Struct core::ptr::NonNull

1.25.0 · source ·

pub struct NonNull<T: ?Sized> { /* private fields */ }

Expand description

*mut T but non-zero and covariant.

This is often the correct thing to use when building data structures using raw pointers, but is ultimately more dangerous to use because of its additional properties. If you’re not sure if you should use NonNull<T>, just use *mut T!

Unlike *mut T, the pointer must always be non-null, even if the pointer is never dereferenced. This is so that enums may use this forbidden value as a discriminant – Option<NonNull<T>> has the same size as *mut T. However the pointer may still dangle if it isn’t dereferenced.

Unlike *mut T, NonNull<T> was chosen to be covariant over T. This makes it possible to use NonNull<T> when building covariant types, but introduces the risk of unsoundness if used in a type that shouldn’t actually be covariant. (The opposite choice was made for *mut T even though technically the unsoundness could only be caused by calling unsafe functions.)

Covariance is correct for most safe abstractions, such as Box, Rc, Arc, Vec, and LinkedList. This is the case because they provide a public API that follows the normal shared XOR mutable rules of Rust.

If your type cannot safely be covariant, you must ensure it contains some additional field to provide invariance. Often this field will be a PhantomData type like PhantomData<Cell<T>> or PhantomData<&'a mut T>.

Notice that NonNull<T> has a From instance for &T. However, this does not change the fact that mutating through a (pointer derived from a) shared reference is undefined behavior unless the mutation happens inside an UnsafeCell<T>. The same goes for creating a mutable reference from a shared reference. When using this From instance without an UnsafeCell<T>, it is your responsibility to ensure that as_mut is never called, and as_ptr is never used for mutation.

§Representation

Thanks to the null pointer optimization, NonNull<T> and Option<NonNull<T>> are guaranteed to have the same size and alignment:

use std::ptr::NonNull;

assert_eq!(size_of::<NonNull<i16>>(), size_of::<Option<NonNull<i16>>>());
assert_eq!(align_of::<NonNull<i16>>(), align_of::<Option<NonNull<i16>>>());

assert_eq!(size_of::<NonNull<str>>(), size_of::<Option<NonNull<str>>>());
assert_eq!(align_of::<NonNull<str>>(), align_of::<Option<NonNull<str>>>());

Implementations§

source §

impl<T: Sized> NonNull<T>

1.25.0 (const: 1.36.0) · source

pub const fn dangling() -> Self

Creates a new NonNull that is dangling, but well-aligned.

This is useful for initializing types which lazily allocate, like Vec::new does.

Note that the pointer value may potentially represent a valid pointer to a T, which means this must not be used as a “not yet initialized” sentinel value. Types that lazily allocate must track initialization by some other means.

§Examples

use std::ptr::NonNull;

let ptr = NonNull::<u32>::dangling();
// Important: don't try to access the value of `ptr` without
// initializing it first! The pointer is not null but isn't valid either!

source

pub const unsafe fn as_uninit_ref<'a>(self) -> &'a MaybeUninit<T>

🔬This is a nightly-only experimental API. (ptr_as_uninit #75402)

Returns a shared references to the value. In contrast to as_ref, this does not require that the value has to be initialized.

For the mutable counterpart see as_uninit_mut.

§Safety

When calling this method, you have to ensure that the pointer is convertible to a reference. Note that because the created reference is to MaybeUninit<T>, the source pointer can point to uninitialized memory.

source

pub const unsafe fn as_uninit_mut<'a>(self) -> &'a mut MaybeUninit<T>

🔬This is a nightly-only experimental API. (ptr_as_uninit #75402)

Returns a unique references to the value. In contrast to as_mut, this does not require that the value has to be initialized.

For the shared counterpart see as_uninit_ref.

§Safety

When calling this method, you have to ensure that the pointer is convertible to a reference. Note that because the created reference is to MaybeUninit<T>, the source pointer can point to uninitialized memory.

source §

impl<T: ?Sized> NonNull<T>

1.25.0 (const: 1.25.0) · source

pub const unsafe fn new_unchecked(ptr: *mut T) -> Self

Creates a new NonNull.

§Safety

ptr must be non-null.

§Examples

use std::ptr::NonNull;

let mut x = 0u32;
let ptr = unsafe { NonNull::new_unchecked(&mut x as *mut _) };

Incorrect usage of this function:

use std::ptr::NonNull;

// NEVER DO THAT!!! This is undefined behavior. ⚠️
let ptr = unsafe { NonNull::<u32>::new_unchecked(std::ptr::null_mut()) };

1.25.0 (const: unstable) · source

pub fn new(ptr: *mut T) -> Option<Self>

Creates a new NonNull if ptr is non-null.

§Examples

use std::ptr::NonNull;

let mut x = 0u32;
let ptr = NonNull::<u32>::new(&mut x as *mut _).expect("ptr is null!");

if let Some(ptr) = NonNull::<u32>::new(std::ptr::null_mut()) {
    unreachable!();
}

source

pub const fn from_raw_parts( data_pointer: NonNull<()>, metadata: <T as Pointee>::Metadata, ) -> NonNull<T>

🔬This is a nightly-only experimental API. (ptr_metadata #81513)

Performs the same functionality as std::ptr::from_raw_parts, except that a NonNull pointer is returned, as opposed to a raw *const pointer.

See the documentation of std::ptr::from_raw_parts for more details.

source

pub const fn to_raw_parts(self) -> (NonNull<()>, <T as Pointee>::Metadata)

🔬This is a nightly-only experimental API. (ptr_metadata #81513)

Decompose a (possibly wide) pointer into its data pointer and metadata components.

The pointer can be later reconstructed with NonNull::from_raw_parts.

source

pub fn addr(self) -> NonZero<usize>

🔬This is a nightly-only experimental API. (strict_provenance #95228)

Gets the “address” portion of the pointer.

For more details see the equivalent method on a raw pointer, pointer::addr.

This API and its claimed semantics are part of the Strict Provenance experiment, see the ptr module documentation.

source

pub fn with_addr(self, addr: NonZero<usize>) -> Self

🔬This is a nightly-only experimental API. (strict_provenance #95228)

Creates a new pointer with the given address.

For more details see the equivalent method on a raw pointer, pointer::with_addr.

This API and its claimed semantics are part of the Strict Provenance experiment, see the ptr module documentation.

source

pub fn map_addr(self, f: impl FnOnce(NonZero<usize>) -> NonZero<usize>) -> Self

🔬This is a nightly-only experimental API. (strict_provenance #95228)

Creates a new pointer by mapping self’s address to a new one.

For more details see the equivalent method on a raw pointer, pointer::map_addr.

This API and its claimed semantics are part of the Strict Provenance experiment, see the ptr module documentation.

1.25.0 (const: 1.32.0) · source

pub const fn as_ptr(self) -> *mut T

Acquires the underlying *mut pointer.

§Examples

use std::ptr::NonNull;

let mut x = 0u32;
let ptr = NonNull::new(&mut x).expect("ptr is null!");

let x_value = unsafe { *ptr.as_ptr() };
assert_eq!(x_value, 0);

unsafe { *ptr.as_ptr() += 2; }
let x_value = unsafe { *ptr.as_ptr() };
assert_eq!(x_value, 2);

1.25.0 (const: 1.73.0) · source

pub const unsafe fn as_ref<'a>(&self) -> &'a T

Returns a shared reference to the value. If the value may be uninitialized, as_uninit_ref must be used instead.

For the mutable counterpart see as_mut.

§Safety

When calling this method, you have to ensure that the pointer is convertible to a reference.

§Examples

use std::ptr::NonNull;

let mut x = 0u32;
let ptr = NonNull::new(&mut x as *mut _).expect("ptr is null!");

let ref_x = unsafe { ptr.as_ref() };
println!("{ref_x}");

1.25.0 (const: unstable) · source

pub unsafe fn as_mut<'a>(&mut self) -> &'a mut T

Returns a unique reference to the value. If the value may be uninitialized, as_uninit_mut must be used instead.

For the shared counterpart see as_ref.

§Safety

When calling this method, you have to ensure that the pointer is convertible to a reference.

§Examples

use std::ptr::NonNull;

let mut x = 0u32;
let mut ptr = NonNull::new(&mut x).expect("null pointer");

let x_ref = unsafe { ptr.as_mut() };
assert_eq!(*x_ref, 0);
*x_ref += 2;
assert_eq!(*x_ref, 2);

1.27.0 (const: 1.36.0) · source

pub const fn cast(self) -> NonNull

Casts to a pointer of another type.

§Examples

use std::ptr::NonNull;

let mut x = 0u32;
let ptr = NonNull::new(&mut x as *mut _).expect("null pointer");

let casted_ptr = ptr.cast::<i8>();
let raw_ptr: *mut i8 = casted_ptr.as_ptr();

1.80.0 (const: 1.80.0) · source

pub const unsafe fn offset(self, count: isize) -> Self
where T: Sized,

Adds an offset to 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:

The computed offset, count * size_of::<T>() bytes, must not overflow 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.

Allocated objects can never be larger than isize::MAX bytes, so if the computed offset 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 safe.

§Examples

use std::ptr::NonNull;

let mut s = [1, 2, 3];
let ptr: NonNull<u32> = NonNull::new(s.as_mut_ptr()).unwrap();

unsafe {
    println!("{}", ptr.offset(1).read());
    println!("{}", ptr.offset(2).read());
}

1.80.0 (const: 1.80.0) · source

pub const unsafe fn byte_offset(self, count: isize) -> Self

Calculates the offset from a pointer in bytes.

count is in units of bytes.

This is purely a convenience for casting to a u8 pointer and using offset on it. See that method for documentation and safety requirements.

For non-Sized pointees this operation changes only the data pointer, leaving the metadata untouched.

1.80.0 (const: 1.80.0) · source

pub const unsafe fn add(self, count: usize) -> Self
where T: Sized,

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 offset of 3 * size_of::<T>() bytes.

§Safety

If any of the following conditions are violated, the result is Undefined Behavior:

The computed offset, count * size_of::<T>() bytes, must not overflow 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.

Allocated objects can never be larger than isize::MAX bytes, so if the computed offset 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 safe.

§Examples

use std::ptr::NonNull;

let s: &str = "123";
let ptr: NonNull<u8> = NonNull::new(s.as_ptr().cast_mut()).unwrap();

unsafe {
    println!("{}", ptr.add(1).read() as char);
    println!("{}", ptr.add(2).read() as char);
}

1.80.0 (const: 1.80.0) · source

pub const unsafe fn byte_add(self, count: usize) -> Self

Calculates the offset from a pointer in bytes (convenience for .byte_offset(count as isize)).

count is in units of bytes.

This is purely a convenience for casting to a u8 pointer and using add on it. See that method for documentation and safety requirements.

For non-Sized pointees this operation changes only the data pointer, leaving the metadata untouched.

1.80.0 (const: 1.80.0) · source

pub const unsafe fn sub(self, count: usize) -> Self
where T: Sized,

Subtracts an 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:

The computed offset, count * size_of::<T>() bytes, must not overflow 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.

Allocated objects can never be larger than isize::MAX bytes, so if the computed offset 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 safe.

§Examples

use std::ptr::NonNull;

let s: &str = "123";

unsafe {
    let end: NonNull<u8> = NonNull::new(s.as_ptr().cast_mut()).unwrap().add(3);
    println!("{}", end.sub(1).read() as char);
    println!("{}", end.sub(2).read() as char);
}

1.80.0 (const: 1.80.0) · source

pub const unsafe fn byte_sub(self, count: usize) -> Self

Calculates the offset from a pointer in bytes (convenience for .byte_offset((count as isize).wrapping_neg())).

count is in units of bytes.

This is purely a convenience for casting to a u8 pointer and using sub on it. See that method for documentation and safety requirements.

For non-Sized pointees this operation changes only the data pointer, leaving the metadata untouched.

1.80.0 (const: 1.80.0) · source

pub const unsafe fn offset_from(self, origin: NonNull<T>) -> isize
where T: Sized,

Calculates the distance between two pointers. The returned value is in units of T: the distance in bytes divided by mem::size_of::<T>().

This is equivalent to (self as isize - origin as isize) / (mem::size_of::<T>() as isize), except that it has a lot more opportunities for UB, in exchange for the compiler better understanding what you are doing.

The primary motivation of this method is for computing the len of an array/slice of T that you are currently representing as a “start” and “end” pointer (and “end” is “one past the end” of the array). In that case, end.offset_from(start) gets you the length of the array.

All of the following safety requirements are trivially satisfied for this usecase.

§Safety

If any of the following conditions are violated, the result is Undefined Behavior:

self and origin must either
- point to the same address, or
- both be derived from a pointer to the same allocated object, and the memory range between the two pointers must be in bounds of that object. (See below for an example.)
The distance between the pointers, in bytes, must be an exact multiple of the size of T.

As a consequence, the absolute distance between the pointers, in bytes, computed on 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 than isize::MAX bytes.

The requirement for pointers to be derived from the same allocated object is primarily needed for const-compatibility: the distance between pointers into different allocated objects is not known at compile-time. However, the requirement also exists at runtime and may be exploited by optimizations. If you wish to compute the difference between pointers that are not guaranteed to be from the same allocation, use (self as isize - origin as isize) / mem::size_of::<T>().

§Panics

This function panics if T is a Zero-Sized Type (“ZST”).

§Examples

Basic usage:

use std::ptr::NonNull;

let a = [0; 5];
let ptr1: NonNull<u32> = NonNull::from(&a[1]);
let ptr2: NonNull<u32> = NonNull::from(&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);
}

Incorrect usage:

#![feature(strict_provenance)]
use std::ptr::NonNull;

let ptr1 = NonNull::new(Box::into_raw(Box::new(0u8))).unwrap();
let ptr2 = NonNull::new(Box::into_raw(Box::new(1u8))).unwrap();
let diff = (ptr2.addr().get() as isize).wrapping_sub(ptr1.addr().get() as isize);
// Make ptr2_other an "alias" of ptr2.add(1), but derived from ptr1.
let diff_plus_1 = diff.wrapping_add(1);
let ptr2_other = NonNull::new(ptr1.as_ptr().wrapping_byte_offset(diff_plus_1)).unwrap();
assert_eq!(ptr2.addr(), ptr2_other.addr());
// Since ptr2_other and ptr2 are derived from pointers to different objects,
// computing their offset is undefined behavior, even though
// they point to addresses that are in-bounds of the same object!

let one = unsafe { ptr2_other.offset_from(ptr2) }; // Undefined Behavior! ⚠️

1.80.0 (const: 1.80.0) · source

pub const unsafe fn byte_offset_from<U: ?Sized>( self, origin: NonNull, ) -> isize

Calculates the distance between two pointers. The returned value is in units of bytes.

This is purely a convenience for casting to a u8 pointer and using offset_from on it. See that method for documentation and safety requirements.

For non-Sized pointees this operation considers only the data pointers, ignoring the metadata.

source

pub const unsafe fn sub_ptr(self, subtracted: NonNull<T>) -> usize
where T: Sized,

🔬This is a nightly-only experimental API. (ptr_sub_ptr #95892)

Calculates the distance between two pointers, where it’s known that self is equal to or greater than origin. The returned value is in units of T: the distance in bytes is divided by mem::size_of::<T>().

This computes the same value that offset_from would compute, but with the added precondition that the offset is guaranteed to be non-negative. This method is equivalent to usize::try_from(self.offset_from(origin)).unwrap_unchecked(), but it provides slightly more information to the optimizer, which can sometimes allow it to optimize slightly better with some backends.

This method can be though of as recovering the count that was passed to add (or, with the parameters in the other order, to sub). The following are all equivalent, assuming that their safety preconditions are met:

ptr.sub_ptr(origin) == count
origin.add(count) == ptr
ptr.sub(count) == origin

§Safety

The distance between the pointers must be non-negative (self >= origin)
All the safety conditions of offset_from apply to this method as well; see it for the full details.

Importantly, despite the return type of this method being able to represent a larger offset, it’s still not permitted to pass pointers which differ by more than isize::MAX bytes. As such, the result of this method will always be less than or equal to isize::MAX as usize.

§Panics

This function panics if T is a Zero-Sized Type (“ZST”).

§Examples

#![feature(ptr_sub_ptr)]
use std::ptr::NonNull;

let a = [0; 5];
let ptr1: NonNull<u32> = NonNull::from(&a[1]);
let ptr2: NonNull<u32> = NonNull::from(&a[3]);
unsafe {
    assert_eq!(ptr2.sub_ptr(ptr1), 2);
    assert_eq!(ptr1.add(2), ptr2);
    assert_eq!(ptr2.sub(2), ptr1);
    assert_eq!(ptr2.sub_ptr(ptr2), 0);
}

// This would be incorrect, as the pointers are not correctly ordered:
// ptr1.sub_ptr(ptr2)

1.80.0 (const: 1.80.0) · source

pub const unsafe fn read(self) -> T
where T: Sized,

Reads the value from self without moving it. This leaves the memory in self unchanged.

See ptr::read for safety concerns and examples.

1.80.0 · source

pub unsafe fn read_volatile(self) -> T
where T: Sized,

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.

1.80.0 (const: 1.80.0) · source

pub const unsafe fn read_unaligned(self) -> T
where T: Sized,

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.

1.80.0 (const: unstable) · source

pub unsafe fn copy_to(self, dest: NonNull<T>, count: usize)
where T: Sized,

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.

1.80.0 (const: unstable) · source

pub unsafe fn copy_to_nonoverlapping(self, dest: NonNull<T>, count: usize)
where T: Sized,

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.

1.80.0 (const: unstable) · source

pub unsafe fn copy_from(self, src: NonNull<T>, count: usize)
where T: Sized,

Copies count * size_of<T> bytes from src to self. The source and destination may overlap.

NOTE: this has the opposite argument order of ptr::copy.

See ptr::copy for safety concerns and examples.

1.80.0 (const: unstable) · source

pub unsafe fn copy_from_nonoverlapping(self, src: NonNull<T>, count: usize)
where T: Sized,

Copies count * size_of<T> bytes from src to self. The source and destination may not overlap.

NOTE: this has the opposite argument order of ptr::copy_nonoverlapping.

See ptr::copy_nonoverlapping for safety concerns and examples.

1.80.0 · source

pub unsafe fn drop_in_place(self)

Executes the destructor (if any) of the pointed-to value.

See ptr::drop_in_place for safety concerns and examples.

1.80.0 (const: unstable) · source

pub unsafe fn write(self, val: T)
where T: Sized,

Overwrites a memory location with the given value without reading or dropping the old value.

See ptr::write for safety concerns and examples.

1.80.0 (const: unstable) · source

pub unsafe fn write_bytes(self, val: u8, count: usize)
where T: Sized,

Invokes memset on the specified pointer, setting count * size_of::<T>() bytes of memory starting at self to val.

See ptr::write_bytes for safety concerns and examples.

1.80.0 · source

pub unsafe fn write_volatile(self, val: T)
where T: Sized,

Performs a volatile write of a memory location with the given value without reading or dropping the old value.

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::write_volatile for safety concerns and examples.

1.80.0 (const: unstable) · source

pub unsafe fn write_unaligned(self, val: T)
where T: Sized,

Overwrites a memory location with the given value without reading or dropping the old value.

Unlike write, the pointer may be unaligned.

See ptr::write_unaligned for safety concerns and examples.

1.80.0 · source

pub unsafe fn replace(self, src: T) -> T
where T: Sized,

Replaces the value at self with src, returning the old value, without dropping either.

See ptr::replace for safety concerns and examples.

1.80.0 (const: unstable) · source

pub unsafe fn swap(self, with: NonNull<T>)
where T: Sized,

Swaps the values at two mutable locations of the same type, without deinitializing either. They may overlap, unlike mem::swap which is otherwise equivalent.

See ptr::swap for safety concerns and examples.

1.80.0 (const: unstable) · source

pub fn align_offset(self, align: usize) -> usize
where T: Sized,

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.

The offset is expressed in number of T elements, and not bytes.

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.

When this is called during compile-time evaluation (which is unstable), the implementation may return usize::MAX in cases where that can never happen at runtime. This is because the actual alignment of pointers is not known yet during compile-time, so an offset with guaranteed alignment can sometimes not be computed. For example, a buffer declared as [u8; N] might be allocated at an odd or an even address, but at compile-time this is not yet known, so the execution has to be correct for either choice. It is therefore impossible to find an offset that is guaranteed to be 2-aligned. (This behavior is subject to change, as usual for unstable APIs.)

§Panics

The function panics if align is not a power-of-two.

§Examples

Accessing adjacent u8 as u16

use std::mem::align_of;
use std::ptr::NonNull;

let x = [5_u8, 6, 7, 8, 9];
let ptr = NonNull::new(x.as_ptr() as *mut u8).unwrap();
let offset = ptr.align_offset(align_of::<u16>());

if offset < x.len() - 1 {
    let u16_ptr = ptr.add(offset).cast::<u16>();
    assert!(u16_ptr.read() == u16::from_ne_bytes([5, 6]) || u16_ptr.read() == u16::from_ne_bytes([6, 7]));
} else {
    // while the pointer can be aligned via `offset`, it would point
    // outside the allocation
}

1.79.0 (const: unstable) · source

pub fn is_aligned(self) -> bool
where T: Sized,

Returns whether the pointer is properly aligned for T.

§Examples

use std::ptr::NonNull;

// On some platforms, the alignment of i32 is less than 4.
#[repr(align(4))]
struct AlignedI32(i32);

let data = AlignedI32(42);
let ptr = NonNull::<AlignedI32>::from(&data);

assert!(ptr.is_aligned());
assert!(!NonNull::new(ptr.as_ptr().wrapping_byte_add(1)).unwrap().is_aligned());

§At compiletime

Note: Alignment at compiletime is experimental and subject to change. See the tracking issue for details.

At compiletime, the compiler may not know where a value will end up in memory. Calling this function on a pointer created from a reference at compiletime will only return true if the pointer is guaranteed to be aligned. This means that the pointer is never aligned if cast to a type with a stricter alignment than the reference’s underlying allocation.

#![feature(const_nonnull_new)]
#![feature(const_option)]
#![feature(const_pointer_is_aligned)]
use std::ptr::NonNull;

// On some platforms, the alignment of primitives is less than their size.
#[repr(align(4))]
struct AlignedI32(i32);
#[repr(align(8))]
struct AlignedI64(i64);

const _: () = {
    let data = [AlignedI32(42), AlignedI32(42)];
    let ptr = NonNull::<AlignedI32>::new(&data[0] as *const _ as *mut _).unwrap();
    assert!(ptr.is_aligned());

    // At runtime either `ptr1` or `ptr2` would be aligned, but at compiletime neither is aligned.
    let ptr1 = ptr.cast::<AlignedI64>();
    let ptr2 = unsafe { ptr.add(1).cast::<AlignedI64>() };
    assert!(!ptr1.is_aligned());
    assert!(!ptr2.is_aligned());
};

Due to this behavior, it is possible that a runtime pointer derived from a compiletime pointer is aligned, even if the compiletime pointer wasn’t aligned.

#![feature(const_pointer_is_aligned)]

// On some platforms, the alignment of primitives is less than their size.
#[repr(align(4))]
struct AlignedI32(i32);
#[repr(align(8))]
struct AlignedI64(i64);

// At compiletime, neither `COMPTIME_PTR` nor `COMPTIME_PTR + 1` is aligned.
const COMPTIME_PTR: *const AlignedI32 = &AlignedI32(42);
const _: () = assert!(!COMPTIME_PTR.cast::<AlignedI64>().is_aligned());
const _: () = assert!(!COMPTIME_PTR.wrapping_add(1).cast::<AlignedI64>().is_aligned());

// At runtime, either `runtime_ptr` or `runtime_ptr + 1` is aligned.
let runtime_ptr = COMPTIME_PTR;
assert_ne!(
    runtime_ptr.cast::<AlignedI64>().is_aligned(),
    runtime_ptr.wrapping_add(1).cast::<AlignedI64>().is_aligned(),
);

If a pointer is created from a fixed address, this function behaves the same during runtime and compiletime.

#![feature(const_pointer_is_aligned)]
#![feature(const_option)]
#![feature(const_nonnull_new)]
use std::ptr::NonNull;

// On some platforms, the alignment of primitives is less than their size.
#[repr(align(4))]
struct AlignedI32(i32);
#[repr(align(8))]
struct AlignedI64(i64);

const _: () = {
    let ptr = NonNull::new(40 as *mut AlignedI32).unwrap();
    assert!(ptr.is_aligned());

    // For pointers with a known address, runtime and compiletime behavior are identical.
    let ptr1 = ptr.cast::<AlignedI64>();
    let ptr2 = NonNull::new(ptr.as_ptr().wrapping_add(1)).unwrap().cast::<AlignedI64>();
    assert!(ptr1.is_aligned());
    assert!(!ptr2.is_aligned());
};

source

pub const fn is_aligned_to(self, align: usize) -> bool

🔬This is a nightly-only experimental API. (pointer_is_aligned_to #96284)

Returns whether the pointer is aligned to align.

For non-Sized pointees this operation considers only the data pointer, ignoring the metadata.

§Panics

The function panics if align is not a power-of-two (this includes 0).

§Examples

#![feature(pointer_is_aligned_to)]

// On some platforms, the alignment of i32 is less than 4.
#[repr(align(4))]
struct AlignedI32(i32);

let data = AlignedI32(42);
let ptr = &data as *const AlignedI32;

assert!(ptr.is_aligned_to(1));
assert!(ptr.is_aligned_to(2));
assert!(ptr.is_aligned_to(4));

assert!(ptr.wrapping_byte_add(2).is_aligned_to(2));
assert!(!ptr.wrapping_byte_add(2).is_aligned_to(4));

assert_ne!(ptr.is_aligned_to(8), ptr.wrapping_add(1).is_aligned_to(8));

§At compiletime

Note: Alignment at compiletime is experimental and subject to change. See the tracking issue for details.

At compiletime, the compiler may not know where a value will end up in memory. Calling this function on a pointer created from a reference at compiletime will only return true if the pointer is guaranteed to be aligned. This means that the pointer cannot be stricter aligned than the reference’s underlying allocation.

#![feature(pointer_is_aligned_to)]
#![feature(const_pointer_is_aligned)]

// On some platforms, the alignment of i32 is less than 4.
#[repr(align(4))]
struct AlignedI32(i32);

const _: () = {
    let data = AlignedI32(42);
    let ptr = &data as *const AlignedI32;

    assert!(ptr.is_aligned_to(1));
    assert!(ptr.is_aligned_to(2));
    assert!(ptr.is_aligned_to(4));

    // At compiletime, we know for sure that the pointer isn't aligned to 8.
    assert!(!ptr.is_aligned_to(8));
    assert!(!ptr.wrapping_add(1).is_aligned_to(8));
};

Due to this behavior, it is possible that a runtime pointer derived from a compiletime pointer is aligned, even if the compiletime pointer wasn’t aligned.

#![feature(pointer_is_aligned_to)]
#![feature(const_pointer_is_aligned)]

// On some platforms, the alignment of i32 is less than 4.
#[repr(align(4))]
struct AlignedI32(i32);

// At compiletime, neither `COMPTIME_PTR` nor `COMPTIME_PTR + 1` is aligned.
const COMPTIME_PTR: *const AlignedI32 = &AlignedI32(42);
const _: () = assert!(!COMPTIME_PTR.is_aligned_to(8));
const _: () = assert!(!COMPTIME_PTR.wrapping_add(1).is_aligned_to(8));

// At runtime, either `runtime_ptr` or `runtime_ptr + 1` is aligned.
let runtime_ptr = COMPTIME_PTR;
assert_ne!(
    runtime_ptr.is_aligned_to(8),
    runtime_ptr.wrapping_add(1).is_aligned_to(8),
);

If a pointer is created from a fixed address, this function behaves the same during runtime and compiletime.

#![feature(pointer_is_aligned_to)]
#![feature(const_pointer_is_aligned)]

const _: () = {
    let ptr = 40 as *const u8;
    assert!(ptr.is_aligned_to(1));
    assert!(ptr.is_aligned_to(2));
    assert!(ptr.is_aligned_to(4));
    assert!(ptr.is_aligned_to(8));
    assert!(!ptr.is_aligned_to(16));
};

source §

impl<T> NonNull<[T]>

1.70.0 (const: unstable) · source

pub fn slice_from_raw_parts(data: NonNull<T>, len: usize) -> Self

Creates a non-null raw slice from a thin pointer and a length.

The len argument is the number of elements, not the number of bytes.

This function is safe, but dereferencing the return value is unsafe. See the documentation of slice::from_raw_parts for slice safety requirements.

§Examples

use std::ptr::NonNull;

// create a slice pointer when starting out with a pointer to the first element
let mut x = [5, 6, 7];
let nonnull_pointer = NonNull::new(x.as_mut_ptr()).unwrap();
let slice = NonNull::slice_from_raw_parts(nonnull_pointer, 3);
assert_eq!(unsafe { slice.as_ref()[2] }, 7);

(Note that this example artificially demonstrates a use of this method, but let slice = NonNull::from(&x[..]); would be a better way to write code like this.)

1.63.0 (const: 1.63.0) · source

pub const fn len(self) -> usize

Returns the length of a non-null raw slice.

The returned value is the number of elements, not the number of bytes.

This function is safe, even when the non-null raw slice cannot be dereferenced to a slice because the pointer does not have a valid address.

§Examples

use std::ptr::NonNull;

let slice: NonNull<[i8]> = NonNull::slice_from_raw_parts(NonNull::dangling(), 3);
assert_eq!(slice.len(), 3);

1.79.0 (const: 1.79.0) · source

pub const fn is_empty(self) -> bool

Returns true if the non-null raw slice has a length of 0.

§Examples

use std::ptr::NonNull;

let slice: NonNull<[i8]> = NonNull::slice_from_raw_parts(NonNull::dangling(), 3);
assert!(!slice.is_empty());

source

pub const fn as_non_null_ptr(self) -> NonNull<T>

🔬This is a nightly-only experimental API. (slice_ptr_get #74265)

Returns a non-null pointer to the slice’s buffer.

§Examples

#![feature(slice_ptr_get)]
use std::ptr::NonNull;

let slice: NonNull<[i8]> = NonNull::slice_from_raw_parts(NonNull::dangling(), 3);
assert_eq!(slice.as_non_null_ptr(), NonNull::<i8>::dangling());

source

pub const fn as_mut_ptr(self) -> *mut T

🔬This is a nightly-only experimental API. (slice_ptr_get #74265)

Returns a raw pointer to the slice’s buffer.

§Examples

#![feature(slice_ptr_get)]
use std::ptr::NonNull;

let slice: NonNull<[i8]> = NonNull::slice_from_raw_parts(NonNull::dangling(), 3);
assert_eq!(slice.as_mut_ptr(), NonNull::<i8>::dangling().as_ptr());

source

pub const unsafe fn as_uninit_slice<'a>(self) -> &'a [MaybeUninit<T>]

🔬This is a nightly-only experimental API. (ptr_as_uninit #75402)

Returns a shared reference to a slice of possibly uninitialized values. In contrast to as_ref, this does not require that the value has to be initialized.

For the mutable counterpart see as_uninit_slice_mut.

§Safety

When calling this method, you have to ensure that all of the following is true:

The pointer must be valid for reads for ptr.len() * mem::size_of::<T>() many bytes, and it must be properly aligned. This means in particular:
- The entire memory range of this slice must be contained within a single allocated object! Slices can never span across multiple allocated objects.
- The pointer must be aligned even for zero-length slices. One reason for this is that enum layout optimizations may rely on references (including slices of any length) being aligned and non-null to distinguish them from other data. You can obtain a pointer that is usable as data for zero-length slices using NonNull::dangling().
The total size ptr.len() * mem::size_of::<T>() of the slice must be no larger than isize::MAX. See the safety documentation of pointer::offset.
You must enforce Rust’s aliasing rules, since the returned lifetime 'a is arbitrarily chosen and does not necessarily reflect the actual lifetime of the data. In particular, while this reference exists, the memory the pointer points to must not get mutated (except inside UnsafeCell).

This applies even if the result of this method is unused!

pub const unsafe fn as_uninit_slice_mut<'a>(self) -> &'a mut [MaybeUninit<T>]

🔬This is a nightly-only experimental API. (ptr_as_uninit #75402)

Returns a unique reference to a slice of possibly uninitialized values. In contrast to as_mut, this does not require that the value has to be initialized.

For the shared counterpart see as_uninit_slice.

§Safety

When calling this method, you have to ensure that all of the following is true:

The pointer must be valid for reads and writes for ptr.len() * mem::size_of::<T>() many bytes, and it must be properly aligned. This means in particular:
- The entire memory range of this slice must be contained within a single allocated object! Slices can never span across multiple allocated objects.
- The pointer must be aligned even for zero-length slices. One reason for this is that enum layout optimizations may rely on references (including slices of any length) being aligned and non-null to distinguish them from other data. You can obtain a pointer that is usable as data for zero-length slices using NonNull::dangling().
The total size ptr.len() * mem::size_of::<T>() of the slice must be no larger than isize::MAX. See the safety documentation of pointer::offset.
You must enforce Rust’s aliasing rules, since the returned lifetime 'a is arbitrarily chosen and does not necessarily reflect the actual lifetime of the data. In particular, while this reference exists, the memory the pointer points to must not get accessed (read or written) through any other pointer.

This applies even if the result of this method is unused!

§Examples

#![feature(allocator_api, ptr_as_uninit)]

use std::alloc::{Allocator, Layout, Global};
use std::mem::MaybeUninit;
use std::ptr::NonNull;

let memory: NonNull<[u8]> = Global.allocate(Layout::new::<[u8; 32]>())?;
// This is safe as `memory` is valid for reads and writes for `memory.len()` many bytes.
// Note that calling `memory.as_mut()` is not allowed here as the content may be uninitialized.
let slice: &mut [MaybeUninit<u8>] = unsafe { memory.as_uninit_slice_mut() };

source

pub unsafe fn get_unchecked_mut(self, index: I) -> NonNull<I::Output>
where I: SliceIndex<[T]>,

🔬This is a nightly-only experimental API. (slice_ptr_get #74265)

Returns a raw pointer to an element or subslice, without doing bounds checking.

Calling this method with an out-of-bounds index or when self is not dereferenceable is undefined behavior even if the resulting pointer is not used.

§Examples

#![feature(slice_ptr_get)]
use std::ptr::NonNull;

let x = &mut [1, 2, 4];
let x = NonNull::slice_from_raw_parts(NonNull::new(x.as_mut_ptr()).unwrap(), x.len());

unsafe {
    assert_eq!(x.get_unchecked_mut(1).as_ptr(), x.as_non_null_ptr().as_ptr().add(1));
}