//! Basic functions for dealing with memory.

//!

//! This module contains functions for querying the size and alignment of

//! types, initializing and manipulating memory.

#![stable(feature = "rust1", since = "1.0.0")]

use crate::alloc::Layout;

#[cfg(feature = "ferrocene_certified")]

use crate::intrinsics;

#[cfg(not(feature = "ferrocene_certified"))]

use crate::marker::DiscriminantKind;

#[cfg(not(feature = "ferrocene_certified"))]

use crate::{clone, cmp, fmt, hash, intrinsics, ptr};

#[cfg(not(feature = "ferrocene_certified"))]

mod manually_drop;

#[stable(feature = "manually_drop", since = "1.20.0")]

#[cfg(not(feature = "ferrocene_certified"))]

pub use manually_drop::ManuallyDrop;

#[cfg(not(feature = "ferrocene_certified"))]

mod maybe_uninit;

#[stable(feature = "maybe_uninit", since = "1.36.0")]

#[cfg(not(feature = "ferrocene_certified"))]

pub use maybe_uninit::MaybeUninit;

#[cfg(not(feature = "ferrocene_certified"))]

mod transmutability;

#[unstable(feature = "transmutability", issue = "99571")]

#[cfg(not(feature = "ferrocene_certified"))]

pub use transmutability::{Assume, TransmuteFrom};

#[cfg(not(feature = "ferrocene_certified"))]

mod drop_guard;

#[unstable(feature = "drop_guard", issue = "144426")]

#[cfg(not(feature = "ferrocene_certified"))]

pub use drop_guard::DropGuard;

// This one has to be a re-export (rather than wrapping the underlying intrinsic) so that we can do

// the special magic "types have equal size" check at the call site.

#[stable(feature = "rust1", since = "1.0.0")]

#[doc(inline)]

pub use crate::intrinsics::transmute;

/// Takes ownership and "forgets" about the value **without running its destructor**.

///

/// Any resources the value manages, such as heap memory or a file handle, will linger

/// forever in an unreachable state. However, it does not guarantee that pointers

/// to this memory will remain valid.

///

/// * If you want to leak memory, see [`Box::leak`].

/// * If you want to obtain a raw pointer to the memory, see [`Box::into_raw`].

/// * If you want to dispose of a value properly, running its destructor, see

///   [`mem::drop`].

///

/// # Safety

///

/// `forget` is not marked as `unsafe`, because Rust's safety guarantees

/// do not include a guarantee that destructors will always run. For example,

/// a program can create a reference cycle using [`Rc`][rc], or call

/// [`process::exit`][exit] to exit without running destructors. Thus, allowing

/// `mem::forget` from safe code does not fundamentally change Rust's safety

/// guarantees.

///

/// That said, leaking resources such as memory or I/O objects is usually undesirable.

/// The need comes up in some specialized use cases for FFI or unsafe code, but even

/// then, [`ManuallyDrop`] is typically preferred.

///

/// Because forgetting a value is allowed, any `unsafe` code you write must

/// allow for this possibility. You cannot return a value and expect that the

/// caller will necessarily run the value's destructor.

///

/// [rc]: ../../std/rc/struct.Rc.html

/// [exit]: ../../std/process/fn.exit.html

///

/// # Examples

///

/// The canonical safe use of `mem::forget` is to circumvent a value's destructor

/// implemented by the `Drop` trait. For example, this will leak a `File`, i.e. reclaim

/// the space taken by the variable but never close the underlying system resource:

///

/// ```no_run

/// use std::mem;

/// use std::fs::File;

///

/// let file = File::open("foo.txt").unwrap();

/// mem::forget(file);

/// ```

///

/// This is useful when the ownership of the underlying resource was previously

/// transferred to code outside of Rust, for example by transmitting the raw

/// file descriptor to C code.

///

/// # Relationship with `ManuallyDrop`

///

/// While `mem::forget` can also be used to transfer *memory* ownership, doing so is error-prone.

/// [`ManuallyDrop`] should be used instead. Consider, for example, this code:

///

/// ```

/// use std::mem;

///

/// let mut v = vec![65, 122];

/// // Build a `String` using the contents of `v`

/// let s = unsafe { String::from_raw_parts(v.as_mut_ptr(), v.len(), v.capacity()) };

/// // leak `v` because its memory is now managed by `s`

/// mem::forget(v);  // ERROR - v is invalid and must not be passed to a function

/// assert_eq!(s, "Az");

/// // `s` is implicitly dropped and its memory deallocated.

/// ```

///

/// There are two issues with the above example:

///

/// * If more code were added between the construction of `String` and the invocation of

///   `mem::forget()`, a panic within it would cause a double free because the same memory

///   is handled by both `v` and `s`.

/// * After calling `v.as_mut_ptr()` and transmitting the ownership of the data to `s`,

///   the `v` value is invalid. Even when a value is just moved to `mem::forget` (which won't

///   inspect it), some types have strict requirements on their values that

///   make them invalid when dangling or no longer owned. Using invalid values in any

///   way, including passing them to or returning them from functions, constitutes

///   undefined behavior and may break the assumptions made by the compiler.

///

/// Switching to `ManuallyDrop` avoids both issues:

///

/// ```

/// use std::mem::ManuallyDrop;

///

/// let v = vec![65, 122];

/// // Before we disassemble `v` into its raw parts, make sure it

/// // does not get dropped!

/// let mut v = ManuallyDrop::new(v);

/// // Now disassemble `v`. These operations cannot panic, so there cannot be a leak.

/// let (ptr, len, cap) = (v.as_mut_ptr(), v.len(), v.capacity());

/// // Finally, build a `String`.

/// let s = unsafe { String::from_raw_parts(ptr, len, cap) };

/// assert_eq!(s, "Az");

/// // `s` is implicitly dropped and its memory deallocated.

/// ```

///

/// `ManuallyDrop` robustly prevents double-free because we disable `v`'s destructor

/// before doing anything else. `mem::forget()` doesn't allow this because it consumes its

/// argument, forcing us to call it only after extracting anything we need from `v`. Even

/// if a panic were introduced between construction of `ManuallyDrop` and building the

/// string (which cannot happen in the code as shown), it would result in a leak and not a

/// double free. In other words, `ManuallyDrop` errs on the side of leaking instead of

/// erring on the side of (double-)dropping.

///

/// Also, `ManuallyDrop` prevents us from having to "touch" `v` after transferring the

/// ownership to `s` — the final step of interacting with `v` to dispose of it without

/// running its destructor is entirely avoided.

///

/// [`Box`]: ../../std/boxed/struct.Box.html

/// [`Box::leak`]: ../../std/boxed/struct.Box.html#method.leak

/// [`Box::into_raw`]: ../../std/boxed/struct.Box.html#method.into_raw

/// [`mem::drop`]: drop

/// [ub]: ../../reference/behavior-considered-undefined.html

#[inline]

#[rustc_const_stable(feature = "const_forget", since = "1.46.0")]

#[stable(feature = "rust1", since = "1.0.0")]

#[rustc_diagnostic_item = "mem_forget"]

#[cfg(not(feature = "ferrocene_certified"))]

pub const fn forget<T>(t: T) {

    let _ = ManuallyDrop::new(t);

}

/// Like [`forget`], but also accepts unsized values.

///

/// While Rust does not permit unsized locals since its removal in [#111942] it is

/// still possible to call functions with unsized values from a function argument

/// or place expression.

///

/// ```rust

/// #![feature(unsized_fn_params, forget_unsized)]

/// #![allow(internal_features)]

///

/// use std::mem::forget_unsized;

///

/// pub fn in_place() {

///     forget_unsized(*Box::<str>::from("str"));

/// }

///

/// pub fn param(x: str) {

///     forget_unsized(x);

/// }

/// ```

///

/// This works because the compiler will alter these functions to pass the parameter

/// by reference instead. This trick is necessary to support `Box<dyn FnOnce()>: FnOnce()`.

/// See [#68304] and [#71170] for more information.

///

/// [#111942]: https://github.com/rust-lang/rust/issues/111942

/// [#68304]: https://github.com/rust-lang/rust/issues/68304

/// [#71170]: https://github.com/rust-lang/rust/pull/71170

#[inline]

#[unstable(feature = "forget_unsized", issue = "none")]

#[cfg(not(feature = "ferrocene_certified"))]

pub fn forget_unsized<T: ?Sized>(t: T) {

    intrinsics::forget(t)

}

/// Returns the size of a type in bytes.

///

/// More specifically, this is the offset in bytes between successive elements

/// in an array with that item type including alignment padding. Thus, for any

/// type `T` and length `n`, `[T; n]` has a size of `n * size_of::<T>()`.

///

/// In general, the size of a type is not stable across compilations, but

/// specific types such as primitives are.

///

/// The following table gives the size for primitives.

///

/// Type | `size_of::<Type>()`

/// ---- | ---------------

/// () | 0

/// bool | 1

/// u8 | 1

/// u16 | 2

/// u32 | 4

/// u64 | 8

/// u128 | 16

/// i8 | 1

/// i16 | 2

/// i32 | 4

/// i64 | 8

/// i128 | 16

/// f32 | 4

/// f64 | 8

/// char | 4

///

/// Furthermore, `usize` and `isize` have the same size.

///

/// The types [`*const T`], `&T`, [`Box<T>`], [`Option<&T>`], and `Option<Box<T>>` all have

/// the same size. If `T` is `Sized`, all of those types have the same size as `usize`.

///

/// The mutability of a pointer does not change its size. As such, `&T` and `&mut T`

/// have the same size. Likewise for `*const T` and `*mut T`.

///

/// # Size of `#[repr(C)]` items

///

/// The `C` representation for items has a defined layout. With this layout,

/// the size of items is also stable as long as all fields have a stable size.

///

/// ## Size of Structs

///

/// For `struct`s, the size is determined by the following algorithm.

///

/// For each field in the struct ordered by declaration order:

///

/// 1. Add the size of the field.

/// 2. Round up the current size to the nearest multiple of the next field's [alignment].

///

/// Finally, round the size of the struct to the nearest multiple of its [alignment].

/// The alignment of the struct is usually the largest alignment of all its

/// fields; this can be changed with the use of `repr(align(N))`.

///

/// Unlike `C`, zero sized structs are not rounded up to one byte in size.

///

/// ## Size of Enums

///

/// Enums that carry no data other than the discriminant have the same size as C enums

/// on the platform they are compiled for.

///

/// ## Size of Unions

///

/// The size of a union is the size of its largest field.

///

/// Unlike `C`, zero sized unions are not rounded up to one byte in size.

///

/// # Examples

///

/// ```

/// // Some primitives

/// assert_eq!(4, size_of::<i32>());

/// assert_eq!(8, size_of::<f64>());

/// assert_eq!(0, size_of::<()>());

///

/// // Some arrays

/// assert_eq!(8, size_of::<[i32; 2]>());

/// assert_eq!(12, size_of::<[i32; 3]>());

/// assert_eq!(0, size_of::<[i32; 0]>());

///

///

/// // Pointer size equality

/// assert_eq!(size_of::<&i32>(), size_of::<*const i32>());

/// assert_eq!(size_of::<&i32>(), size_of::<Box<i32>>());

/// assert_eq!(size_of::<&i32>(), size_of::<Option<&i32>>());

/// assert_eq!(size_of::<Box<i32>>(), size_of::<Option<Box<i32>>>());

/// ```

///

/// Using `#[repr(C)]`.

///

/// ```

/// #[repr(C)]

/// struct FieldStruct {

///     first: u8,

///     second: u16,

///     third: u8

/// }

///

/// // The size of the first field is 1, so add 1 to the size. Size is 1.

/// // The alignment of the second field is 2, so add 1 to the size for padding. Size is 2.

/// // The size of the second field is 2, so add 2 to the size. Size is 4.

/// // The alignment of the third field is 1, so add 0 to the size for padding. Size is 4.

/// // The size of the third field is 1, so add 1 to the size. Size is 5.

/// // Finally, the alignment of the struct is 2 (because the largest alignment amongst its

/// // fields is 2), so add 1 to the size for padding. Size is 6.

/// assert_eq!(6, size_of::<FieldStruct>());

///

/// #[repr(C)]

/// struct TupleStruct(u8, u16, u8);

///

/// // Tuple structs follow the same rules.

/// assert_eq!(6, size_of::<TupleStruct>());

///

/// // Note that reordering the fields can lower the size. We can remove both padding bytes

/// // by putting `third` before `second`.

/// #[repr(C)]

/// struct FieldStructOptimized {

///     first: u8,

///     third: u8,

///     second: u16

/// }

///

/// assert_eq!(4, size_of::<FieldStructOptimized>());

///

/// // Union size is the size of the largest field.

/// #[repr(C)]

/// union ExampleUnion {

///     smaller: u8,

///     larger: u16

/// }

///

/// assert_eq!(2, size_of::<ExampleUnion>());

/// ```

///

/// [alignment]: align_of

/// [`*const T`]: primitive@pointer

/// [`Box<T>`]: ../../std/boxed/struct.Box.html

/// [`Option<&T>`]: crate::option::Option

///

#[inline(always)]

#[must_use]

#[stable(feature = "rust1", since = "1.0.0")]

#[rustc_promotable]

#[rustc_const_stable(feature = "const_mem_size_of", since = "1.24.0")]

#[rustc_diagnostic_item = "mem_size_of"]

/// Returns the size of the pointed-to value in bytes.

///

/// This is usually the same as [`size_of::<T>()`]. However, when `T` *has* no

/// statically-known size, e.g., a slice [`[T]`][slice] or a [trait object],

/// then `size_of_val` can be used to get the dynamically-known size.

///

/// [trait object]: ../../book/ch17-02-trait-objects.html

///

/// # Examples

///

/// ```

/// assert_eq!(4, size_of_val(&5i32));

///

/// let x: [u8; 13] = [0; 13];

/// let y: &[u8] = &x;

/// assert_eq!(13, size_of_val(y));

/// ```

///

/// [`size_of::<T>()`]: size_of

#[inline]

#[must_use]

#[stable(feature = "rust1", since = "1.0.0")]

#[rustc_const_stable(feature = "const_size_of_val", since = "1.85.0")]

#[rustc_diagnostic_item = "mem_size_of_val"]

#[cfg(not(feature = "ferrocene_certified"))]

pub const fn size_of_val<T: ?Sized>(val: &T) -> usize {

    // SAFETY: `val` is a reference, so it's a valid raw pointer

    unsafe { intrinsics::size_of_val(val) }

}

/// Returns the size of the pointed-to value in bytes.

///

/// This is usually the same as [`size_of::<T>()`]. However, when `T` *has* no

/// statically-known size, e.g., a slice [`[T]`][slice] or a [trait object],

/// then `size_of_val_raw` can be used to get the dynamically-known size.

///

/// # Safety

///

/// This function is only safe to call if the following conditions hold:

///

/// - If `T` is `Sized`, this function is always safe to call.

/// - If the unsized tail of `T` is:

///     - a [slice], then the length of the slice tail must be an initialized

///       integer, and the size of the *entire value*

///       (dynamic tail length + statically sized prefix) must fit in `isize`.

///       For the special case where the dynamic tail length is 0, this function

///       is safe to call.

//        NOTE: the reason this is safe is that if an overflow were to occur already with size 0,

//        then we would stop compilation as even the "statically known" part of the type would

//        already be too big (or the call may be in dead code and optimized away, but then it

//        doesn't matter).

///     - a [trait object], then the vtable part of the pointer must point

///       to a valid vtable acquired by an unsizing coercion, and the size

///       of the *entire value* (dynamic tail length + statically sized prefix)

///       must fit in `isize`.

///     - an (unstable) [extern type], then this function is always safe to

///       call, but may panic or otherwise return the wrong value, as the

///       extern type's layout is not known. This is the same behavior as

///       [`size_of_val`] on a reference to a type with an extern type tail.

///     - otherwise, it is conservatively not allowed to call this function.

///

/// [`size_of::<T>()`]: size_of

/// [trait object]: ../../book/ch17-02-trait-objects.html

/// [extern type]: ../../unstable-book/language-features/extern-types.html

///

/// # Examples

///

/// ```

/// #![feature(layout_for_ptr)]

/// use std::mem;

///

/// assert_eq!(4, size_of_val(&5i32));

///

/// let x: [u8; 13] = [0; 13];

/// let y: &[u8] = &x;

/// assert_eq!(13, unsafe { mem::size_of_val_raw(y) });

/// ```

#[inline]

#[must_use]

#[unstable(feature = "layout_for_ptr", issue = "69835")]

#[cfg(not(feature = "ferrocene_certified"))]

pub const unsafe fn size_of_val_raw<T: ?Sized>(val: *const T) -> usize {

    // SAFETY: the caller must provide a valid raw pointer

    unsafe { intrinsics::size_of_val(val) }

}

/// Returns the [ABI]-required minimum alignment of a type in bytes.

///

/// Every reference to a value of the type `T` must be a multiple of this number.

///

/// This is the alignment used for struct fields. It may be smaller than the preferred alignment.

///

/// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface

///

/// # Examples

///

/// ```

/// # #![allow(deprecated)]

/// use std::mem;

///

/// assert_eq!(4, mem::min_align_of::<i32>());

/// ```

#[inline]

#[must_use]

#[stable(feature = "rust1", since = "1.0.0")]

#[deprecated(note = "use `align_of` instead", since = "1.2.0", suggestion = "align_of")]

#[cfg(not(feature = "ferrocene_certified"))]

pub fn min_align_of<T>() -> usize {

    intrinsics::align_of::<T>()

}

/// Returns the [ABI]-required minimum alignment of the type of the value that `val` points to in

/// bytes.

///

/// Every reference to a value of the type `T` must be a multiple of this number.

///

/// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface

///

/// # Examples

///

/// ```

/// # #![allow(deprecated)]

/// use std::mem;

///

/// assert_eq!(4, mem::min_align_of_val(&5i32));

/// ```

#[inline]

#[must_use]

#[stable(feature = "rust1", since = "1.0.0")]

#[deprecated(note = "use `align_of_val` instead", since = "1.2.0", suggestion = "align_of_val")]

#[cfg(not(feature = "ferrocene_certified"))]

pub fn min_align_of_val<T: ?Sized>(val: &T) -> usize {

    // SAFETY: val is a reference, so it's a valid raw pointer

    unsafe { intrinsics::align_of_val(val) }

}

/// Returns the [ABI]-required minimum alignment of a type in bytes.

///

/// Every reference to a value of the type `T` must be a multiple of this number.

///

/// This is the alignment used for struct fields. It may be smaller than the preferred alignment.

///

/// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface

///

/// # Examples

///

/// ```

/// assert_eq!(4, align_of::<i32>());

/// ```

#[inline(always)]

#[must_use]

#[stable(feature = "rust1", since = "1.0.0")]

#[rustc_promotable]

#[rustc_const_stable(feature = "const_align_of", since = "1.24.0")]

#[rustc_diagnostic_item = "mem_align_of"]

/// Returns the [ABI]-required minimum alignment of the type of the value that `val` points to in

/// bytes.

///

/// Every reference to a value of the type `T` must be a multiple of this number.

///

/// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface

///

/// # Examples

///

/// ```

/// assert_eq!(4, align_of_val(&5i32));

/// ```

#[inline]

#[must_use]

#[stable(feature = "rust1", since = "1.0.0")]

#[rustc_const_stable(feature = "const_align_of_val", since = "1.85.0")]

#[cfg(not(feature = "ferrocene_certified"))]

pub const fn align_of_val<T: ?Sized>(val: &T) -> usize {

    // SAFETY: val is a reference, so it's a valid raw pointer

    unsafe { intrinsics::align_of_val(val) }

}

/// Returns the [ABI]-required minimum alignment of the type of the value that `val` points to in

/// bytes.

///

/// Every reference to a value of the type `T` must be a multiple of this number.

///

/// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface

///

/// # Safety

///

/// This function is only safe to call if the following conditions hold:

///

/// - If `T` is `Sized`, this function is always safe to call.

/// - If the unsized tail of `T` is:

///     - a [slice], then the length of the slice tail must be an initialized

///       integer, and the size of the *entire value*

///       (dynamic tail length + statically sized prefix) must fit in `isize`.

///       For the special case where the dynamic tail length is 0, this function

///       is safe to call.

///     - a [trait object], then the vtable part of the pointer must point

///       to a valid vtable acquired by an unsizing coercion, and the size

///       of the *entire value* (dynamic tail length + statically sized prefix)

///       must fit in `isize`.

///     - an (unstable) [extern type], then this function is always safe to

///       call, but may panic or otherwise return the wrong value, as the

///       extern type's layout is not known. This is the same behavior as

///       [`align_of_val`] on a reference to a type with an extern type tail.

///     - otherwise, it is conservatively not allowed to call this function.

///

/// [trait object]: ../../book/ch17-02-trait-objects.html

/// [extern type]: ../../unstable-book/language-features/extern-types.html

///

/// # Examples

///

/// ```

/// #![feature(layout_for_ptr)]

/// use std::mem;

///

/// assert_eq!(4, unsafe { mem::align_of_val_raw(&5i32) });

/// ```

#[inline]

#[must_use]

#[unstable(feature = "layout_for_ptr", issue = "69835")]

#[cfg(not(feature = "ferrocene_certified"))]

pub const unsafe fn align_of_val_raw<T: ?Sized>(val: *const T) -> usize {

    // SAFETY: the caller must provide a valid raw pointer

    unsafe { intrinsics::align_of_val(val) }

}

/// Returns `true` if dropping values of type `T` matters.

///

/// This is purely an optimization hint, and may be implemented conservatively:

/// it may return `true` for types that don't actually need to be dropped.

/// As such always returning `true` would be a valid implementation of

/// this function. However if this function actually returns `false`, then you

/// can be certain dropping `T` has no side effect.

///

/// Low level implementations of things like collections, which need to manually

/// drop their data, should use this function to avoid unnecessarily

/// trying to drop all their contents when they are destroyed. This might not

/// make a difference in release builds (where a loop that has no side-effects

/// is easily detected and eliminated), but is often a big win for debug builds.

///

/// Note that [`drop_in_place`] already performs this check, so if your workload

/// can be reduced to some small number of [`drop_in_place`] calls, using this is

/// unnecessary. In particular note that you can [`drop_in_place`] a slice, and that

/// will do a single needs_drop check for all the values.

///

/// Types like Vec therefore just `drop_in_place(&mut self[..])` without using

/// `needs_drop` explicitly. Types like [`HashMap`], on the other hand, have to drop

/// values one at a time and should use this API.

///

/// [`drop_in_place`]: crate::ptr::drop_in_place

/// [`HashMap`]: ../../std/collections/struct.HashMap.html

///

/// # Examples

///

/// Here's an example of how a collection might make use of `needs_drop`:

///

/// ```

/// use std::{mem, ptr};

///

/// pub struct MyCollection<T> {

/// #   data: [T; 1],

///     /* ... */

/// }

/// # impl<T> MyCollection<T> {

/// #   fn iter_mut(&mut self) -> &mut [T] { &mut self.data }

/// #   fn free_buffer(&mut self) {}

/// # }

///

/// impl<T> Drop for MyCollection<T> {

///     fn drop(&mut self) {

///         unsafe {

///             // drop the data

///             if mem::needs_drop::<T>() {

///                 for x in self.iter_mut() {

///                     ptr::drop_in_place(x);

///                 }

///             }

///             self.free_buffer();

///         }

///     }

/// }

/// ```

#[inline]

#[must_use]

#[stable(feature = "needs_drop", since = "1.21.0")]

#[rustc_const_stable(feature = "const_mem_needs_drop", since = "1.36.0")]

#[rustc_diagnostic_item = "needs_drop"]

#[cfg(not(feature = "ferrocene_certified"))]

pub const fn needs_drop<T: ?Sized>() -> bool {

    const { intrinsics::needs_drop::<T>() }

}

/// Returns the value of type `T` represented by the all-zero byte-pattern.

///

/// This means that, for example, the padding byte in `(u8, u16)` is not

/// necessarily zeroed.

///

/// There is no guarantee that an all-zero byte-pattern represents a valid value

/// of some type `T`. For example, the all-zero byte-pattern is not a valid value

/// for reference types (`&T`, `&mut T`) and function pointers. Using `zeroed`

/// on such types causes immediate [undefined behavior][ub] because [the Rust

/// compiler assumes][inv] that there always is a valid value in a variable it

/// considers initialized.

///

/// This has the same effect as [`MaybeUninit::zeroed().assume_init()`][zeroed].

/// It is useful for FFI sometimes, but should generally be avoided.

///

/// [zeroed]: MaybeUninit::zeroed

/// [ub]: ../../reference/behavior-considered-undefined.html

/// [inv]: MaybeUninit#initialization-invariant

///

/// # Examples

///

/// Correct usage of this function: initializing an integer with zero.

///

/// ```

/// use std::mem;

///

/// let x: i32 = unsafe { mem::zeroed() };

/// assert_eq!(0, x);

/// ```

///

/// *Incorrect* usage of this function: initializing a reference with zero.

///

/// ```rust,no_run

/// # #![allow(invalid_value)]

/// use std::mem;

///

/// let _x: &i32 = unsafe { mem::zeroed() }; // Undefined behavior!

/// let _y: fn() = unsafe { mem::zeroed() }; // And again!

/// ```

#[inline(always)]

#[must_use]

#[stable(feature = "rust1", since = "1.0.0")]

#[rustc_diagnostic_item = "mem_zeroed"]

#[track_caller]

#[rustc_const_stable(feature = "const_mem_zeroed", since = "1.75.0")]

#[cfg(not(feature = "ferrocene_certified"))]

pub const unsafe fn zeroed<T>() -> T {

    // SAFETY: the caller must guarantee that an all-zero value is valid for `T`.

    unsafe {

        intrinsics::assert_zero_valid::<T>();

        MaybeUninit::zeroed().assume_init()

    }

}

/// Bypasses Rust's normal memory-initialization checks by pretending to

/// produce a value of type `T`, while doing nothing at all.

///

/// **This function is deprecated.** Use [`MaybeUninit<T>`] instead.

/// It also might be slower than using `MaybeUninit<T>` due to mitigations that were put in place to

/// limit the potential harm caused by incorrect use of this function in legacy code.

///

/// The reason for deprecation is that the function basically cannot be used

/// correctly: it has the same effect as [`MaybeUninit::uninit().assume_init()`][uninit].

/// As the [`assume_init` documentation][assume_init] explains,

/// [the Rust compiler assumes][inv] that values are properly initialized.

///

/// Truly uninitialized memory like what gets returned here

/// is special in that the compiler knows that it does not have a fixed value.

/// This makes it undefined behavior to have uninitialized data in a variable even

/// if that variable has an integer type.

///

/// Therefore, it is immediate undefined behavior to call this function on nearly all types,

/// including integer types and arrays of integer types, and even if the result is unused.

///

/// [uninit]: MaybeUninit::uninit

/// [assume_init]: MaybeUninit::assume_init

/// [inv]: MaybeUninit#initialization-invariant

#[inline(always)]

#[must_use]

#[deprecated(since = "1.39.0", note = "use `mem::MaybeUninit` instead")]

#[stable(feature = "rust1", since = "1.0.0")]

#[rustc_diagnostic_item = "mem_uninitialized"]

#[track_caller]

#[cfg(not(feature = "ferrocene_certified"))]

pub unsafe fn uninitialized<T>() -> T {

    // SAFETY: the caller must guarantee that an uninitialized value is valid for `T`.

    unsafe {

        intrinsics::assert_mem_uninitialized_valid::<T>();

        let mut val = MaybeUninit::<T>::uninit();

        // Fill memory with 0x01, as an imperfect mitigation for old code that uses this function on

        // bool, nonnull, and noundef types. But don't do this if we actively want to detect UB.

        if !cfg!(any(miri, sanitize = "memory")) {

            val.as_mut_ptr().write_bytes(0x01, 1);

        }

        val.assume_init()

    }

}

/// Swaps the values at two mutable locations, without deinitializing either one.

///

/// * If you want to swap with a default or dummy value, see [`take`].

/// * If you want to swap with a passed value, returning the old value, see [`replace`].

///

/// # Examples

///

/// ```

/// use std::mem;

///

/// let mut x = 5;

/// let mut y = 42;

///

/// mem::swap(&mut x, &mut y);

///

/// assert_eq!(42, x);

/// assert_eq!(5, y);

/// ```

#[inline]

#[stable(feature = "rust1", since = "1.0.0")]

#[rustc_const_stable(feature = "const_swap", since = "1.85.0")]

#[rustc_diagnostic_item = "mem_swap"]

#[cfg(not(feature = "ferrocene_certified"))]

pub const fn swap<T>(x: &mut T, y: &mut T) {

    // SAFETY: `&mut` guarantees these are typed readable and writable

    // as well as non-overlapping.

    unsafe { intrinsics::typed_swap_nonoverlapping(x, y) }

}

/// Replaces `dest` with the default value of `T`, returning the previous `dest` value.

///

/// * If you want to replace the values of two variables, see [`swap`].

/// * If you want to replace with a passed value instead of the default value, see [`replace`].

///

/// # Examples

///

/// A simple example:

///

/// ```

/// use std::mem;

///

/// let mut v: Vec<i32> = vec![1, 2];

///

/// let old_v = mem::take(&mut v);

/// assert_eq!(vec![1, 2], old_v);

/// assert!(v.is_empty());

/// ```

///

/// `take` allows taking ownership of a struct field by replacing it with an "empty" value.

/// Without `take` you can run into issues like these:

///

/// ```compile_fail,E0507

/// struct Buffer<T> { buf: Vec<T> }

///

/// impl<T> Buffer<T> {

///     fn get_and_reset(&mut self) -> Vec<T> {

///         // error: cannot move out of dereference of `&mut`-pointer

///         let buf = self.buf;

///         self.buf = Vec::new();

///         buf

///     }

/// }

/// ```

///

/// Note that `T` does not necessarily implement [`Clone`], so it can't even clone and reset

/// `self.buf`. But `take` can be used to disassociate the original value of `self.buf` from

/// `self`, allowing it to be returned:

///

/// ```

/// use std::mem;

///

/// # struct Buffer<T> { buf: Vec<T> }

/// impl<T> Buffer<T> {

///     fn get_and_reset(&mut self) -> Vec<T> {

///         mem::take(&mut self.buf)

///     }

/// }

///

/// let mut buffer = Buffer { buf: vec![0, 1] };

/// assert_eq!(buffer.buf.len(), 2);

///

/// assert_eq!(buffer.get_and_reset(), vec![0, 1]);

/// assert_eq!(buffer.buf.len(), 0);

/// ```

#[inline]

#[stable(feature = "mem_take", since = "1.40.0")]

#[cfg(not(feature = "ferrocene_certified"))]

pub fn take<T: Default>(dest: &mut T) -> T {

    replace(dest, T::default())

}

/// Moves `src` into the referenced `dest`, returning the previous `dest` value.

///

/// Neither value is dropped.

///

/// * If you want to replace the values of two variables, see [`swap`].

/// * If you want to replace with a default value, see [`take`].

///

/// # Examples

///

/// A simple example:

///

/// ```

/// use std::mem;

///

/// let mut v: Vec<i32> = vec![1, 2];

///

/// let old_v = mem::replace(&mut v, vec![3, 4, 5]);

/// assert_eq!(vec![1, 2], old_v);

/// assert_eq!(vec![3, 4, 5], v);

/// ```

///

/// `replace` allows consumption of a struct field by replacing it with another value.

/// Without `replace` you can run into issues like these:

///

/// ```compile_fail,E0507

/// struct Buffer<T> { buf: Vec<T> }

///

/// impl<T> Buffer<T> {

///     fn replace_index(&mut self, i: usize, v: T) -> T {

///         // error: cannot move out of dereference of `&mut`-pointer

///         let t = self.buf[i];

///         self.buf[i] = v;

///         t

///     }

/// }

/// ```

///

/// Note that `T` does not necessarily implement [`Clone`], so we can't even clone `self.buf[i]` to

/// avoid the move. But `replace` can be used to disassociate the original value at that index from

/// `self`, allowing it to be returned:

///

/// ```

/// # #![allow(dead_code)]

/// use std::mem;

///

/// # struct Buffer<T> { buf: Vec<T> }

/// impl<T> Buffer<T> {

///     fn replace_index(&mut self, i: usize, v: T) -> T {

///         mem::replace(&mut self.buf[i], v)

///     }

/// }

///

/// let mut buffer = Buffer { buf: vec![0, 1] };

/// assert_eq!(buffer.buf[0], 0);

///

/// assert_eq!(buffer.replace_index(0, 2), 0);

/// assert_eq!(buffer.buf[0], 2);

/// ```

#[inline]

#[stable(feature = "rust1", since = "1.0.0")]

#[must_use = "if you don't need the old value, you can just assign the new value directly"]

#[rustc_const_stable(feature = "const_replace", since = "1.83.0")]

#[rustc_diagnostic_item = "mem_replace"]

#[cfg(not(feature = "ferrocene_certified"))]

pub const fn replace<T>(dest: &mut T, src: T) -> T {

    // It may be tempting to use `swap` to avoid `unsafe` here. Don't!

    // The compiler optimizes the implementation below to two `memcpy`s

    // while `swap` would require at least three. See PR#83022 for details.

    // SAFETY: We read from `dest` but directly write `src` into it afterwards,

    // such that the old value is not duplicated. Nothing is dropped and

    // nothing here can panic.

    unsafe {

        // Ideally we wouldn't use the intrinsics here, but going through the

        // `ptr` methods introduces two unnecessary UbChecks, so until we can

        // remove those for pointers that come from references, this uses the

        // intrinsics instead so this stays very cheap in MIR (and debug).

        let result = crate::intrinsics::read_via_copy(dest);

        crate::intrinsics::write_via_move(dest, src);

        result

    }

}

/// Disposes of a value.

///

/// This does so by calling the argument's implementation of [`Drop`][drop].

///

/// This effectively does nothing for types which implement `Copy`, e.g.

/// integers. Such values are copied and _then_ moved into the function, so the

/// value persists after this function call.

///

/// This function is not magic; it is literally defined as

///

/// ```

/// pub fn drop<T>(_x: T) {}

/// ```

///

/// Because `_x` is moved into the function, it is automatically dropped before

/// the function returns.

///

/// [drop]: Drop

///

/// # Examples

///

/// Basic usage:

///

/// ```

/// let v = vec![1, 2, 3];

///

/// drop(v); // explicitly drop the vector

/// ```

///

/// Since [`RefCell`] enforces the borrow rules at runtime, `drop` can

/// release a [`RefCell`] borrow:

///

/// ```

/// use std::cell::RefCell;

///

/// let x = RefCell::new(1);

///

/// let mut mutable_borrow = x.borrow_mut();

/// *mutable_borrow = 1;

///

/// drop(mutable_borrow); // relinquish the mutable borrow on this slot

///

/// let borrow = x.borrow();

/// println!("{}", *borrow);

/// ```

///

/// Integers and other types implementing [`Copy`] are unaffected by `drop`.

///

/// ```

/// # #![allow(dropping_copy_types)]

/// #[derive(Copy, Clone)]

/// struct Foo(u8);

///

/// let x = 1;

/// let y = Foo(2);

/// drop(x); // a copy of `x` is moved and dropped

/// drop(y); // a copy of `y` is moved and dropped

///

/// println!("x: {}, y: {}", x, y.0); // still available

/// ```

///

/// [`RefCell`]: crate::cell::RefCell

#[inline]

#[stable(feature = "rust1", since = "1.0.0")]

#[rustc_diagnostic_item = "mem_drop"]

/// Bitwise-copies a value.

///

/// This function is not magic; it is literally defined as

/// ```

/// pub const fn copy<T: Copy>(x: &T) -> T { *x }

/// ```

///

/// It is useful when you want to pass a function pointer to a combinator, rather than defining a new closure.

///

/// Example:

/// ```

/// #![feature(mem_copy_fn)]

/// use core::mem::copy;

/// let result_from_ffi_function: Result<(), &i32> = Err(&1);

/// let result_copied: Result<(), i32> = result_from_ffi_function.map_err(copy);

/// ```

#[inline]

#[unstable(feature = "mem_copy_fn", issue = "98262")]

#[cfg(not(feature = "ferrocene_certified"))]

pub const fn copy<T: Copy>(x: &T) -> T {

    *x

}

/// Interprets `src` as having type `&Dst`, and then reads `src` without moving

/// the contained value.

///

/// This function will unsafely assume the pointer `src` is valid for [`size_of::<Dst>`][size_of]

/// bytes by transmuting `&Src` to `&Dst` and then reading the `&Dst` (except that this is done

/// in a way that is correct even when `&Dst` has stricter alignment requirements than `&Src`).

/// It will also unsafely create a copy of the contained value instead of moving out of `src`.

///

/// It is not a compile-time error if `Src` and `Dst` have different sizes, but it

/// is highly encouraged to only invoke this function where `Src` and `Dst` have the

/// same size. This function triggers [undefined behavior][ub] if `Dst` is larger than

/// `Src`.

///

/// [ub]: ../../reference/behavior-considered-undefined.html

///

/// # Examples

///

/// ```

/// use std::mem;

///

/// #[repr(packed)]

/// struct Foo {

///     bar: u8,

/// }

///

/// let foo_array = [10u8];

///

/// unsafe {

///     // Copy the data from 'foo_array' and treat it as a 'Foo'

///     let mut foo_struct: Foo = mem::transmute_copy(&foo_array);

///     assert_eq!(foo_struct.bar, 10);

///

///     // Modify the copied data

///     foo_struct.bar = 20;

///     assert_eq!(foo_struct.bar, 20);

/// }

///

/// // The contents of 'foo_array' should not have changed

/// assert_eq!(foo_array, [10]);

/// ```

#[inline]

#[must_use]

#[track_caller]

#[stable(feature = "rust1", since = "1.0.0")]

#[rustc_const_stable(feature = "const_transmute_copy", since = "1.74.0")]

#[cfg(not(feature = "ferrocene_certified"))]

pub const unsafe fn transmute_copy<Src, Dst>(src: &Src) -> Dst {

    assert!(

        size_of::<Src>() >= size_of::<Dst>(),

        "cannot transmute_copy if Dst is larger than Src"

    );

    // If Dst has a higher alignment requirement, src might not be suitably aligned.

    if align_of::<Dst>() > align_of::<Src>() {

        // SAFETY: `src` is a reference which is guaranteed to be valid for reads.

        // The caller must guarantee that the actual transmutation is safe.

        unsafe { ptr::read_unaligned(src as *const Src as *const Dst) }

    } else {

        // SAFETY: `src` is a reference which is guaranteed to be valid for reads.

        // We just checked that `src as *const Dst` was properly aligned.

        // The caller must guarantee that the actual transmutation is safe.

        unsafe { ptr::read(src as *const Src as *const Dst) }

    }

}

/// Opaque type representing the discriminant of an enum.

///

/// See the [`discriminant`] function in this module for more information.

#[stable(feature = "discriminant_value", since = "1.21.0")]

#[cfg(not(feature = "ferrocene_certified"))]

pub struct Discriminant<T>(<T as DiscriminantKind>::Discriminant);

// N.B. These trait implementations cannot be derived because we don't want any bounds on T.

#[stable(feature = "discriminant_value", since = "1.21.0")]

#[cfg(not(feature = "ferrocene_certified"))]

impl<T> Copy for Discriminant<T> {}

#[stable(feature = "discriminant_value", since = "1.21.0")]

#[cfg(not(feature = "ferrocene_certified"))]

impl<T> clone::Clone for Discriminant<T> {

    fn clone(&self) -> Self {

        *self

    }

}

#[stable(feature = "discriminant_value", since = "1.21.0")]

#[cfg(not(feature = "ferrocene_certified"))]

impl<T> cmp::PartialEq for Discriminant<T> {

    fn eq(&self, rhs: &Self) -> bool {

        self.0 == rhs.0

    }

}

#[stable(feature = "discriminant_value", since = "1.21.0")]

#[cfg(not(feature = "ferrocene_certified"))]

impl<T> cmp::Eq for Discriminant<T> {}

#[stable(feature = "discriminant_value", since = "1.21.0")]

#[cfg(not(feature = "ferrocene_certified"))]

impl<T> hash::Hash for Discriminant<T> {

    fn hash<H: hash::Hasher>(&self, state: &mut H) {

        self.0.hash(state);

    }

}

#[stable(feature = "discriminant_value", since = "1.21.0")]

#[cfg(not(feature = "ferrocene_certified"))]

impl<T> fmt::Debug for Discriminant<T> {

    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {

        fmt.debug_tuple("Discriminant").field(&self.0).finish()

    }

}

/// Returns a value uniquely identifying the enum variant in `v`.

///

/// If `T` is not an enum, calling this function will not result in undefined behavior, but the

/// return value is unspecified.

///

/// # Stability

///

/// The discriminant of an enum variant may change if the enum definition changes. A discriminant

/// of some variant will not change between compilations with the same compiler. See the [Reference]

/// for more information.

///

/// [Reference]: ../../reference/items/enumerations.html#custom-discriminant-values-for-fieldless-enumerations

///

/// The value of a [`Discriminant<T>`] is independent of any *free lifetimes* in `T`. As such,

/// reading or writing a `Discriminant<Foo<'a>>` as a `Discriminant<Foo<'b>>` (whether via

/// [`transmute`] or otherwise) is always sound. Note that this is **not** true for other kinds

/// of generic parameters and for higher-ranked lifetimes; `Discriminant<Foo<A>>` and

/// `Discriminant<Foo<B>>` as well as `Discriminant<Bar<dyn for<'a> Trait<'a>>>` and

/// `Discriminant<Bar<dyn Trait<'static>>>` may be incompatible.

///

/// # Examples

///

/// This can be used to compare enums that carry data, while disregarding

/// the actual data:

///

/// ```

/// use std::mem;

///

/// enum Foo { A(&'static str), B(i32), C(i32) }

///

/// assert_eq!(mem::discriminant(&Foo::A("bar")), mem::discriminant(&Foo::A("baz")));

/// assert_eq!(mem::discriminant(&Foo::B(1)), mem::discriminant(&Foo::B(2)));

/// assert_ne!(mem::discriminant(&Foo::B(3)), mem::discriminant(&Foo::C(3)));

/// ```

///

/// ## Accessing the numeric value of the discriminant

///

/// Note that it is *undefined behavior* to [`transmute`] from [`Discriminant`] to a primitive!

///

/// If an enum has only unit variants, then the numeric value of the discriminant can be accessed

/// with an [`as`] cast:

///

/// ```

/// enum Enum {

///     Foo,

///     Bar,

///     Baz,

/// }

///

/// assert_eq!(0, Enum::Foo as isize);

/// assert_eq!(1, Enum::Bar as isize);

/// assert_eq!(2, Enum::Baz as isize);

/// ```

///

/// If an enum has opted-in to having a [primitive representation] for its discriminant,

/// then it's possible to use pointers to read the memory location storing the discriminant.

/// That **cannot** be done for enums using the [default representation], however, as it's

/// undefined what layout the discriminant has and where it's stored — it might not even be

/// stored at all!

///

/// [`as`]: ../../std/keyword.as.html

/// [primitive representation]: ../../reference/type-layout.html#primitive-representations

/// [default representation]: ../../reference/type-layout.html#the-default-representation

/// ```

/// #[repr(u8)]

/// enum Enum {

///     Unit,

///     Tuple(bool),

///     Struct { a: bool },

/// }

///

/// impl Enum {

///     fn discriminant(&self) -> u8 {

///         // SAFETY: Because `Self` is marked `repr(u8)`, its layout is a `repr(C)` `union`

///         // between `repr(C)` structs, each of which has the `u8` discriminant as its first

///         // field, so we can read the discriminant without offsetting the pointer.

///         unsafe { *<*const _>::from(self).cast::<u8>() }

///     }

/// }

///

/// let unit_like = Enum::Unit;

/// let tuple_like = Enum::Tuple(true);

/// let struct_like = Enum::Struct { a: false };

/// assert_eq!(0, unit_like.discriminant());

/// assert_eq!(1, tuple_like.discriminant());

/// assert_eq!(2, struct_like.discriminant());

///

/// // ⚠️ This is undefined behavior. Don't do this. ⚠️

/// // assert_eq!(0, unsafe { std::mem::transmute::<_, u8>(std::mem::discriminant(&unit_like)) });

/// ```

#[stable(feature = "discriminant_value", since = "1.21.0")]

#[rustc_const_stable(feature = "const_discriminant", since = "1.75.0")]

#[rustc_diagnostic_item = "mem_discriminant"]

#[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces

#[cfg(not(feature = "ferrocene_certified"))]

pub const fn discriminant<T>(v: &T) -> Discriminant<T> {

    Discriminant(intrinsics::discriminant_value(v))

}

/// Returns the number of variants in the enum type `T`.

///

/// If `T` is not an enum, calling this function will not result in undefined behavior, but the

/// return value is unspecified. Equally, if `T` is an enum with more variants than `usize::MAX`

/// the return value is unspecified. Uninhabited variants will be counted.

///

/// Note that an enum may be expanded with additional variants in the future

/// as a non-breaking change, for example if it is marked `#[non_exhaustive]`,

/// which will change the result of this function.

///

/// # Examples

///

/// ```

/// # #![feature(never_type)]

/// # #![feature(variant_count)]

///

/// use std::mem;

///

/// enum Void {}

/// enum Foo { A(&'static str), B(i32), C(i32) }

///

/// assert_eq!(mem::variant_count::<Void>(), 0);

/// assert_eq!(mem::variant_count::<Foo>(), 3);

///

/// assert_eq!(mem::variant_count::<Option<!>>(), 2);

/// assert_eq!(mem::variant_count::<Result<!, !>>(), 2);

/// ```

#[inline(always)]

#[must_use]

#[unstable(feature = "variant_count", issue = "73662")]

#[rustc_const_unstable(feature = "variant_count", issue = "73662")]

#[rustc_diagnostic_item = "mem_variant_count"]

#[cfg(not(feature = "ferrocene_certified"))]

pub const fn variant_count<T>() -> usize {

    const { intrinsics::variant_count::<T>() }

}

/// Provides associated constants for various useful properties of types,

/// to give them a canonical form in our code and make them easier to read.

///

/// This is here only to simplify all the ZST checks we need in the library.

/// It's not on a stabilization track right now.

#[doc(hidden)]

#[unstable(feature = "sized_type_properties", issue = "none")]

pub trait SizedTypeProperties: Sized {

    /// `true` if this type requires no storage.

    /// `false` if its [size](size_of) is greater than zero.

    ///

    /// # Examples

    ///

    /// ```

    /// #![feature(sized_type_properties)]

    /// use core::mem::SizedTypeProperties;

    ///

    /// fn do_something_with<T>() {

    ///     if T::IS_ZST {

    ///         // ... special approach ...

    ///     } else {

    ///         // ... the normal thing ...

    ///     }

    /// }

    ///

    /// struct MyUnit;

    /// assert!(MyUnit::IS_ZST);

    ///

    /// // For negative checks, consider using UFCS to emphasize the negation

    /// assert!(!<i32>::IS_ZST);

    /// // As it can sometimes hide in the type otherwise

    /// assert!(!String::IS_ZST);

    /// ```

    #[doc(hidden)]

    #[unstable(feature = "sized_type_properties", issue = "none")]

    const IS_ZST: bool = size_of::<Self>() == 0;

    #[doc(hidden)]

    #[unstable(feature = "sized_type_properties", issue = "none")]

    const LAYOUT: Layout = Layout::new::<Self>();

    /// The largest safe length for a `[Self]`.

    ///

    /// Anything larger than this would make `size_of_val` overflow `isize::MAX`,

    /// which is never allowed for a single object.

    #[doc(hidden)]

    #[unstable(feature = "sized_type_properties", issue = "none")]

    const MAX_SLICE_LEN: usize = match size_of::<Self>() {

        0 => usize::MAX,

        n => (isize::MAX as usize) / n,

    };

}

#[doc(hidden)]

#[unstable(feature = "sized_type_properties", issue = "none")]

impl<T> SizedTypeProperties for T {}

/// Expands to the offset in bytes of a field from the beginning of the given type.

///

/// The type may be a `struct`, `enum`, `union`, or tuple.

///

/// The field may be a nested field (`field1.field2`), but not an array index.

/// The field must be visible to the call site.

///

/// The offset is returned as a [`usize`].

///

/// # Offsets of, and in, dynamically sized types

///

/// The field’s type must be [`Sized`], but it may be located in a [dynamically sized] container.

/// If the field type is dynamically sized, then you cannot use `offset_of!` (since the field's

/// alignment, and therefore its offset, may also be dynamic) and must take the offset from an

/// actual pointer to the container instead.