const_ptr.rs.html -- source
Description: Source to the Rust file `src/libcore/ptr/const_ptr.rs`.
Keywords: rust, rustlang, rust-lang

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    use super::*;
    use crate::cmp::Ordering::{self, Equal, Greater, Less};
    use crate::intrinsics;
    use crate::mem;

    #[lang = "const_ptr"]
    impl<T: ?Sized> *const T {
        /// Returns `true` if the pointer is null.
        ///
        /// Note that unsized types have many possible null pointers, as only the
        /// raw data pointer is considered, not their length, vtable, etc.
        /// Therefore, two pointers that are null may still not compare equal to
        /// each other.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let s: &str = "Follow the rabbit";
        /// let ptr: *const u8 = s.as_ptr();
        /// assert!(!ptr.is_null());
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[inline]
        pub fn is_null(self) -> bool {
            // Compare via a cast to a thin pointer, so fat pointers are only
            // considering their "data" part for null-ness.
            (self as *const u8) == null()
        }

        /// Casts to a pointer of another type.
        #[stable(feature = "ptr_cast", since = "1.38.0")]
        #[rustc_const_stable(feature = "const_ptr_cast", since = "1.38.0")]
        #[inline]
        pub const fn cast<U>(self) -> *const U {
            self as _
        }

        /// Returns `None` if the pointer is null, or else returns a reference to
        /// the value wrapped in `Some`.
        ///
        /// # Safety
        ///
        /// While this method and its mutable counterpart are useful for
        /// null-safety, it is important to note that this is still an unsafe
        /// operation because the returned value could be pointing to invalid
        /// memory.
        ///
        /// When calling this method, you have to ensure that *either* the pointer is NULL *or*
        /// all of the following is true:
        /// - it is properly aligned
        /// - it must point to an initialized instance of T; in particular, the pointer must be
        ///   "dereferenceable" in the sense defined [here].
        ///
        /// This applies even if the result of this method is unused!
        /// (The part about being initialized is not yet fully decided, but until
        /// it is, the only safe approach is to ensure that they are indeed initialized.)
        ///
        /// Additionally, the lifetime `'a` returned is arbitrarily chosen and does
        /// not necessarily reflect the actual lifetime of the data. *You* must enforce
        /// Rust's aliasing rules. In particular, for the duration of this lifetime,
        /// the memory the pointer points to must not get mutated (except inside `UnsafeCell`).
        ///
        /// [here]: crate::ptr#safety
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let ptr: *const u8 = &10u8 as *const u8;
        ///
        /// unsafe {
        ///     if let Some(val_back) = ptr.as_ref() {
        ///         println!("We got back the value: {}!", val_back);
        ///     }
        /// }
        /// ```
        ///
        /// # Null-unchecked version
        ///
        /// If you are sure the pointer can never be null and are looking for some kind of
        /// `as_ref_unchecked` that returns the `&T` instead of `Option<&T>`, know that you can
        /// dereference the pointer directly.
        ///
        /// ```
        /// let ptr: *const u8 = &10u8 as *const u8;
        ///
        /// unsafe {
        ///     let val_back = &*ptr;
        ///     println!("We got back the value: {}!", val_back);
        /// }
        /// ```
        #[stable(feature = "ptr_as_ref", since = "1.9.0")]
        #[inline]
        pub unsafe fn as_ref<'a>(self) -> Option<&'a T> {
            // SAFETY: the caller must guarantee that `self` is valid
            // for a reference if it isn't null.
            if self.is_null() { None } else { unsafe { Some(&*self) } }
        }

        /// Calculates the offset from a pointer.
        ///
        /// `count` is in units of T; e.g., a `count` of 3 represents a pointer
        /// offset of `3 * size_of::<T>()` bytes.
        ///
        /// # Safety
        ///
        /// If any of the following conditions are violated, the result is Undefined
        /// Behavior:
        ///
        /// * Both the starting and resulting pointer must be either in bounds or one
        ///   byte past the end of the same allocated object. Note that in Rust,
        ///   every (stack-allocated) variable is considered a separate allocated object.
        ///
        /// * The computed offset, **in bytes**, cannot overflow an `isize`.
        ///
        /// * The offset being in bounds cannot rely on "wrapping around" the address
        ///   space. That is, the infinite-precision sum, **in bytes** must fit in a usize.
        ///
        /// The compiler and standard library generally tries to ensure allocations
        /// never reach a size where an offset is a concern. For instance, `Vec`
        /// and `Box` ensure they never allocate more than `isize::MAX` bytes, so
        /// `vec.as_ptr().add(vec.len())` is always safe.
        ///
        /// Most platforms fundamentally can't even construct such an allocation.
        /// For instance, no known 64-bit platform can ever serve a request
        /// for 2<sup>63</sup> bytes due to page-table limitations or splitting the address space.
        /// However, some 32-bit and 16-bit platforms may successfully serve a request for
        /// more than `isize::MAX` bytes with things like Physical Address
        /// Extension. As such, memory acquired directly from allocators or memory
        /// mapped files *may* be too large to handle with this function.
        ///
        /// Consider using [`wrapping_offset`] instead if these constraints are
        /// difficult to satisfy. The only advantage of this method is that it
        /// enables more aggressive compiler optimizations.
        ///
        /// [`wrapping_offset`]: #method.wrapping_offset
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let s: &str = "123";
        /// let ptr: *const u8 = s.as_ptr();
        ///
        /// unsafe {
        ///     println!("{}", *ptr.offset(1) as char);
        ///     println!("{}", *ptr.offset(2) as char);
        /// }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        #[must_use = "returns a new pointer rather than modifying its argument"]
        #[rustc_const_unstable(feature = "const_ptr_offset", issue = "71499")]
        #[inline]
        pub const unsafe fn offset(self, count: isize) -> *const T
        where
            T: Sized,
        {
            // SAFETY: the caller must uphold the safety contract for `offset`.
            unsafe { intrinsics::offset(self, count) }
        }

        /// Calculates the offset from a pointer using wrapping arithmetic.
        ///
        /// `count` is in units of T; e.g., a `count` of 3 represents a pointer
        /// offset of `3 * size_of::<T>()` bytes.
        ///
        /// # Safety
        ///
        /// The resulting pointer does not need to be in bounds, but it is
        /// potentially hazardous to dereference (which requires `unsafe`).
        ///
        /// In particular, the resulting pointer remains attached to the same allocated
        /// object that `self` points to. It may *not* be used to access a
        /// different allocated object. Note that in Rust,
        /// every (stack-allocated) variable is considered a separate allocated object.
        ///
        /// In other words, `x.wrapping_offset(y.wrapping_offset_from(x))` is
        /// *not* the same as `y`, and dereferencing it is undefined behavior
        /// unless `x` and `y` point into the same allocated object.
        ///
        /// Compared to [`offset`], this method basically delays the requirement of staying
        /// within the same allocated object: [`offset`] is immediate Undefined Behavior when
        /// crossing object boundaries; `wrapping_offset` produces a pointer but still leads
        /// to Undefined Behavior if that pointer is dereferenced. [`offset`] can be optimized
        /// better and is thus preferable in performance-sensitive code.
        ///
        /// If you need to cross object boundaries, cast the pointer to an integer and
        /// do the arithmetic there.
        ///
        /// [`offset`]: #method.offset
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// // Iterate using a raw pointer in increments of two elements
        /// let data = [1u8, 2, 3, 4, 5];
        /// let mut ptr: *const u8 = data.as_ptr();
        /// let step = 2;
        /// let end_rounded_up = ptr.wrapping_offset(6);
        ///
        /// // This loop prints "1, 3, 5, "
        /// while ptr != end_rounded_up {
        ///     unsafe {
        ///         print!("{}, ", *ptr);
        ///     }
        ///     ptr = ptr.wrapping_offset(step);
        /// }
        /// ```
        #[stable(feature = "ptr_wrapping_offset", since = "1.16.0")]
        #[must_use = "returns a new pointer rather than modifying its argument"]
        #[rustc_const_unstable(feature = "const_ptr_offset", issue = "71499")]
        #[inline]
        pub const fn wrapping_offset(self, count: isize) -> *const T
        where
            T: Sized,
        {
            // SAFETY: the `arith_offset` intrinsic has no prerequisites to be called.
            unsafe { intrinsics::arith_offset(self, count) }
        }

        /// Calculates the distance between two pointers. The returned value is in
        /// units of T: the distance in bytes is divided by `mem::size_of::<T>()`.
        ///
        /// This function is the inverse of [`offset`].
        ///
        /// [`offset`]: #method.offset
        /// [`wrapping_offset_from`]: #method.wrapping_offset_from
        ///
        /// # Safety
        ///
        /// If any of the following conditions are violated, the result is Undefined
        /// Behavior:
        ///
        /// * Both the starting and other pointer must be either in bounds or one
        ///   byte past the end of the same allocated object. Note that in Rust,
        ///   every (stack-allocated) variable is considered a separate allocated object.
        ///
        /// * The distance between the pointers, **in bytes**, cannot overflow an `isize`.
        ///
        /// * The distance between the pointers, in bytes, must be an exact multiple
        ///   of the size of `T`.
        ///
        /// * The distance being in bounds cannot rely on "wrapping around" the address space.
        ///
        /// The compiler and standard library generally try to ensure allocations
        /// never reach a size where an offset is a concern. For instance, `Vec`
        /// and `Box` ensure they never allocate more than `isize::MAX` bytes, so
        /// `ptr_into_vec.offset_from(vec.as_ptr())` is always safe.
        ///
        /// Most platforms fundamentally can't even construct such an allocation.
        /// For instance, no known 64-bit platform can ever serve a request
        /// for 2<sup>63</sup> bytes due to page-table limitations or splitting the address space.
        /// However, some 32-bit and 16-bit platforms may successfully serve a request for
        /// more than `isize::MAX` bytes with things like Physical Address
        /// Extension. As such, memory acquired directly from allocators or memory
        /// mapped files *may* be too large to handle with this function.
        ///
        /// Consider using [`wrapping_offset_from`] instead if these constraints are
        /// difficult to satisfy. The only advantage of this method is that it
        /// enables more aggressive compiler optimizations.
        ///
        /// # Panics
        ///
        /// This function panics if `T` is a Zero-Sized Type ("ZST").
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// #![feature(ptr_offset_from)]
        ///
        /// let a = [0; 5];
        /// let ptr1: *const i32 = &a[1];
        /// let ptr2: *const i32 = &a[3];
        /// unsafe {
        ///     assert_eq!(ptr2.offset_from(ptr1), 2);
        ///     assert_eq!(ptr1.offset_from(ptr2), -2);
        ///     assert_eq!(ptr1.offset(2), ptr2);
        ///     assert_eq!(ptr2.offset(-2), ptr1);
        /// }
        /// ```
        #[unstable(feature = "ptr_offset_from", issue = "41079")]
        #[rustc_const_unstable(feature = "const_ptr_offset_from", issue = "41079")]
        #[inline]
        pub const unsafe fn offset_from(self, origin: *const T) -> isize
        where
            T: Sized,
        {
            let pointee_size = mem::size_of::<T>();
            assert!(0 < pointee_size && pointee_size <= isize::MAX as usize);
            // SAFETY: the caller must uphold the safety contract for `ptr_offset_from`.
            unsafe { intrinsics::ptr_offset_from(self, origin) }
        }

        /// Returns whether two pointers are guaranteed to be equal.
        ///
        /// At runtime this function behaves like `self == other`.
        /// However, in some contexts (e.g., compile-time evaluation),
        /// it is not always possible to determine equality of two pointers, so this function may
        /// spuriously return `false` for pointers that later actually turn out to be equal.
        /// But when it returns `true`, the pointers are guaranteed to be equal.
        ///
        /// This function is the mirror of [`guaranteed_ne`], but not its inverse. There are pointer
        /// comparisons for which both functions return `false`.
        ///
        /// [`guaranteed_ne`]: #method.guaranteed_ne
        ///
        /// The return value may change depending on the compiler version and unsafe code may not
        /// rely on the result of this function for soundness. It is suggested to only use this function
        /// for performance optimizations where spurious `false` return values by this function do not
        /// affect the outcome, but just the performance.
        /// The consequences of using this method to make runtime and compile-time code behave
        /// differently have not been explored. This method should not be used to introduce such
        /// differences, and it should also not be stabilized before we have a better understanding
        /// of this issue.
        #[unstable(feature = "const_raw_ptr_comparison", issue = "53020")]
        #[rustc_const_unstable(feature = "const_raw_ptr_comparison", issue = "53020")]
        #[inline]
        #[cfg(not(bootstrap))]
        pub const fn guaranteed_eq(self, other: *const T) -> bool
        where
            T: Sized,
        {
            intrinsics::ptr_guaranteed_eq(self, other)
        }

        /// Returns whether two pointers are guaranteed to be inequal.
        ///
        /// At runtime this function behaves like `self != other`.
        /// However, in some contexts (e.g., compile-time evaluation),
        /// it is not always possible to determine the inequality of two pointers, so this function may
        /// spuriously return `false` for pointers that later actually turn out to be inequal.
        /// But when it returns `true`, the pointers are guaranteed to be inequal.
        ///
        /// This function is the mirror of [`guaranteed_eq`], but not its inverse. There are pointer
        /// comparisons for which both functions return `false`.
        ///
        /// [`guaranteed_eq`]: #method.guaranteed_eq
        ///
        /// The return value may change depending on the compiler version and unsafe code may not
        /// rely on the result of this function for soundness. It is suggested to only use this function
        /// for performance optimizations where spurious `false` return values by this function do not
        /// affect the outcome, but just the performance.
        /// The consequences of using this method to make runtime and compile-time code behave
        /// differently have not been explored. This method should not be used to introduce such
        /// differences, and it should also not be stabilized before we have a better understanding
        /// of this issue.
        #[unstable(feature = "const_raw_ptr_comparison", issue = "53020")]
        #[rustc_const_unstable(feature = "const_raw_ptr_comparison", issue = "53020")]
        #[inline]
        #[cfg(not(bootstrap))]
        pub const fn guaranteed_ne(self, other: *const T) -> bool
        where
            T: Sized,
        {
            intrinsics::ptr_guaranteed_ne(self, other)
        }

        /// Calculates the distance between two pointers. The returned value is in
        /// units of T: the distance in bytes is divided by `mem::size_of::<T>()`.
        ///
        /// If the address different between the two pointers is not a multiple of
        /// `mem::size_of::<T>()` then the result of the division is rounded towards
        /// zero.
        ///
        /// Though this method is safe for any two pointers, note that its result
        /// will be mostly useless if the two pointers aren't into the same allocated
        /// object, for example if they point to two different local variables.
        ///
        /// # Panics
        ///
        /// This function panics if `T` is a zero-sized type.
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// #![feature(ptr_wrapping_offset_from)]
        ///
        /// let a = [0; 5];
        /// let ptr1: *const i32 = &a[1];
        /// let ptr2: *const i32 = &a[3];
        /// assert_eq!(ptr2.wrapping_offset_from(ptr1), 2);
        /// assert_eq!(ptr1.wrapping_offset_from(ptr2), -2);
        /// assert_eq!(ptr1.wrapping_offset(2), ptr2);
        /// assert_eq!(ptr2.wrapping_offset(-2), ptr1);
        ///
        /// let ptr1: *const i32 = 3 as _;
        /// let ptr2: *const i32 = 13 as _;
        /// assert_eq!(ptr2.wrapping_offset_from(ptr1), 2);
        /// ```
        #[unstable(feature = "ptr_wrapping_offset_from", issue = "41079")]
        #[rustc_deprecated(
            since = "1.46.0",
            reason = "Pointer distances across allocation \
            boundaries are not typically meaningful. \
            Use integer subtraction if you really need this."
        )]
        #[inline]
        pub fn wrapping_offset_from(self, origin: *const T) -> isize
        where
            T: Sized,
        {
            let pointee_size = mem::size_of::<T>();
            assert!(0 < pointee_size && pointee_size <= isize::MAX as usize);

            let d = isize::wrapping_sub(self as _, origin as _);
            d.wrapping_div(pointee_size as _)
        }

        /// Calculates the offset from a pointer (convenience for `.offset(count as isize)`).
        ///
        /// `count` is in units of T; e.g., a `count` of 3 represents a pointer
        /// offset of `3 * size_of::<T>()` bytes.
        ///
        /// # Safety
        ///
        /// If any of the following conditions are violated, the result is Undefined
        /// Behavior:
        ///
        /// * Both the starting and resulting pointer must be either in bounds or one
        ///   byte past the end of the same allocated object. Note that in Rust,
        ///   every (stack-allocated) variable is considered a separate allocated object.
        ///
        /// * The computed offset, **in bytes**, cannot overflow an `isize`.
        ///
        /// * The offset being in bounds cannot rely on "wrapping around" the address
        ///   space. That is, the infinite-precision sum must fit in a `usize`.
        ///
        /// The compiler and standard library generally tries to ensure allocations
        /// never reach a size where an offset is a concern. For instance, `Vec`
        /// and `Box` ensure they never allocate more than `isize::MAX` bytes, so
        /// `vec.as_ptr().add(vec.len())` is always safe.
        ///
        /// Most platforms fundamentally can't even construct such an allocation.
        /// For instance, no known 64-bit platform can ever serve a request
        /// for 2<sup>63</sup> bytes due to page-table limitations or splitting the address space.
        /// However, some 32-bit and 16-bit platforms may successfully serve a request for
        /// more than `isize::MAX` bytes with things like Physical Address
        /// Extension. As such, memory acquired directly from allocators or memory
        /// mapped files *may* be too large to handle with this function.
        ///
        /// Consider using [`wrapping_add`] instead if these constraints are
        /// difficult to satisfy. The only advantage of this method is that it
        /// enables more aggressive compiler optimizations.
        ///
        /// [`wrapping_add`]: #method.wrapping_add
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let s: &str = "123";
        /// let ptr: *const u8 = s.as_ptr();
        ///
        /// unsafe {
        ///     println!("{}", *ptr.add(1) as char);
        ///     println!("{}", *ptr.add(2) as char);
        /// }
        /// ```
        #[stable(feature = "pointer_methods", since = "1.26.0")]
        #[must_use = "returns a new pointer rather than modifying its argument"]
        #[rustc_const_unstable(feature = "const_ptr_offset", issue = "71499")]
        #[inline]
        pub const unsafe fn add(self, count: usize) -> Self
        where
            T: Sized,
        {
            // SAFETY: the caller must uphold the safety contract for `offset`.
            unsafe { self.offset(count as isize) }
        }

        /// Calculates the offset from a pointer (convenience for
        /// `.offset((count as isize).wrapping_neg())`).
        ///
        /// `count` is in units of T; e.g., a `count` of 3 represents a pointer
        /// offset of `3 * size_of::<T>()` bytes.
        ///
        /// # Safety
        ///
        /// If any of the following conditions are violated, the result is Undefined
        /// Behavior:
        ///
        /// * Both the starting and resulting pointer must be either in bounds or one
        ///   byte past the end of the same allocated object. Note that in Rust,
        ///   every (stack-allocated) variable is considered a separate allocated object.
        ///
        /// * The computed offset cannot exceed `isize::MAX` **bytes**.
        ///
        /// * The offset being in bounds cannot rely on "wrapping around" the address
        ///   space. That is, the infinite-precision sum must fit in a usize.
        ///
        /// The compiler and standard library generally tries to ensure allocations
        /// never reach a size where an offset is a concern. For instance, `Vec`
        /// and `Box` ensure they never allocate more than `isize::MAX` bytes, so
        /// `vec.as_ptr().add(vec.len()).sub(vec.len())` is always safe.
        ///
        /// Most platforms fundamentally can't even construct such an allocation.
        /// For instance, no known 64-bit platform can ever serve a request
        /// for 2<sup>63</sup> bytes due to page-table limitations or splitting the address space.
        /// However, some 32-bit and 16-bit platforms may successfully serve a request for
        /// more than `isize::MAX` bytes with things like Physical Address
        /// Extension. As such, memory acquired directly from allocators or memory
        /// mapped files *may* be too large to handle with this function.
        ///
        /// Consider using [`wrapping_sub`] instead if these constraints are
        /// difficult to satisfy. The only advantage of this method is that it
        /// enables more aggressive compiler optimizations.
        ///
        /// [`wrapping_sub`]: #method.wrapping_sub
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// let s: &str = "123";
        ///
        /// unsafe {
        ///     let end: *const u8 = s.as_ptr().add(3);
        ///     println!("{}", *end.sub(1) as char);
        ///     println!("{}", *end.sub(2) as char);
        /// }
        /// ```
        #[stable(feature = "pointer_methods", since = "1.26.0")]
        #[must_use = "returns a new pointer rather than modifying its argument"]
        #[rustc_const_unstable(feature = "const_ptr_offset", issue = "71499")]
        #[inline]
        pub const unsafe fn sub(self, count: usize) -> Self
        where
            T: Sized,
        {
            // SAFETY: the caller must uphold the safety contract for `offset`.
            unsafe { self.offset((count as isize).wrapping_neg()) }
        }

        /// Calculates the offset from a pointer using wrapping arithmetic.
        /// (convenience for `.wrapping_offset(count as isize)`)
        ///
        /// `count` is in units of T; e.g., a `count` of 3 represents a pointer
        /// offset of `3 * size_of::<T>()` bytes.
        ///
        /// # Safety
        ///
        /// The resulting pointer does not need to be in bounds, but it is
        /// potentially hazardous to dereference (which requires `unsafe`).
        ///
        /// In particular, the resulting pointer remains attached to the same allocated
        /// object that `self` points to. It may *not* be used to access a
        /// different allocated object. Note that in Rust,
        /// every (stack-allocated) variable is considered a separate allocated object.
        ///
        /// Compared to [`add`], this method basically delays the requirement of staying
        /// within the same allocated object: [`add`] is immediate Undefined Behavior when
        /// crossing object boundaries; `wrapping_add` produces a pointer but still leads
        /// to Undefined Behavior if that pointer is dereferenced. [`add`] can be optimized
        /// better and is thus preferable in performance-sensitive code.
        ///
        /// If you need to cross object boundaries, cast the pointer to an integer and
        /// do the arithmetic there.
        ///
        /// [`add`]: #method.add
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// // Iterate using a raw pointer in increments of two elements
        /// let data = [1u8, 2, 3, 4, 5];
        /// let mut ptr: *const u8 = data.as_ptr();
        /// let step = 2;
        /// let end_rounded_up = ptr.wrapping_add(6);
        ///
        /// // This loop prints "1, 3, 5, "
        /// while ptr != end_rounded_up {
        ///     unsafe {
        ///         print!("{}, ", *ptr);
        ///     }
        ///     ptr = ptr.wrapping_add(step);
        /// }
        /// ```
        #[stable(feature = "pointer_methods", since = "1.26.0")]
        #[must_use = "returns a new pointer rather than modifying its argument"]
        #[rustc_const_unstable(feature = "const_ptr_offset", issue = "71499")]
        #[inline]
        pub const fn wrapping_add(self, count: usize) -> Self
        where
            T: Sized,
        {
            self.wrapping_offset(count as isize)
        }

        /// Calculates the offset from a pointer using wrapping arithmetic.
        /// (convenience for `.wrapping_offset((count as isize).wrapping_sub())`)
        ///
        /// `count` is in units of T; e.g., a `count` of 3 represents a pointer
        /// offset of `3 * size_of::<T>()` bytes.
        ///
        /// # Safety
        ///
        /// The resulting pointer does not need to be in bounds, but it is
        /// potentially hazardous to dereference (which requires `unsafe`).
        ///
        /// In particular, the resulting pointer remains attached to the same allocated
        /// object that `self` points to. It may *not* be used to access a
        /// different allocated object. Note that in Rust,
        /// every (stack-allocated) variable is considered a separate allocated object.
        ///
        /// Compared to [`sub`], this method basically delays the requirement of staying
        /// within the same allocated object: [`sub`] is immediate Undefined Behavior when
        /// crossing object boundaries; `wrapping_sub` produces a pointer but still leads
        /// to Undefined Behavior if that pointer is dereferenced. [`sub`] can be optimized
        /// better and is thus preferable in performance-sensitive code.
        ///
        /// If you need to cross object boundaries, cast the pointer to an integer and
        /// do the arithmetic there.
        ///
        /// [`sub`]: #method.sub
        ///
        /// # Examples
        ///
        /// Basic usage:
        ///
        /// ```
        /// // Iterate using a raw pointer in increments of two elements (backwards)
        /// let data = [1u8, 2, 3, 4, 5];
        /// let mut ptr: *const u8 = data.as_ptr();
        /// let start_rounded_down = ptr.wrapping_sub(2);
        /// ptr = ptr.wrapping_add(4);
        /// let step = 2;
        /// // This loop prints "5, 3, 1, "
        /// while ptr != start_rounded_down {
        ///     unsafe {
        ///         print!("{}, ", *ptr);
        ///     }
        ///     ptr = ptr.wrapping_sub(step);
        /// }
        /// ```
        #[stable(feature = "pointer_methods", since = "1.26.0")]
        #[must_use = "returns a new pointer rather than modifying its argument"]
        #[rustc_const_unstable(feature = "const_ptr_offset", issue = "71499")]
        #[inline]
        pub const fn wrapping_sub(self, count: usize) -> Self
        where
            T: Sized,
        {
            self.wrapping_offset((count as isize).wrapping_neg())
        }

        /// Reads the value from `self` without moving it. This leaves the
        /// memory in `self` unchanged.
        ///
        /// See [`ptr::read`] for safety concerns and examples.
        ///
        /// [`ptr::read`]: ./ptr/fn.read.html
        #[stable(feature = "pointer_methods", since = "1.26.0")]
        #[inline]
        pub unsafe fn read(self) -> T
        where
            T: Sized,
        {
            // SAFETY: the caller must uphold the safety contract for `read`.
            unsafe { read(self) }
        }

        /// Performs a volatile read of the value from `self` without moving it. This
        /// leaves the memory in `self` unchanged.
        ///
        /// Volatile operations are intended to act on I/O memory, and are guaranteed
        /// to not be elided or reordered by the compiler across other volatile
        /// operations.
        ///
        /// See [`ptr::read_volatile`] for safety concerns and examples.
        ///
        /// [`ptr::read_volatile`]: ./ptr/fn.read_volatile.html
        #[stable(feature = "pointer_methods", since = "1.26.0")]
        #[inline]
        pub unsafe fn read_volatile(self) -> T
        where
            T: Sized,
        {
            // SAFETY: the caller must uphold the safety contract for `read_volatile`.
            unsafe { read_volatile(self) }
        }

        /// Reads the value from `self` without moving it. This leaves the
        /// memory in `self` unchanged.
        ///
        /// Unlike `read`, the pointer may be unaligned.
        ///
        /// See [`ptr::read_unaligned`] for safety concerns and examples.
        ///
        /// [`ptr::read_unaligned`]: ./ptr/fn.read_unaligned.html
        #[stable(feature = "pointer_methods", since = "1.26.0")]
        #[inline]
        pub unsafe fn read_unaligned(self) -> T
        where
            T: Sized,
        {
            // SAFETY: the caller must uphold the safety contract for `read_unaligned`.
            unsafe { read_unaligned(self) }
        }

        /// Copies `count * size_of<T>` bytes from `self` to `dest`. The source
        /// and destination may overlap.
        ///
        /// NOTE: this has the *same* argument order as [`ptr::copy`].
        ///
        /// See [`ptr::copy`] for safety concerns and examples.
        ///
        /// [`ptr::copy`]: ./ptr/fn.copy.html
        #[stable(feature = "pointer_methods", since = "1.26.0")]
        #[inline]
        pub unsafe fn copy_to(self, dest: *mut T, count: usize)
        where
            T: Sized,
        {
            // SAFETY: the caller must uphold the safety contract for `copy`.
            unsafe { copy(self, dest, count) }
        }

        /// Copies `count * size_of<T>` bytes from `self` to `dest`. The source
        /// and destination may *not* overlap.
        ///
        /// NOTE: this has the *same* argument order as [`ptr::copy_nonoverlapping`].
        ///
        /// See [`ptr::copy_nonoverlapping`] for safety concerns and examples.
        ///
        /// [`ptr::copy_nonoverlapping`]: ./ptr/fn.copy_nonoverlapping.html
        #[stable(feature = "pointer_methods", since = "1.26.0")]
        #[inline]
        pub unsafe fn copy_to_nonoverlapping(self, dest: *mut T, count: usize)
        where
            T: Sized,
        {
            // SAFETY: the caller must uphold the safety contract for `copy_nonoverlapping`.
            unsafe { copy_nonoverlapping(self, dest, count) }
        }

        /// Computes the offset that needs to be applied to the pointer in order to make it aligned to
        /// `align`.
        ///
        /// If it is not possible to align the pointer, the implementation returns
        /// `usize::MAX`. It is permissible for the implementation to *always*
        /// return `usize::MAX`. Only your algorithm's performance can depend
        /// on getting a usable offset here, not its correctness.
        ///
        /// The offset is expressed in number of `T` elements, and not bytes. The value returned can be
        /// used with the `wrapping_add` method.
        ///
        /// There are no guarantees whatsoever that offsetting the pointer will not overflow or go
        /// beyond the allocation that the pointer points into. It is up to the caller to ensure that
        /// the returned offset is correct in all terms other than alignment.
        ///
        /// # Panics
        ///
        /// The function panics if `align` is not a power-of-two.
        ///
        /// # Examples
        ///
        /// Accessing adjacent `u8` as `u16`
        ///
        /// ```
        /// # fn foo(n: usize) {
        /// # use std::mem::align_of;
        /// # unsafe {
        /// let x = [5u8, 6u8, 7u8, 8u8, 9u8];
        /// let ptr = &x[n] as *const u8;
        /// let offset = ptr.align_offset(align_of::<u16>());
        /// if offset < x.len() - n - 1 {
        ///     let u16_ptr = ptr.add(offset) as *const u16;
        ///     assert_ne!(*u16_ptr, 500);
        /// } else {
        ///     // while the pointer can be aligned via `offset`, it would point
        ///     // outside the allocation
        /// }
        /// # } }
        /// ```
        #[stable(feature = "align_offset", since = "1.36.0")]
        pub fn align_offset(self, align: usize) -> usize
        where
            T: Sized,
        {
            if !align.is_power_of_two() {
                panic!("align_offset: align is not a power-of-two");
            }
            // SAFETY: `align` has been checked to be a power of 2 above
            unsafe { align_offset(self, align) }
        }
    }

    #[lang = "const_slice_ptr"]
    impl<T> *const [T] {
        /// Returns the length of a raw slice.
        ///
        /// The returned value is the number of **elements**, not the number of bytes.
        ///
        /// This function is safe, even when the raw slice cannot be cast to a slice
        /// reference because the pointer is null or unaligned.
        ///
        /// # Examples
        ///
        /// ```rust
        /// #![feature(slice_ptr_len)]
        ///
        /// use std::ptr;
        ///
        /// let slice: *const [i8] = ptr::slice_from_raw_parts(ptr::null(), 3);
        /// assert_eq!(slice.len(), 3);
        /// ```
        #[inline]
        #[unstable(feature = "slice_ptr_len", issue = "71146")]
        #[rustc_const_unstable(feature = "const_slice_ptr_len", issue = "71146")]
        pub const fn len(self) -> usize {
            // SAFETY: this is safe because `*const [T]` and `FatPtr<T>` have the same layout.
            // Only `std` can make this guarantee.
            unsafe { Repr { rust: self }.raw }.len
        }
    }

    // Equality for pointers
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized> PartialEq for *const T {
        #[inline]
        fn eq(&self, other: &*const T) -> bool {
            *self == *other
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized> Eq for *const T {}

    // Comparison for pointers
    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized> Ord for *const T {
        #[inline]
        fn cmp(&self, other: &*const T) -> Ordering {
            if self < other {
                Less
            } else if self == other {
                Equal
            } else {
                Greater
            }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: ?Sized> PartialOrd for *const T {
        #[inline]
        fn partial_cmp(&self, other: &*const T) -> Option<Ordering> {
            Some(self.cmp(other))
        }

        #[inline]
        fn lt(&self, other: &*const T) -> bool {
            *self < *other
        }

        #[inline]
        fn le(&self, other: &*const T) -> bool {
            *self <= *other
        }

        #[inline]
        fn gt(&self, other: &*const T) -> bool {
            *self > *other
        }

        #[inline]
        fn ge(&self, other: &*const T) -> bool {
            *self >= *other
        }
    }