";
  31     /// let ptr: *const u8 = s.as_ptr();
  32     /// assert!(!ptr.is_null());
  33     /// ```
  34     #[stable(feature = "rust1", since = "1.0.0")]
  35     #[rustc_const_unstable(feature = "const_ptr_is_null", issue = "74939")]
  36     #[inline]
  37     pub const fn is_null(self) -> bool {
  38         // Compare via a cast to a thin pointer, so fat pointers are only
  39         // considering their "data" part for null-ness.
  40         (self as *const u8).guaranteed_eq(null())
  41     }
  42
  43     /// Casts to a pointer of another type.
  44     #[stable(feature = "ptr_cast", since = "1.38.0")]
  45     #[rustc_const_stable(feature = "const_ptr_cast", since = "1.38.0")]
  46     #[inline]
  47     pub const fn cast<U>(self) -> *const U {
  48         self as _
  49     }
  50
  51     /// Decompose a (possibly wide) pointer into is address and metadata components.
  52     ///
  53     /// The pointer can be later reconstructed with [`from_raw_parts`].
  54     #[unstable(feature = "ptr_metadata", issue = "81513")]
  55     #[rustc_const_unstable(feature = "ptr_metadata", issue = "81513")]
  56     #[inline]
  57     pub const fn to_raw_parts(self) -> (*const (), <T as super::Pointee>::Metadata) {
  58         (self.cast(), metadata(self))
  59     }
  60
  61     /// Returns `None` if the pointer is null, or else returns a shared reference to
  62     /// the value wrapped in `Some`. If the value may be uninitialized, [`as_uninit_ref`]
  63     /// must be used instead.
  64     ///
  65     /// [`as_uninit_ref`]: #method.as_uninit_ref
  66     ///
  67     /// # Safety
  68     ///
  69     /// When calling this method, you have to ensure that *either* the pointer is NULL *or*
  70     /// all of the following is true:
  71     ///
  72     /// * The pointer must be properly aligned.
  73     ///
  74     /// * It must be "dereferencable" in the sense defined in [the module documentation].
  75     ///
  76     /// * The pointer must point to an initialized instance of `T`.
  77     ///
  78     /// * You must enforce Rust's aliasing rules, since the returned lifetime `'a` is
  79     ///   arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
  80     ///   In particular, for the duration of this lifetime, the memory the pointer points to must
  81     ///   not get mutated (except inside `UnsafeCell`).
  82     ///
  83     /// This applies even if the result of this method is unused!
  84     /// (The part about being initialized is not yet fully decided, but until
  85     /// it is, the only safe approach is to ensure that they are indeed initialized.)
  86     ///
  87     /// [the module documentation]: crate::ptr#safety
  88     ///
  89     /// # Examples
  90     ///
  91     /// Basic usage:
  92     ///
  93     /// ```
  94     /// let ptr: *const u8 = &10u8 as *const u8;
  95     ///
  96     /// unsafe {
  97     ///     if let Some(val_back) = ptr.as_ref() {
  98     ///         println!("We got back the value: {}!", val_back);
  99     ///     }
 100     /// }
 101     /// ```
 102     ///
 103     /// # Null-unchecked version
 104     ///
 105     /// If you are sure the pointer can never be null and are looking for some kind of
 106     /// `as_ref_unchecked` that returns the `&T` instead of `Option<&T>`, know that you can
 107     /// dereference the pointer directly.
 108     ///
 109     /// ```
 110     /// let ptr: *const u8 = &10u8 as *const u8;
 111     ///
 112     /// unsafe {
 113     ///     let val_back = &*ptr;
 114     ///     println!("We got back the value: {}!", val_back);
 115     /// }
 116     /// ```
 117     #[stable(feature = "ptr_as_ref", since = "1.9.0")]
 118     #[inline]
 119     pub unsafe fn as_ref<'a>(self) -> Option<&'a T> {
 120         // SAFETY: the caller must guarantee that `self` is valid
 121         // for a reference if it isn't null.
 122         if self.is_null() { None } else { unsafe { Some(&*self) } }
 123     }
 124
 125     /// Returns `None` if the pointer is null, or else returns a shared reference to
 126     /// the value wrapped in `Some`. In contrast to [`as_ref`], this does not require
 127     /// that the value has to be initialized.
 128     ///
 129     /// [`as_ref`]: #method.as_ref
 130     ///
 131     /// # Safety
 132     ///
 133     /// When calling this method, you have to ensure that *either* the pointer is NULL *or*
 134     /// all of the following is true:
 135     ///
 136     /// * The pointer must be properly aligned.
 137     ///
 138     /// * It must be "dereferencable" in the sense defined in [the module documentation].
 139     ///
 140     /// * You must enforce Rust's aliasing rules, since the returned lifetime `'a` is
 141     ///   arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
 142     ///   In particular, for the duration of this lifetime, the memory the pointer points to must
 143     ///   not get mutated (except inside `UnsafeCell`).
 144     ///
 145     /// This applies even if the result of this method is unused!
 146     ///
 147     /// [the module documentation]: crate::ptr#safety
 148     ///
 149     /// # Examples
 150     ///
 151     /// Basic usage:
 152     ///
 153     /// ```
 154     /// #![feature(ptr_as_uninit)]
 155     ///
 156     /// let ptr: *const u8 = &10u8 as *const u8;
 157     ///
 158     /// unsafe {
 159     ///     if let Some(val_back) = ptr.as_uninit_ref() {
 160     ///         println!("We got back the value: {}!", val_back.assume_init());
 161     ///     }
 162     /// }
 163     /// ```
 164     #[inline]
 165     #[unstable(feature = "ptr_as_uninit", issue = "75402")]
 166     pub unsafe fn as_uninit_ref<'a>(self) -> Option<&'a MaybeUninit<T>>
 167     where
 168         T: Sized,
 169     {
 170         // SAFETY: the caller must guarantee that `self` meets all the
 171         // requirements for a reference.
 172         if self.is_null() { None } else { Some(unsafe { &*(self as *const MaybeUninit<T>) }) }
 173     }
 174
 175     /// Calculates the offset from a pointer.
 176     ///
 177     /// `count` is in units of T; e.g., a `count` of 3 represents a pointer
 178     /// offset of `3 * size_of::<T>()` bytes.
 179     ///
 180     /// # Safety
 181     ///
 182     /// If any of the following conditions are violated, the result is Undefined
 183     /// Behavior:
 184     ///
 185     /// * Both the starting and resulting pointer must be either in bounds or one
 186     ///   byte past the end of the same [allocated object].
 187     ///
 188     /// * The computed offset, **in bytes**, cannot overflow an `isize`.
 189     ///
 190     /// * The offset being in bounds cannot rely on "wrapping around" the address
 191     ///   space. That is, the infinite-precision sum, **in bytes** must fit in a usize.
 192     ///
 193     /// The compiler and standard library generally tries to ensure allocations
 194     /// never reach a size where an offset is a concern. For instance, `Vec`
 195     /// and `Box` ensure they never allocate more than `isize::MAX` bytes, so
 196     /// `vec.as_ptr().add(vec.len())` is always safe.
 197     ///
 198     /// Most platforms fundamentally can't even construct such an allocation.
 199     /// For instance, no known 64-bit platform can ever serve a request
 200     /// for 2<sup>63</sup> bytes due to page-table limitations or splitting the address space.
 201     /// However, some 32-bit and 16-bit platforms may successfully serve a request for
 202     /// more than `isize::MAX` bytes with things like Physical Address
 203     /// Extension. As such, memory acquired directly from allocators or memory
 204     /// mapped files *may* be too large to handle with this function.
 205     ///
 206     /// Consider using [`wrapping_offset`] instead if these constraints are
 207     /// difficult to satisfy. The only advantage of this method is that it
 208     /// enables more aggressive compiler optimizations.
 209     ///
 210     /// [`wrapping_offset`]: #method.wrapping_offset
 211     /// [allocated object]: crate::ptr#allocated-object
 212     ///
 213     /// # Examples
 214     ///
 215     /// Basic usage:
 216     ///
 217     /// ```
 218     /// let s: &str = "123";
 219     /// let ptr: *const u8 = s.as_ptr();
 220     ///
 221     /// unsafe {
 222     ///     println!("{}", *ptr.offset(1) as char);
 223     ///     println!("{}", *ptr.offset(2) as char);
 224     /// }
 225     /// ```
 226     #[stable(feature = "rust1", since = "1.0.0")]
 227     #[must_use = "returns a new pointer rather than modifying its argument"]
 228     #[rustc_const_unstable(feature = "const_ptr_offset", issue = "71499")]
 229     #[inline(always)]
 230     pub const unsafe fn offset(self, count: isize) -> *const T
 231     where
 232         T: Sized,
 233     {
 234         // SAFETY: the caller must uphold the safety contract for `offset`.
 235         unsafe { intrinsics::offset(self, count) }
 236     }
 237
 238     /// Calculates the offset from a pointer using wrapping arithmetic.
 239     ///
 240     /// `count` is in units of T; e.g., a `count` of 3 represents a pointer
 241     /// offset of `3 * size_of::<T>()` bytes.
 242     ///
 243     /// # Safety
 244     ///
 245     /// This operation itself is always safe, but using the resulting pointer is not.
 246     ///
 247     /// The resulting pointer "remembers" the [allocated object] that `self` points to; it may not
 248     /// be used to read or write other allocated objects.
 249     ///
 250     /// In other words, `let z = x.wrapping_offset((y as isize) - (x as isize))` does *not* make `z`
 251     /// the same as `y` even if we assume `T` has size `1` and there is no overflow: `z` is still
 252     /// attached to the object `x` is attached to, and dereferencing it is Undefined Behavior unless
 253     /// `x` and `y` point into the same allocated object.
 254     ///
 255     /// Compared to [`offset`], this method basically delays the requirement of staying within the
 256     /// same allocated object: [`offset`] is immediate Undefined Behavior when crossing object
 257     /// boundaries; `wrapping_offset` produces a pointer but still leads to Undefined Behavior if a
 258     /// pointer is dereferenced when it is out-of-bounds of the object it is attached to. [`offset`]
 259     /// can be optimized better and is thus preferable in performance-sensitive code.
 260     ///
 261     /// The delayed check only considers the value of the pointer that was dereferenced, not the
 262     /// intermediate values used during the computation of the final result. For example,
 263     /// `x.wrapping_offset(o).wrapping_offset(o.wrapping_neg())` is always the same as `x`. In other
 264     /// words, leaving the allocated object and then re-entering it later is permitted.
 265     ///
 266     /// [`offset`]: #method.offset
 267     /// [allocated object]: crate::ptr#allocated-object
 268     ///
 269     /// # Examples
 270     ///
 271     /// Basic usage:
 272     ///
 273     /// ```
 274     /// // Iterate using a raw pointer in increments of two elements
 275     /// let data = [1u8, 2, 3, 4, 5];
 276     /// let mut ptr: *const u8 = data.as_ptr();
 277     /// let step = 2;
 278     /// let end_rounded_up = ptr.wrapping_offset(6);
 279     ///
 280     /// // This loop prints "1, 3, 5, "
 281     /// while ptr != end_rounded_up {
 282     ///     unsafe {
 283     ///         print!("{}, ", *ptr);
 284     ///     }
 285     ///     ptr = ptr.wrapping_offset(step);
 286     /// }
 287     /// ```
 288     #[stable(feature = "ptr_wrapping_offset", since = "1.16.0")]
 289     #[must_use = "returns a new pointer rather than modifying its argument"]
 290     #[rustc_const_unstable(feature = "const_ptr_offset", issue = "71499")]
 291     #[inline(always)]
 292     pub const fn wrapping_offset(self, count: isize) -> *const T
 293     where
 294         T: Sized,
 295     {
 296         // SAFETY: the `arith_offset` intrinsic has no prerequisites to be called.
 297         unsafe { intrinsics::arith_offset(self, count) }
 298     }
 299
 300     /// Calculates the distance between two pointers. The returned value is in
 301     /// units of T: the distance in bytes is divided by `mem::size_of::<T>()`.
 302     ///
 303     /// This function is the inverse of [`offset`].
 304     ///
 305     /// [`offset`]: #method.offset
 306     ///
 307     /// # Safety
 308     ///
 309     /// If any of the following conditions are violated, the result is Undefined
 310     /// Behavior:
 311     ///
 312     /// * Both the starting and other pointer must be either in bounds or one
 313     ///   byte past the end of the same [allocated object].
 314     ///
 315     /// * Both pointers must be *derived from* a pointer to the same object.
 316     ///   (See below for an example.)
 317     ///
 318     /// * The distance between the pointers, in bytes, must be an exact multiple
 319     ///   of the size of `T`.
 320     ///
 321     /// * The distance between the pointers, **in bytes**, cannot overflow an `isize`.
 322     ///
 323     /// * The distance being in bounds cannot rely on "wrapping around" the address space.
 324     ///
 325     /// Rust types are never larger than `isize::MAX` and Rust allocations never wrap around the
 326     /// address space, so two pointers within some value of any Rust type `T` will always satisfy
 327     /// the last two conditions. The standard library also generally ensures that allocations
 328     /// never reach a size where an offset is a concern. For instance, `Vec` and `Box` ensure they
 329     /// never allocate more than `isize::MAX` bytes, so `ptr_into_vec.offset_from(vec.as_ptr())`
 330     /// always satisfies the last two conditions.
 331     ///
 332     /// Most platforms fundamentally can't even construct such a large allocation.
 333     /// For instance, no known 64-bit platform can ever serve a request
 334     /// for 2<sup>63</sup> bytes due to page-table limitations or splitting the address space.
 335     /// However, some 32-bit and 16-bit platforms may successfully serve a request for
 336     /// more than `isize::MAX` bytes with things like Physical Address
 337     /// Extension. As such, memory acquired directly from allocators or memory
 338     /// mapped files *may* be too large to handle with this function.
 339     /// (Note that [`offset`] and [`add`] also have a similar limitation and hence cannot be used on
 340     /// such large allocations either.)
 341     ///
 342     /// [`add`]: #method.add
 343     /// [allocated object]: crate::ptr#allocated-object
 344     ///
 345     /// # Panics
 346     ///
 347     /// This function panics if `T` is a Zero-Sized Type ("ZST").
 348     ///
 349     /// # Examples
 350     ///
 351     /// Basic usage:
 352     ///
 353     /// ```
 354     /// let a = [0; 5];
 355     /// let ptr1: *const i32 = &a[1];
 356     /// let ptr2: *const i32 = &a[3];
 357     /// unsafe {
 358     ///     assert_eq!(ptr2.offset_from(ptr1), 2);
 359     ///     assert_eq!(ptr1.offset_from(ptr2), -2);
 360     ///     assert_eq!(ptr1.offset(2), ptr2);
 361     ///     assert_eq!(ptr2.offset(-2), ptr1);
 362     /// }
 363     /// ```
 364     ///
 365     /// *Incorrect* usage:
 366     ///
 367     /// ```rust,no_run
 368     /// let ptr1 = Box::into_raw(Box::new(0u8)) as *const u8;
 369     /// let ptr2 = Box::into_raw(Box::new(1u8)) as *const u8;
 370     /// let diff = (ptr2 as isize).wrapping_sub(ptr1 as isize);
 371     /// // Make ptr2_other an "alias" of ptr2, but derived from ptr1.
 372     /// let ptr2_other = (ptr1 as *const u8).wrapping_offset(diff);
 373     /// assert_eq!(ptr2 as usize, ptr2_other as usize);
 374     /// // Since ptr2_other and ptr2 are derived from pointers to different objects,
 375     /// // computing their offset is undefined behavior, even though
 376     /// // they point to the same address!
 377     /// unsafe {
 378     ///     let zero = ptr2_other.offset_from(ptr2); // Undefined Behavior
 379     /// }
 380     /// ```
 381     #[stable(feature = "ptr_offset_from", since = "1.47.0")]
 382     #[rustc_const_unstable(feature = "const_ptr_offset_from", issue = "41079")]
 383     #[inline]
 384     pub const unsafe fn offset_from(self, origin: *const T) -> isize
 385     where
 386         T: Sized,
 387     {
 388         let pointee_size = mem::size_of::<T>();
 389         assert!(0 < pointee_size && pointee_size <= isize::MAX as usize);
 390         // SAFETY: the caller must uphold the safety contract for `ptr_offset_from`.
 391         unsafe { intrinsics::ptr_offset_from(self, origin) }
 392     }
 393
 394     /// Returns whether two pointers are guaranteed to be equal.
 395     ///
 396     /// At runtime this function behaves like `self == other`.
 397     /// However, in some contexts (e.g., compile-time evaluation),
 398     /// it is not always possible to determine equality of two pointers, so this function may
 399     /// spuriously return `false` for pointers that later actually turn out to be equal.
 400     /// But when it returns `true`, the pointers are guaranteed to be equal.
 401     ///
 402     /// This function is the mirror of [`guaranteed_ne`], but not its inverse. There are pointer
 403     /// comparisons for which both functions return `false`.
 404     ///
 405     /// [`guaranteed_ne`]: #method.guaranteed_ne
 406     ///
 407     /// The return value may change depending on the compiler version and unsafe code may not
 408     /// rely on the result of this function for soundness. It is suggested to only use this function
 409     /// for performance optimizations where spurious `false` return values by this function do not
 410     /// affect the outcome, but just the performance.
 411     /// The consequences of using this method to make runtime and compile-time code behave
 412     /// differently have not been explored. This method should not be used to introduce such
 413     /// differences, and it should also not be stabilized before we have a better understanding
 414     /// of this issue.
 415     #[unstable(feature = "const_raw_ptr_comparison", issue = "53020")]
 416     #[rustc_const_unstable(feature = "const_raw_ptr_comparison", issue = "53020")]
 417     #[inline]
 418     pub const fn guaranteed_eq(self, other: *const T) -> bool
 419     where
 420         T: Sized,
 421     {
 422         intrinsics::ptr_guaranteed_eq(self, other)
 423     }
 424
 425     /// Returns whether two pointers are guaranteed to be unequal.
 426     ///
 427     /// At runtime this function behaves like `self != other`.
 428     /// However, in some contexts (e.g., compile-time evaluation),
 429     /// it is not always possible to determine the inequality of two pointers, so this function may
 430     /// spuriously return `false` for pointers that later actually turn out to be unequal.
 431     /// But when it returns `true`, the pointers are guaranteed to be unequal.
 432     ///
 433     /// This function is the mirror of [`guaranteed_eq`], but not its inverse. There are pointer
 434     /// comparisons for which both functions return `false`.
 435     ///
 436     /// [`guaranteed_eq`]: #method.guaranteed_eq
 437     ///
 438     /// The return value may change depending on the compiler version and unsafe code may not
 439     /// rely on the result of this function for soundness. It is suggested to only use this function
 440     /// for performance optimizations where spurious `false` return values by this function do not
 441     /// affect the outcome, but just the performance.
 442     /// The consequences of using this method to make runtime and compile-time code behave
 443     /// differently have not been explored. This method should not be used to introduce such
 444     /// differences, and it should also not be stabilized before we have a better understanding
 445     /// of this issue.
 446     #[unstable(feature = "const_raw_ptr_comparison", issue = "53020")]
 447     #[rustc_const_unstable(feature = "const_raw_ptr_comparison", issue = "53020")]
 448     #[inline]
 449     pub const fn guaranteed_ne(self, other: *const T) -> bool
 450     where
 451         T: Sized,
 452     {
 453         intrinsics::ptr_guaranteed_ne(self, other)
 454     }
 455
 456     /// Calculates the offset from a pointer (convenience for `.offset(count as isize)`).
 457     ///
 458     /// `count` is in units of T; e.g., a `count` of 3 represents a pointer
 459     /// offset of `3 * size_of::<T>()` bytes.
 460     ///
 461     /// # Safety
 462     ///
 463     /// If any of the following conditions are violated, the result is Undefined
 464     /// Behavior:
 465     ///
 466     /// * Both the starting and resulting pointer must be either in bounds or one
 467     ///   byte past the end of the same [allocated object].
 468     ///
 469     /// * The computed offset, **in bytes**, cannot overflow an `isize`.
 470     ///
 471     /// * The offset being in bounds cannot rely on "wrapping around" the address
 472     ///   space. That is, the infinite-precision sum must fit in a `usize`.
 473     ///
 474     /// The compiler and standard library generally tries to ensure allocations
 475     /// never reach a size where an offset is a concern. For instance, `Vec`
 476     /// and `Box` ensure they never allocate more than `isize::MAX` bytes, so
 477     /// `vec.as_ptr().add(vec.len())` is always safe.
 478     ///
 479     /// Most platforms fundamentally can't even construct such an allocation.
 480     /// For instance, no known 64-bit platform can ever serve a request
 481     /// for 2<sup>63</sup> bytes due to page-table limitations or splitting the address space.
 482     /// However, some 32-bit and 16-bit platforms may successfully serve a request for
 483     /// more than `isize::MAX` bytes with things like Physical Address
 484     /// Extension. As such, memory acquired directly from allocators or memory
 485     /// mapped files *may* be too large to handle with this function.
 486     ///
 487     /// Consider using [`wrapping_add`] instead if these constraints are
 488     /// difficult to satisfy. The only advantage of this method is that it
 489     /// enables more aggressive compiler optimizations.
 490     ///
 491     /// [`wrapping_add`]: #method.wrapping_add
 492     /// [allocated object]: crate::ptr#allocated-object
 493     ///
 494     /// # Examples
 495     ///
 496     /// Basic usage:
 497     ///
 498     /// ```
 499     /// let s: &str = "123";
 500     /// let ptr: *const u8 = s.as_ptr();
 501     ///
 502     /// unsafe {
 503     ///     println!("{}", *ptr.add(1) as char);
 504     ///     println!("{}", *ptr.add(2) as char);
 505     /// }
 506     /// ```
 507     #[stable(feature = "pointer_methods", since = "1.26.0")]
 508     #[must_use = "returns a new pointer rather than modifying its argument"]
 509     #[rustc_const_unstable(feature = "const_ptr_offset", issue = "71499")]
 510     #[inline(always)]
 511     pub const unsafe fn add(self, count: usize) -> Self
 512     where
 513         T: Sized,
 514     {
 515         // SAFETY: the caller must uphold the safety contract for `offset`.
 516         unsafe { self.offset(count as isize) }
 517     }
 518
 519     /// Calculates the offset from a pointer (convenience for
 520     /// `.offset((count as isize).wrapping_neg())`).
 521     ///
 522     /// `count` is in units of T; e.g., a `count` of 3 represents a pointer
 523     /// offset of `3 * size_of::<T>()` bytes.
 524     ///
 525     /// # Safety
 526     ///
 527     /// If any of the following conditions are violated, the result is Undefined
 528     /// Behavior:
 529     ///
 530     /// * Both the starting and resulting pointer must be either in bounds or one
 531     ///   byte past the end of the same [allocated object].
 532     ///
 533     /// * The computed offset cannot exceed `isize::MAX` **bytes**.
 534     ///
 535     /// * The offset being in bounds cannot rely on "wrapping around" the address
 536     ///   space. That is, the infinite-precision sum must fit in a usize.
 537     ///
 538     /// The compiler and standard library generally tries to ensure allocations
 539     /// never reach a size where an offset is a concern. For instance, `Vec`
 540     /// and `Box` ensure they never allocate more than `isize::MAX` bytes, so
 541     /// `vec.as_ptr().add(vec.len()).sub(vec.len())` is always safe.
 542     ///
 543     /// Most platforms fundamentally can't even construct such an allocation.
 544     /// For instance, no known 64-bit platform can ever serve a request
 545     /// for 2<sup>63</sup> bytes due to page-table limitations or splitting the address space.
 546     /// However, some 32-bit and 16-bit platforms may successfully serve a request for
 547     /// more than `isize::MAX` bytes with things like Physical Address
 548     /// Extension. As such, memory acquired directly from allocators or memory
 549     /// mapped files *may* be too large to handle with this function.
 550     ///
 551     /// Consider using [`wrapping_sub`] instead if these constraints are
 552     /// difficult to satisfy. The only advantage of this method is that it
 553     /// enables more aggressive compiler optimizations.
 554     ///
 555     /// [`wrapping_sub`]: #method.wrapping_sub
 556     /// [allocated object]: crate::ptr#allocated-object
 557     ///
 558     /// # Examples
 559     ///
 560     /// Basic usage:
 561     ///
 562     /// ```
 563     /// let s: &str = "123";
 564     ///
 565     /// unsafe {
 566     ///     let end: *const u8 = s.as_ptr().add(3);
 567     ///     println!("{}", *end.sub(1) as char);
 568     ///     println!("{}", *end.sub(2) as char);
 569     /// }
 570     /// ```
 571     #[stable(feature = "pointer_methods", since = "1.26.0")]
 572     #[must_use = "returns a new pointer rather than modifying its argument"]
 573     #[rustc_const_unstable(feature = "const_ptr_offset", issue = "71499")]
 574     #[inline]
 575     pub const unsafe fn sub(self, count: usize) -> Self
 576     where
 577         T: Sized,
 578     {
 579         // SAFETY: the caller must uphold the safety contract for `offset`.
 580         unsafe { self.offset((count as isize).wrapping_neg()) }
 581     }
 582
 583     /// Calculates the offset from a pointer using wrapping arithmetic.
 584     /// (convenience for `.wrapping_offset(count as isize)`)
 585     ///
 586     /// `count` is in units of T; e.g., a `count` of 3 represents a pointer
 587     /// offset of `3 * size_of::<T>()` bytes.
 588     ///
 589     /// # Safety
 590     ///
 591     /// This operation itself is always safe, but using the resulting pointer is not.
 592     ///
 593     /// The resulting pointer "remembers" the [allocated object] that `self` points to; it may not
 594     /// be used to read or write other allocated objects.
 595     ///
 596     /// In other words, `let z = x.wrapping_add((y as usize) - (x as usize))` does *not* make `z`
 597     /// the same as `y` even if we assume `T` has size `1` and there is no overflow: `z` is still
 598     /// attached to the object `x` is attached to, and dereferencing it is Undefined Behavior unless
 599     /// `x` and `y` point into the same allocated object.
 600     ///
 601     /// Compared to [`add`], this method basically delays the requirement of staying within the
 602     /// same allocated object: [`add`] is immediate Undefined Behavior when crossing object
 603     /// boundaries; `wrapping_add` produces a pointer but still leads to Undefined Behavior if a
 604     /// pointer is dereferenced when it is out-of-bounds of the object it is attached to. [`add`]
 605     /// can be optimized better and is thus preferable in performance-sensitive code.
 606     ///
 607     /// The delayed check only considers the value of the pointer that was dereferenced, not the
 608     /// intermediate values used during the computation of the final result. For example,
 609     /// `x.wrapping_add(o).wrapping_sub(o)` is always the same as `x`. In other words, leaving the
 610     /// allocated object and then re-entering it later is permitted.
 611     ///
 612     /// [`add`]: #method.add
 613     /// [allocated object]: crate::ptr#allocated-object
 614     ///
 615     /// # Examples
 616     ///
 617     /// Basic usage:
 618     ///
 619     /// ```
 620     /// // Iterate using a raw pointer in increments of two elements
 621     /// let data = [1u8, 2, 3, 4, 5];
 622     /// let mut ptr: *const u8 = data.as_ptr();
 623     /// let step = 2;
 624     /// let end_rounded_up = ptr.wrapping_add(6);
 625     ///
 626     /// // This loop prints "1, 3, 5, "
 627     /// while ptr != end_rounded_up {
 628     ///     unsafe {
 629     ///         print!("{}, ", *ptr);
 630     ///     }
 631     ///     ptr = ptr.wrapping_add(step);
 632     /// }
 633     /// ```
 634     #[stable(feature = "pointer_methods", since = "1.26.0")]
 635     #[must_use = "returns a new pointer rather than modifying its argument"]
 636     #[rustc_const_unstable(feature = "const_ptr_offset", issue = "71499")]
 637     #[inline(always)]
 638     pub const fn wrapping_add(self, count: usize) -> Self
 639     where
 640         T: Sized,
 641     {
 642         self.wrapping_offset(count as isize)
 643     }
 644
 645     /// Calculates the offset from a pointer using wrapping arithmetic.
 646     /// (convenience for `.wrapping_offset((count as isize).wrapping_neg())`)
 647     ///
 648     /// `count` is in units of T; e.g., a `count` of 3 represents a pointer
 649     /// offset of `3 * size_of::<T>()` bytes.
 650     ///
 651     /// # Safety
 652     ///
 653     /// This operation itself is always safe, but using the resulting pointer is not.
 654     ///
 655     /// The resulting pointer "remembers" the [allocated object] that `self` points to; it may not
 656     /// be used to read or write other allocated objects.
 657     ///
 658     /// In other words, `let z = x.wrapping_sub((x as usize) - (y as usize))` does *not* make `z`
 659     /// the same as `y` even if we assume `T` has size `1` and there is no overflow: `z` is still
 660     /// attached to the object `x` is attached to, and dereferencing it is Undefined Behavior unless
 661     /// `x` and `y` point into the same allocated object.
 662     ///
 663     /// Compared to [`sub`], this method basically delays the requirement of staying within the
 664     /// same allocated object: [`sub`] is immediate Undefined Behavior when crossing object
 665     /// boundaries; `wrapping_sub` produces a pointer but still leads to Undefined Behavior if a
 666     /// pointer is dereferenced when it is out-of-bounds of the object it is attached to. [`sub`]
 667     /// can be optimized better and is thus preferable in performance-sensitive code.
 668     ///
 669     /// The delayed check only considers the value of the pointer that was dereferenced, not the
 670     /// intermediate values used during the computation of the final result. For example,
 671     /// `x.wrapping_add(o).wrapping_sub(o)` is always the same as `x`. In other words, leaving the
 672     /// allocated object and then re-entering it later is permitted.
 673     ///
 674     /// [`sub`]: #method.sub
 675     /// [allocated object]: crate::ptr#allocated-object
 676     ///
 677     /// # Examples
 678     ///
 679     /// Basic usage:
 680     ///
 681     /// ```
 682     /// // Iterate using a raw pointer in increments of two elements (backwards)
 683     /// let data = [1u8, 2, 3, 4, 5];
 684     /// let mut ptr: *const u8 = data.as_ptr();
 685     /// let start_rounded_down = ptr.wrapping_sub(2);
 686     /// ptr = ptr.wrapping_add(4);
 687     /// let step = 2;
 688     /// // This loop prints "5, 3, 1, "
 689     /// while ptr != start_rounded_down {
 690     ///     unsafe {
 691     ///         print!("{}, ", *ptr);
 692     ///     }
 693     ///     ptr = ptr.wrapping_sub(step);
 694     /// }
 695     /// ```
 696     #[stable(feature = "pointer_methods", since = "1.26.0")]
 697     #[must_use = "returns a new pointer rather than modifying its argument"]
 698     #[rustc_const_unstable(feature = "const_ptr_offset", issue = "71499")]
 699     #[inline]
 700     pub const fn wrapping_sub(self, count: usize) -> Self
 701     where
 702         T: Sized,
 703     {
 704         self.wrapping_offset((count as isize).wrapping_neg())
 705     }
 706
 707     /// Sets the pointer value to `ptr`.
 708     ///
 709     /// In case `self` is a (fat) pointer to an unsized type, this operation
 710     /// will only affect the pointer part, whereas for (thin) pointers to
 711     /// sized types, this has the same effect as a simple assignment.
 712     ///
 713     /// The resulting pointer will have provenance of `val`, i.e., for a fat
 714     /// pointer, this operation is semantically the same as creating a new
 715     /// fat pointer with the data pointer value of `val` but the metadata of
 716     /// `self`.
 717     ///
 718     /// # Examples
 719     ///
 720     /// This function is primarily useful for allowing byte-wise pointer
 721     /// arithmetic on potentially fat pointers:
 722     ///
 723     /// ```
 724     /// #![feature(set_ptr_value)]
 725     /// # use core::fmt::Debug;
 726     /// let arr: [i32; 3] = [1, 2, 3];
 727     /// let mut ptr = arr.as_ptr() as *const dyn Debug;
 728     /// let thin = ptr as *const u8;
 729     /// unsafe {
 730     ///     ptr = ptr.set_ptr_value(thin.add(8));
 731     ///     # assert_eq!(*(ptr as *const i32), 3);
 732     ///     println!("{:?}", &*ptr); // will print "3"
 733     /// }
 734     /// ```
 735     #[unstable(feature = "set_ptr_value", issue = "75091")]
 736     #[must_use = "returns a new pointer rather than modifying its argument"]
 737     #[inline]
 738     pub fn set_ptr_value(mut self, val: *const u8) -> Self {
 739         let thin = &mut self as *mut *const T as *mut *const u8;
 740         // SAFETY: In case of a thin pointer, this operations is identical
 741         // to a simple assignment. In case of a fat pointer, with the current
 742         // fat pointer layout implementation, the first field of such a
 743         // pointer is always the data pointer, which is likewise assigned.
 744         unsafe { *thin = val };
 745         self
 746     }
 747
 748     /// Reads the value from `self` without moving it. This leaves the
 749     /// memory in `self` unchanged.
 750     ///
 751     /// See [`ptr::read`] for safety concerns and examples.
 752     ///
 753     /// [`ptr::read`]: crate::ptr::read()
 754     #[stable(feature = "pointer_methods", since = "1.26.0")]
 755     #[rustc_const_unstable(feature = "const_ptr_read", issue = "80377")]
 756     #[inline]
 757     pub const unsafe fn read(self) -> T
 758     where
 759         T: Sized,
 760     {
 761         // SAFETY: the caller must uphold the safety contract for `read`.
 762         unsafe { read(self) }
 763     }
 764
 765     /// Performs a volatile read of the value from `self` without moving it. This
 766     /// leaves the memory in `self` unchanged.
 767     ///
 768     /// Volatile operations are intended to act on I/O memory, and are guaranteed
 769     /// to not be elided or reordered by the compiler across other volatile
 770     /// operations.
 771     ///
 772     /// See [`ptr::read_volatile`] for safety concerns and examples.
 773     ///
 774     /// [`ptr::read_volatile`]: crate::ptr::read_volatile()
 775     #[stable(feature = "pointer_methods", since = "1.26.0")]
 776     #[inline]
 777     pub unsafe fn read_volatile(self) -> T
 778     where
 779         T: Sized,
 780     {
 781         // SAFETY: the caller must uphold the safety contract for `read_volatile`.
 782         unsafe { read_volatile(self) }
 783     }
 784
 785     /// Reads the value from `self` without moving it. This leaves the
 786     /// memory in `self` unchanged.
 787     ///
 788     /// Unlike `read`, the pointer may be unaligned.
 789     ///
 790     /// See [`ptr::read_unaligned`] for safety concerns and examples.
 791     ///
 792     /// [`ptr::read_unaligned`]: crate::ptr::read_unaligned()
 793     #[stable(feature = "pointer_methods", since = "1.26.0")]
 794     #[rustc_const_unstable(feature = "const_ptr_read", issue = "80377")]
 795     #[inline]
 796     pub const unsafe fn read_unaligned(self) -> T
 797     where
 798         T: Sized,
 799     {
 800         // SAFETY: the caller must uphold the safety contract for `read_unaligned`.
 801         unsafe { read_unaligned(self) }
 802     }
 803
 804     /// Copies `count * size_of<T>` bytes from `self` to `dest`. The source
 805     /// and destination may overlap.
 806     ///
 807     /// NOTE: this has the *same* argument order as [`ptr::copy`].
 808     ///
 809     /// See [`ptr::copy`] for safety concerns and examples.
 810     ///
 811     /// [`ptr::copy`]: crate::ptr::copy()
 812     #[rustc_const_unstable(feature = "const_intrinsic_copy", issue = "80697")]
 813     #[stable(feature = "pointer_methods", since = "1.26.0")]
 814     #[inline]
 815     pub const unsafe fn copy_to(self, dest: *mut T, count: usize)
 816     where
 817         T: Sized,
 818     {
 819         // SAFETY: the caller must uphold the safety contract for `copy`.
 820         unsafe { copy(self, dest, count) }
 821     }
 822
 823     /// Copies `count * size_of<T>` bytes from `self` to `dest`. The source
 824     /// and destination may *not* overlap.
 825     ///
 826     /// NOTE: this has the *same* argument order as [`ptr::copy_nonoverlapping`].
 827     ///
 828     /// See [`ptr::copy_nonoverlapping`] for safety concerns and examples.
 829     ///
 830     /// [`ptr::copy_nonoverlapping`]: crate::ptr::copy_nonoverlapping()
 831     #[rustc_const_unstable(feature = "const_intrinsic_copy", issue = "80697")]
 832     #[stable(feature = "pointer_methods", since = "1.26.0")]
 833     #[inline]
 834     pub const unsafe fn copy_to_nonoverlapping(self, dest: *mut T, count: usize)
 835     where
 836         T: Sized,
 837     {
 838         // SAFETY: the caller must uphold the safety contract for `copy_nonoverlapping`.
 839         unsafe { copy_nonoverlapping(self, dest, count) }
 840     }
 841
 842     /// Computes the offset that needs to be applied to the pointer in order to make it aligned to
 843     /// `align`.
 844     ///
 845     /// If it is not possible to align the pointer, the implementation returns
 846     /// `usize::MAX`. It is permissible for the implementation to *always*
 847     /// return `usize::MAX`. Only your algorithm's performance can depend
 848     /// on getting a usable offset here, not its correctness.
 849     ///
 850     /// The offset is expressed in number of `T` elements, and not bytes. The value returned can be
 851     /// used with the `wrapping_add` method.
 852     ///
 853     /// There are no guarantees whatsoever that offsetting the pointer will not overflow or go
 854     /// beyond the allocation that the pointer points into. It is up to the caller to ensure that
 855     /// the returned offset is correct in all terms other than alignment.
 856     ///
 857     /// # Panics
 858     ///
 859     /// The function panics if `align` is not a power-of-two.
 860     ///
 861     /// # Examples
 862     ///
 863     /// Accessing adjacent `u8` as `u16`
 864     ///
 865     /// ```
 866     /// # fn foo(n: usize) {
 867     /// # use std::mem::align_of;
 868     /// # unsafe {
 869     /// let x = [5u8, 6u8, 7u8, 8u8, 9u8];
 870     /// let ptr = x.as_ptr().add(n) as *const u8;
 871     /// let offset = ptr.align_offset(align_of::<u16>());
 872     /// if offset < x.len() - n - 1 {
 873     ///     let u16_ptr = ptr.add(offset) as *const u16;
 874     ///     assert_ne!(*u16_ptr, 500);
 875     /// } else {
 876     ///     // while the pointer can be aligned via `offset`, it would point
 877     ///     // outside the allocation
 878     /// }
 879     /// # } }
 880     /// ```
 881     #[stable(feature = "align_offset", since = "1.36.0")]
 882     pub fn align_offset(self, align: usize) -> usize
 883     where
 884         T: Sized,
 885     {
 886         if !align.is_power_of_two() {
 887             panic!("align_offset: align is not a power-of-two");
 888         }
 889         // SAFETY: `align` has been checked to be a power of 2 above
 890         unsafe { align_offset(self, align) }
 891     }
 892 }
 893
 894 #[lang = "const_slice_ptr"]
 895 impl<T> *const [T] {
 896     /// Returns the length of a raw slice.
 897     ///
 898     /// The returned value is the number of **elements**, not the number of bytes.
 899     ///
 900     /// This function is safe, even when the raw slice cannot be cast to a slice
 901     /// reference because the pointer is null or unaligned.
 902     ///
 903     /// # Examples
 904     ///
 905     /// ```rust
 906     /// #![feature(slice_ptr_len)]
 907     ///
 908     /// use std::ptr;
 909     ///
 910     /// let slice: *const [i8] = ptr::slice_from_raw_parts(ptr::null(), 3);
 911     /// assert_eq!(slice.len(), 3);
 912     /// ```
 913     #[inline]
 914     #[unstable(feature = "slice_ptr_len", issue = "71146")]
 915     #[rustc_const_unstable(feature = "const_slice_ptr_len", issue = "71146")]
 916     pub const fn len(self) -> usize {
 917         metadata(self)
 918     }
 919
 920     /// Returns a raw pointer to the slice's buffer.
 921     ///
 922     /// This is equivalent to casting `self` to `*const T`, but more type-safe.
 923     ///
 924     /// # Examples
 925     ///
 926     /// ```rust
 927     /// #![feature(slice_ptr_get)]
 928     /// use std::ptr;
 929     ///
 930     /// let slice: *const [i8] = ptr::slice_from_raw_parts(ptr::null(), 3);
 931     /// assert_eq!(slice.as_ptr(), 0 as *const i8);
 932     /// ```
 933     #[inline]
 934     #[unstable(feature = "slice_ptr_get", issue = "74265")]
 935     #[rustc_const_unstable(feature = "slice_ptr_get", issue = "74265")]
 936     pub const fn as_ptr(self) -> *const T {
 937         self as *const T
 938     }
 939
 940     /// Returns a raw pointer to an element or subslice, without doing bounds
 941     /// checking.
 942     ///
 943     /// Calling this method with an out-of-bounds index or when `self` is not dereferencable
 944     /// is *[undefined behavior]* even if the resulting pointer is not used.
 945     ///
 946     /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
 947     ///
 948     /// # Examples
 949     ///
 950     /// ```
 951     /// #![feature(slice_ptr_get)]
 952     ///
 953     /// let x = &[1, 2, 4] as *const [i32];
 954     ///
 955     /// unsafe {
 956     ///     assert_eq!(x.get_unchecked(1), x.as_ptr().add(1));
 957     /// }
 958     /// ```
 959     #[unstable(feature = "slice_ptr_get", issue = "74265")]
 960     #[inline]
 961     pub unsafe fn get_unchecked<I>(self, index: I) -> *const I::Output
 962     where
 963         I: SliceIndex<[T]>,
 964     {
 965         // SAFETY: the caller ensures that `self` is dereferencable and `index` in-bounds.
 966         unsafe { index.get_unchecked(self) }
 967     }
 968
 969     /// Returns `None` if the pointer is null, or else returns a shared slice to
 970     /// the value wrapped in `Some`. In contrast to [`as_ref`], this does not require
 971     /// that the value has to be initialized.
 972     ///
 973     /// [`as_ref`]: #method.as_ref
 974     ///
 975     /// # Safety
 976     ///
 977     /// When calling this method, you have to ensure that *either* the pointer is NULL *or*
 978     /// all of the following is true:
 979     ///
 980     /// * The pointer must be [valid] for reads for `ptr.len() * mem::size_of::<T>()` many bytes,
 981     ///   and it must be properly aligned. This means in particular:
 982     ///
 983     ///     * The entire memory range of this slice must be contained within a single [allocated object]!
 984     ///       Slices can never span across multiple allocated objects.
 985     ///
 986     ///     * The pointer must be aligned even for zero-length slices. One
 987     ///       reason for this is that enum layout optimizations may rely on references
 988     ///       (including slices of any length) being aligned and non-null to distinguish
 989     ///       them from other data. You can obtain a pointer that is usable as `data`
 990     ///       for zero-length slices using [`NonNull::dangling()`].
 991     ///
 992     /// * The total size `ptr.len() * mem::size_of::<T>()` of the slice must be no larger than `isize::MAX`.
 993     ///   See the safety documentation of [`pointer::offset`].
 994     ///
 995     /// * You must enforce Rust's aliasing rules, since the returned lifetime `'a` is
 996     ///   arbitrarily chosen and does not necessarily reflect the actual lifetime of the data.
 997     ///   In particular, for the duration of this lifetime, the memory the pointer points to must
 998     ///   not get mutated (except inside `UnsafeCell`).
 999     ///
1000     /// This applies even if the result of this method is unused!
1001     ///
1002     /// See also [`slice::from_raw_parts`][].
1003     ///
1004     /// [valid]: crate::ptr#safety
1005     /// [allocated object]: crate::ptr#allocated-object
1006     #[inline]
1007     #[unstable(feature = "ptr_as_uninit", issue = "75402")]
1008     pub unsafe fn as_uninit_slice<'a>(self) -> Option<&'a [MaybeUninit<T>]> {
1009         if self.is_null() {
1010             None
1011         } else {
1012             // SAFETY: the caller must uphold the safety contract for `as_uninit_slice`.
1013             Some(unsafe { slice::from_raw_parts(self as *const MaybeUninit<T>, self.len()) })
1014         }
1015     }
1016 }
1017
1018 // Equality for pointers
1019 #[stable(feature = "rust1", since = "1.0.0")]
1020 impl<T: ?Sized> PartialEq for *const T {
1021     #[inline]
1022     fn eq(&self, other: &*const T) -> bool {
1023         *self == *other
1024     }
1025 }
1026
1027 #[stable(feature = "rust1", since = "1.0.0")]
1028 impl<T: ?Sized> Eq for *const T {}
1029
1030 // Comparison for pointers
1031 #[stable(feature = "rust1", since = "1.0.0")]
1032 impl<T: ?Sized> Ord for *const T {
1033     #[inline]
1034     fn cmp(&self, other: &*const T) -> Ordering {
1035         if self < other {
1036             Less
1037         } else if self == other {
1038             Equal
1039         } else {
1040             Greater
1041         }
1042     }
1043 }
1044
1045 #[stable(feature = "rust1", since = "1.0.0")]
1046 impl<T: ?Sized> PartialOrd for *const T {
1047     #[inline]
1048     fn partial_cmp(&self, other: &*const T) -> Option<Ordering> {
1049         Some(self.cmp(other))
1050     }
1051
1052     #[inline]
1053     fn lt(&self, other: &*const T) -> bool {
1054         *self < *other
1055     }
1056
1057     #[inline]
1058     fn le(&self, other: &*const T) -> bool {
1059         *self <= *other
1060     }
1061
1062     #[inline]
1063     fn gt(&self, other: &*const T) -> bool {
1064         *self > *other
1065     }
1066
1067     #[inline]
1068     fn ge(&self, other: &*const T) -> bool {
1069         *self >= *other
1070     }
1071 }