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also remove redundant requirements from offset()

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This commit is contained in:
Ralf Jung 2024-07-03 16:42:33 +02:00
parent 273d253ce6
commit 9ba492f279
3 changed files with 100 additions and 196 deletions
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101
library/core/src/ptr/const_ptr.rs
View File

@ -390,37 +390,26 @@ impl<T: ?Sized> *const T {
if self.is_null() { None } else { Some(unsafe { &*(self as *const MaybeUninit<T>) }) } if self.is_null() { None } else { Some(unsafe { &*(self as *const MaybeUninit<T>) }) }
} }
/// Calculates the offset from a pointer. /// Adds an offset to a pointer.
/// ///
/// `count` is in units of T; e.g., a `count` of 3 represents a pointer /// `count` is in units of T; e.g., a `count` of 3 represents a pointer
/// offset of `3 * size_of::<T>()` bytes. /// offset of `3 * size_of::<T>()` bytes.
/// ///
/// # Safety /// # Safety
/// ///
/// If any of the following conditions are violated, the result is Undefined /// If any of the following conditions are violated, the result is Undefined Behavior:
/// Behavior:
/// ///
/// * If the computed offset, **in bytes**, is non-zero, then both the starting and resulting /// * The computed offset, `count * size_of::<T>()` bytes, must not overflow `isize`.
/// pointer must be either in bounds or at the end of the same [allocated object].
/// (If it is zero, then the function is always well-defined.)
/// ///
/// * The computed offset, **in bytes**, cannot overflow an `isize`. /// * If the computed offset is non-zero, then `self` must be derived from a pointer to some
/// [allocated object], and the entire memory range between `self` and the result must be in
/// bounds of that allocated object. In particular, this range must not "wrap around" the edge
/// of the address space.
/// ///
/// * The offset being in bounds cannot rely on "wrapping around" the address /// Allocated objects can never be larger than `isize::MAX` bytes, so if the computed offset
/// space. That is, the infinite-precision sum, **in bytes** must fit in a usize. /// stays in bounds of the allocated object, it is guaranteed to satisfy the first requirement.
/// /// This implies, for instance, that `vec.as_ptr().add(vec.len())` (for `vec: Vec<T>`) is always
/// The compiler and standard library generally tries to ensure allocations /// safe.
/// 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 /// Consider using [`wrapping_offset`] instead if these constraints are
/// difficult to satisfy. The only advantage of this method is that it /// difficult to satisfy. The only advantage of this method is that it
@ -622,7 +611,7 @@ impl<T: ?Sized> *const T {
/// * The distance between the pointers, in bytes, must be an exact multiple /// * The distance between the pointers, in bytes, must be an exact multiple
/// of the size of `T`. /// of the size of `T`.
/// ///
/// As a consequence, the absolute distance between the pointers, **in bytes**, computed on /// As a consequence, the absolute distance between the pointers, in bytes, computed on
/// mathematical integers (without "wrapping around"), cannot overflow an `isize`. This is /// mathematical integers (without "wrapping around"), cannot overflow an `isize`. This is
/// implied by the in-bounds requirement, and the fact that no allocated object can be larger /// implied by the in-bounds requirement, and the fact that no allocated object can be larger
/// than `isize::MAX` bytes. /// than `isize::MAX` bytes.
@ -862,37 +851,26 @@ impl<T: ?Sized> *const T {
} }
} }
/// Calculates the offset from a pointer (convenience for `.offset(count as isize)`). /// Adds an offset to a pointer (convenience for `.offset(count as isize)`).
/// ///
/// `count` is in units of T; e.g., a `count` of 3 represents a pointer /// `count` is in units of T; e.g., a `count` of 3 represents a pointer
/// offset of `3 * size_of::<T>()` bytes. /// offset of `3 * size_of::<T>()` bytes.
/// ///
/// # Safety /// # Safety
/// ///
/// If any of the following conditions are violated, the result is Undefined /// If any of the following conditions are violated, the result is Undefined Behavior:
/// Behavior:
/// ///
/// * If the computed offset, **in bytes**, is non-zero, then both the starting and resulting /// * The computed offset, `count * size_of::<T>()` bytes, must not overflow `isize`.
/// pointer must be either in bounds or at the end of the same [allocated object].
/// (If it is zero, then the function is always well-defined.)
/// ///
/// * The computed offset, **in bytes**, cannot overflow an `isize`. /// * If the computed offset is non-zero, then `self` must be derived from a pointer to some
/// [allocated object], and the entire memory range between `self` and the result must be in
/// bounds of that allocated object. In particular, this range must not "wrap around" the edge
/// of the address space.
/// ///
/// * The offset being in bounds cannot rely on "wrapping around" the address /// Allocated objects can never be larger than `isize::MAX` bytes, so if the computed offset
/// space. That is, the infinite-precision sum must fit in a `usize`. /// stays in bounds of the allocated object, it is guaranteed to satisfy the first requirement.
/// /// This implies, for instance, that `vec.as_ptr().add(vec.len())` (for `vec: Vec<T>`) is always
/// The compiler and standard library generally tries to ensure allocations /// safe.
/// 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 /// Consider using [`wrapping_add`] instead if these constraints are
/// difficult to satisfy. The only advantage of this method is that it /// difficult to satisfy. The only advantage of this method is that it
@ -946,7 +924,7 @@ impl<T: ?Sized> *const T {
unsafe { self.cast::<u8>().add(count).with_metadata_of(self) } unsafe { self.cast::<u8>().add(count).with_metadata_of(self) }
} }
/// Calculates the offset from a pointer (convenience for /// Subtracts an offset from a pointer (convenience for
/// `.offset((count as isize).wrapping_neg())`). /// `.offset((count as isize).wrapping_neg())`).
/// ///
/// `count` is in units of T; e.g., a `count` of 3 represents a pointer /// `count` is in units of T; e.g., a `count` of 3 represents a pointer
@ -954,30 +932,19 @@ impl<T: ?Sized> *const T {
/// ///
/// # Safety /// # Safety
/// ///
/// If any of the following conditions are violated, the result is Undefined /// If any of the following conditions are violated, the result is Undefined Behavior:
/// Behavior:
/// ///
/// * If the computed offset, **in bytes**, is non-zero, then both the starting and resulting /// * The computed offset, `count * size_of::<T>()` bytes, must not overflow `isize`.
/// pointer must be either in bounds or at the end of the same [allocated object].
/// (If it is zero, then the function is always well-defined.)
/// ///
/// * The computed offset cannot exceed `isize::MAX` **bytes**. /// * If the computed offset is non-zero, then `self` must be derived from a pointer to some
/// [allocated object], and the entire memory range between `self` and the result must be in
/// bounds of that allocated object. In particular, this range must not "wrap around" the edge
/// of the address space.
/// ///
/// * The offset being in bounds cannot rely on "wrapping around" the address /// Allocated objects can never be larger than `isize::MAX` bytes, so if the computed offset
/// space. That is, the infinite-precision sum must fit in a usize. /// stays in bounds of the allocated object, it is guaranteed to satisfy the first requirement.
/// /// This implies, for instance, that `vec.as_ptr().add(vec.len())` (for `vec: Vec<T>`) is always
/// The compiler and standard library generally tries to ensure allocations /// safe.
/// 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 /// Consider using [`wrapping_sub`] instead if these constraints are
/// difficult to satisfy. The only advantage of this method is that it /// difficult to satisfy. The only advantage of this method is that it

99
library/core/src/ptr/mut_ptr.rs
View File

@ -404,37 +404,26 @@ impl<T: ?Sized> *mut T {
if self.is_null() { None } else { Some(unsafe { &*(self as *const MaybeUninit<T>) }) } if self.is_null() { None } else { Some(unsafe { &*(self as *const MaybeUninit<T>) }) }
} }
/// Calculates the offset from a pointer. /// Adds an offset to a pointer.
/// ///
/// `count` is in units of T; e.g., a `count` of 3 represents a pointer /// `count` is in units of T; e.g., a `count` of 3 represents a pointer
/// offset of `3 * size_of::<T>()` bytes. /// offset of `3 * size_of::<T>()` bytes.
/// ///
/// # Safety /// # Safety
/// ///
/// If any of the following conditions are violated, the result is Undefined /// If any of the following conditions are violated, the result is Undefined Behavior:
/// Behavior:
/// ///
/// * If the computed offset, **in bytes**, is non-zero, then both the starting and resulting /// * The computed offset, `count * size_of::<T>()` bytes, must not overflow `isize`.
/// pointer must be either in bounds or at the end of the same [allocated object].
/// (If it is zero, then the function is always well-defined.)
/// ///
/// * The computed offset, **in bytes**, cannot overflow an `isize`. /// * If the computed offset is non-zero, then `self` must be derived from a pointer to some
/// [allocated object], and the entire memory range between `self` and the result must be in
/// bounds of that allocated object. In particular, this range must not "wrap around" the edge
/// of the address space.
/// ///
/// * The offset being in bounds cannot rely on "wrapping around" the address /// Allocated objects can never be larger than `isize::MAX` bytes, so if the computed offset
/// space. That is, the infinite-precision sum, **in bytes** must fit in a usize. /// stays in bounds of the allocated object, it is guaranteed to satisfy the first requirement.
/// /// This implies, for instance, that `vec.as_ptr().add(vec.len())` (for `vec: Vec<T>`) is always
/// The compiler and standard library generally tries to ensure allocations /// safe.
/// 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 /// Consider using [`wrapping_offset`] instead if these constraints are
/// difficult to satisfy. The only advantage of this method is that it /// difficult to satisfy. The only advantage of this method is that it
@ -1003,37 +992,26 @@ impl<T: ?Sized> *mut T {
unsafe { (self as *const T).sub_ptr(origin) } unsafe { (self as *const T).sub_ptr(origin) }
} }
/// Calculates the offset from a pointer (convenience for `.offset(count as isize)`). /// Adds an offset to a pointer (convenience for `.offset(count as isize)`).
/// ///
/// `count` is in units of T; e.g., a `count` of 3 represents a pointer /// `count` is in units of T; e.g., a `count` of 3 represents a pointer
/// offset of `3 * size_of::<T>()` bytes. /// offset of `3 * size_of::<T>()` bytes.
/// ///
/// # Safety /// # Safety
/// ///
/// If any of the following conditions are violated, the result is Undefined /// If any of the following conditions are violated, the result is Undefined Behavior:
/// Behavior:
/// ///
/// * If the computed offset, **in bytes**, is non-zero, then both the starting and resulting /// * The computed offset, `count * size_of::<T>()` bytes, must not overflow `isize`.
/// pointer must be either in bounds or at the end of the same [allocated object].
/// (If it is zero, then the function is always well-defined.)
/// ///
/// * The computed offset, **in bytes**, cannot overflow an `isize`. /// * If the computed offset is non-zero, then `self` must be derived from a pointer to some
/// [allocated object], and the entire memory range between `self` and the result must be in
/// bounds of that allocated object. In particular, this range must not "wrap around" the edge
/// of the address space.
/// ///
/// * The offset being in bounds cannot rely on "wrapping around" the address /// Allocated objects can never be larger than `isize::MAX` bytes, so if the computed offset
/// space. That is, the infinite-precision sum must fit in a `usize`. /// stays in bounds of the allocated object, it is guaranteed to satisfy the first requirement.
/// /// This implies, for instance, that `vec.as_ptr().add(vec.len())` (for `vec: Vec<T>`) is always
/// The compiler and standard library generally tries to ensure allocations /// safe.
/// 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 /// Consider using [`wrapping_add`] instead if these constraints are
/// difficult to satisfy. The only advantage of this method is that it /// difficult to satisfy. The only advantage of this method is that it
@ -1087,7 +1065,7 @@ impl<T: ?Sized> *mut T {
unsafe { self.cast::<u8>().add(count).with_metadata_of(self) } unsafe { self.cast::<u8>().add(count).with_metadata_of(self) }
} }
/// Calculates the offset from a pointer (convenience for /// Subtracts an offset from a pointer (convenience for
/// `.offset((count as isize).wrapping_neg())`). /// `.offset((count as isize).wrapping_neg())`).
/// ///
/// `count` is in units of T; e.g., a `count` of 3 represents a pointer /// `count` is in units of T; e.g., a `count` of 3 represents a pointer
@ -1095,30 +1073,19 @@ impl<T: ?Sized> *mut T {
/// ///
/// # Safety /// # Safety
/// ///
/// If any of the following conditions are violated, the result is Undefined /// If any of the following conditions are violated, the result is Undefined Behavior:
/// Behavior:
/// ///
/// * If the computed offset, **in bytes**, is non-zero, then both the starting and resulting /// * The computed offset, `count * size_of::<T>()` bytes, must not overflow `isize`.
/// pointer must be either in bounds or at the end of the same [allocated object].
/// (If it is zero, then the function is always well-defined.)
/// ///
/// * The computed offset cannot exceed `isize::MAX` **bytes**. /// * If the computed offset is non-zero, then `self` must be derived from a pointer to some
/// [allocated object], and the entire memory range between `self` and the result must be in
/// bounds of that allocated object. In particular, this range must not "wrap around" the edge
/// of the address space.
/// ///
/// * The offset being in bounds cannot rely on "wrapping around" the address /// Allocated objects can never be larger than `isize::MAX` bytes, so if the computed offset
/// space. That is, the infinite-precision sum must fit in a usize. /// stays in bounds of the allocated object, it is guaranteed to satisfy the first requirement.
/// /// This implies, for instance, that `vec.as_ptr().add(vec.len())` (for `vec: Vec<T>`) is always
/// The compiler and standard library generally tries to ensure allocations /// safe.
/// 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 /// Consider using [`wrapping_sub`] instead if these constraints are
/// difficult to satisfy. The only advantage of this method is that it /// difficult to satisfy. The only advantage of this method is that it

96
library/core/src/ptr/non_null.rs
View File

@ -476,36 +476,26 @@ impl<T: ?Sized> NonNull<T> {
unsafe { NonNull { pointer: self.as_ptr() as *mut U } } unsafe { NonNull { pointer: self.as_ptr() as *mut U } }
} }
/// Calculates the offset from a pointer. /// Adds an offset to a pointer.
/// ///
/// `count` is in units of T; e.g., a `count` of 3 represents a pointer /// `count` is in units of T; e.g., a `count` of 3 represents a pointer
/// offset of `3 * size_of::<T>()` bytes. /// offset of `3 * size_of::<T>()` bytes.
/// ///
/// # Safety /// # Safety
/// ///
/// If any of the following conditions are violated, the result is Undefined /// If any of the following conditions are violated, the result is Undefined Behavior:
/// Behavior:
/// ///
/// * Both the starting and resulting pointer must be either in bounds or one /// * The computed offset, `count * size_of::<T>()` bytes, must not overflow `isize`.
/// byte past the end of the same [allocated object].
/// ///
/// * The computed offset, **in bytes**, cannot overflow an `isize`. /// * If the computed offset is non-zero, then `self` must be derived from a pointer to some
/// [allocated object], and the entire memory range between `self` and the result must be in
/// bounds of that allocated object. In particular, this range must not "wrap around" the edge
/// of the address space.
/// ///
/// * The offset being in bounds cannot rely on "wrapping around" the address /// Allocated objects can never be larger than `isize::MAX` bytes, so if the computed offset
/// space. That is, the infinite-precision sum, **in bytes** must fit in a usize. /// stays in bounds of the allocated object, it is guaranteed to satisfy the first requirement.
/// /// This implies, for instance, that `vec.as_ptr().add(vec.len())` (for `vec: Vec<T>`) is always
/// The compiler and standard library generally tries to ensure allocations /// safe.
/// 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.
/// ///
/// [allocated object]: crate::ptr#allocated-object /// [allocated object]: crate::ptr#allocated-object
/// ///
@ -562,36 +552,26 @@ impl<T: ?Sized> NonNull<T> {
unsafe { NonNull { pointer: self.pointer.byte_offset(count) } } unsafe { NonNull { pointer: self.pointer.byte_offset(count) } }
} }
/// Calculates the offset from a pointer (convenience for `.offset(count as isize)`). /// Adds an offset to a pointer (convenience for `.offset(count as isize)`).
/// ///
/// `count` is in units of T; e.g., a `count` of 3 represents a pointer /// `count` is in units of T; e.g., a `count` of 3 represents a pointer
/// offset of `3 * size_of::<T>()` bytes. /// offset of `3 * size_of::<T>()` bytes.
/// ///
/// # Safety /// # Safety
/// ///
/// If any of the following conditions are violated, the result is Undefined /// If any of the following conditions are violated, the result is Undefined Behavior:
/// Behavior:
/// ///
/// * Both the starting and resulting pointer must be either in bounds or one /// * The computed offset, `count * size_of::<T>()` bytes, must not overflow `isize`.
/// byte past the end of the same [allocated object].
/// ///
/// * The computed offset, **in bytes**, cannot overflow an `isize`. /// * If the computed offset is non-zero, then `self` must be derived from a pointer to some
/// [allocated object], and the entire memory range between `self` and the result must be in
/// bounds of that allocated object. In particular, this range must not "wrap around" the edge
/// of the address space.
/// ///
/// * The offset being in bounds cannot rely on "wrapping around" the address /// Allocated objects can never be larger than `isize::MAX` bytes, so if the computed offset
/// space. That is, the infinite-precision sum must fit in a `usize`. /// stays in bounds of the allocated object, it is guaranteed to satisfy the first requirement.
/// /// This implies, for instance, that `vec.as_ptr().add(vec.len())` (for `vec: Vec<T>`) is always
/// The compiler and standard library generally tries to ensure allocations /// safe.
/// 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.
/// ///
/// [allocated object]: crate::ptr#allocated-object /// [allocated object]: crate::ptr#allocated-object
/// ///
@ -649,7 +629,7 @@ impl<T: ?Sized> NonNull<T> {
unsafe { NonNull { pointer: self.pointer.byte_add(count) } } unsafe { NonNull { pointer: self.pointer.byte_add(count) } }
} }
/// Calculates the offset from a pointer (convenience for /// Subtracts an offset from a pointer (convenience for
/// `.offset((count as isize).wrapping_neg())`). /// `.offset((count as isize).wrapping_neg())`).
/// ///
/// `count` is in units of T; e.g., a `count` of 3 represents a pointer /// `count` is in units of T; e.g., a `count` of 3 represents a pointer
@ -657,29 +637,19 @@ impl<T: ?Sized> NonNull<T> {
/// ///
/// # Safety /// # Safety
/// ///
/// If any of the following conditions are violated, the result is Undefined /// If any of the following conditions are violated, the result is Undefined Behavior:
/// Behavior:
/// ///
/// * Both the starting and resulting pointer must be either in bounds or one /// * The computed offset, `count * size_of::<T>()` bytes, must not overflow `isize`.
/// byte past the end of the same [allocated object].
/// ///
/// * The computed offset cannot exceed `isize::MAX` **bytes**. /// * If the computed offset is non-zero, then `self` must be derived from a pointer to some
/// [allocated object], and the entire memory range between `self` and the result must be in
/// bounds of that allocated object. In particular, this range must not "wrap around" the edge
/// of the address space.
/// ///
/// * The offset being in bounds cannot rely on "wrapping around" the address /// Allocated objects can never be larger than `isize::MAX` bytes, so if the computed offset
/// space. That is, the infinite-precision sum must fit in a usize. /// stays in bounds of the allocated object, it is guaranteed to satisfy the first requirement.
/// /// This implies, for instance, that `vec.as_ptr().add(vec.len())` (for `vec: Vec<T>`) is always
/// The compiler and standard library generally tries to ensure allocations /// safe.
/// 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.
/// ///
/// [allocated object]: crate::ptr#allocated-object /// [allocated object]: crate::ptr#allocated-object
/// ///