core/mem/

mod.rs

//! Basic functions for dealing with memory, values, and types.
//!
//! The contents of this module can be seen as belonging to a few families:
//!
//! * [`drop`], [`replace`], [`swap`], and [`take`]
//!   are safe functions for moving values in particular ways.
//!   They are useful in everyday Rust code.
//!
//! * [`size_of`], [`size_of_val`], [`align_of`], [`align_of_val`], and [`offset_of`]
//!   give information about the representation of values in memory.
//!
//! * [`discriminant`]
//!   allows comparing the variants of [`enum`] values while ignoring their fields.
//!
//! * [`forget`] and [`ManuallyDrop`]
//!   prevent destructors from running, which is used in certain kinds of ownership transfer.
//!   [`needs_drop`]
//!   tells you whether a type’s destructor even does anything.
//!
//! * [`transmute`], [`transmute_copy`], and [`MaybeUninit`]
//!   convert and construct values in [`unsafe`] ways.
//!
//! See also the [`alloc`] and [`ptr`] modules for more primitive operations on memory.
//!
// core::alloc exists but doesn’t contain all the items we want to discuss
//! [`alloc`]: ../../std/alloc/index.html
//! [`enum`]: ../../std/keyword.enum.html
//! [`ptr`]: crate::ptr
//! [`unsafe`]: ../../std/keyword.unsafe.html

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

use crate::alloc::Layout;
use crate::clone::TrivialClone;
use crate::marker::{Destruct, DiscriminantKind};
use crate::panic::const_assert;
use crate::ptr::Alignment;
use crate::{clone, cmp, fmt, hash, intrinsics, ptr};

mod manually_drop;
#[stable(feature = "manually_drop", since = "1.20.0")]
pub use manually_drop::ManuallyDrop;

mod maybe_uninit;
#[stable(feature = "maybe_uninit", since = "1.36.0")]
pub use maybe_uninit::MaybeUninit;

mod maybe_dangling;
#[unstable(feature = "maybe_dangling", issue = "118166")]
pub use maybe_dangling::MaybeDangling;

mod transmutability;
#[unstable(feature = "transmutability", issue = "99571")]
pub use transmutability::{Assume, TransmuteFrom};

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

#[unstable(feature = "type_info", issue = "146922")]
pub mod type_info;

/// 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"]
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")]
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"]
pub const fn size_of<T>() -> usize {
    <T as SizedTypeProperties>::SIZE
}

/// 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"]
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")]
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")]
pub fn min_align_of<T>() -> usize {
    <T as SizedTypeProperties>::ALIGN
}

/// 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")]
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"]
pub const fn align_of<T>() -> usize {
    <T as SizedTypeProperties>::ALIGN
}

/// 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")]
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")]
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"]
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")]
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]
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"]
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")]
#[rustc_const_unstable(feature = "const_default", issue = "143894")]
pub const fn take<T: [const] 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"]
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 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][drop] 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_const_unstable(feature = "const_destruct", issue = "133214")]
#[rustc_diagnostic_item = "mem_drop"]
pub const fn drop<T>(_x: T)
where
    T: [const] Destruct,
{
}

/// 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")]
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")]
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")]
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")]
impl<T> Copy for Discriminant<T> {}

#[stable(feature = "discriminant_value", since = "1.21.0")]
impl<T> clone::Clone for Discriminant<T> {
    fn clone(&self) -> Self {
        *self
    }
}

#[doc(hidden)]
#[unstable(feature = "trivial_clone", issue = "none")]
unsafe impl<T> TrivialClone for Discriminant<T> {}

#[stable(feature = "discriminant_value", since = "1.21.0")]
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")]
impl<T> cmp::Eq for Discriminant<T> {}

#[stable(feature = "discriminant_value", since = "1.21.0")]
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")]
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
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"]
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 {
    #[doc(hidden)]
    #[unstable(feature = "sized_type_properties", issue = "none")]
    #[lang = "mem_size_const"]
    const SIZE: usize = intrinsics::size_of::<Self>();

    #[doc(hidden)]
    #[unstable(feature = "sized_type_properties", issue = "none")]
    #[lang = "mem_align_const"]
    const ALIGN: usize = intrinsics::align_of::<Self>();

    #[doc(hidden)]
    #[unstable(feature = "ptr_alignment_type", issue = "102070")]
    const ALIGNMENT: Alignment = {
        // This can't panic since type alignment is always a power of two.
        Alignment::new(Self::ALIGN).unwrap()
    };

    /// `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 = Self::SIZE == 0;

    #[doc(hidden)]
    #[unstable(feature = "sized_type_properties", issue = "none")]
    const LAYOUT: Layout = {
        // SAFETY: if the type is instantiated, rustc already ensures that its
        // layout is valid. Use the unchecked constructor to avoid inserting a
        // panicking codepath that needs to be optimized out.
        unsafe { Layout::from_size_align_unchecked(Self::SIZE, Self::ALIGN) }
    };

    /// 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 Self::SIZE {
        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.
///
/// ```
/// # use core::mem;
/// # use core::fmt::Debug;
/// #[repr(C)]
/// pub struct Struct<T: ?Sized> {
///     a: u8,
///     b: T,
/// }
///
/// #[derive(Debug)]
/// #[repr(C, align(4))]
/// struct Align4(u32);
///
/// assert_eq!(mem::offset_of!(Struct<dyn Debug>, a), 0); // OK — Sized field
/// assert_eq!(mem::offset_of!(Struct<Align4>, b), 4); // OK — not DST
///
/// // assert_eq!(mem::offset_of!(Struct<dyn Debug>, b), 1);
/// // ^^^ error[E0277]: ... cannot be known at compilation time
///
/// // To obtain the offset of a !Sized field, examine a concrete value
/// // instead of using offset_of!.
/// let value: Struct<Align4> = Struct { a: 1, b: Align4(2) };
/// let ref_unsized: &Struct<dyn Debug> = &value;
/// let offset_of_b = unsafe {
///     (&raw const ref_unsized.b).byte_offset_from_unsigned(ref_unsized)
/// };
/// assert_eq!(offset_of_b, 4);
/// ```
///
/// If you need to obtain the offset of a field of a `!Sized` type, then, since the offset may
/// depend on the particular value being stored (in particular, `dyn Trait` values have a
/// dynamically-determined alignment), you must retrieve the offset from a specific reference
/// or pointer, and so you cannot use `offset_of!` to work without one.
///
/// # Layout is subject to change
///
/// Note that type layout is, in general, [subject to change and
/// platform-specific](https://doc.rust-lang.org/reference/type-layout.html). If
/// layout stability is required, consider using an [explicit `repr` attribute].
///
/// Rust guarantees that the offset of a given field within a given type will not
/// change over the lifetime of the program. However, two different compilations of
/// the same program may result in different layouts. Also, even within a single
/// program execution, no guarantees are made about types which are *similar* but
/// not *identical*, e.g.:
///
/// ```
/// struct Wrapper<T, U>(T, U);
///
/// type A = Wrapper<u8, u8>;
/// type B = Wrapper<u8, i8>;
///
/// // Not necessarily identical even though `u8` and `i8` have the same layout!
/// // assert_eq!(mem::offset_of!(A, 1), mem::offset_of!(B, 1));
///
/// #[repr(transparent)]
/// struct U8(u8);
///
/// type C = Wrapper<u8, U8>;
///
/// // Not necessarily identical even though `u8` and `U8` have the same layout!
/// // assert_eq!(mem::offset_of!(A, 1), mem::offset_of!(C, 1));
///
/// struct Empty<T>(core::marker::PhantomData<T>);
///
/// // Not necessarily identical even though `PhantomData` always has the same layout!
/// // assert_eq!(mem::offset_of!(Empty<u8>, 0), mem::offset_of!(Empty<i8>, 0));
/// ```
///
/// [explicit `repr` attribute]: https://doc.rust-lang.org/reference/type-layout.html#representations
///
/// # Unstable features
///
/// The following unstable features expand the functionality of `offset_of!`:
///
/// * [`offset_of_enum`] — allows `enum` variants to be traversed as if they were fields.
/// * [`offset_of_slice`] — allows getting the offset of a field of type `[T]`.
///
/// # Examples
///
/// ```
/// use std::mem;
/// #[repr(C)]
/// struct FieldStruct {
///     first: u8,
///     second: u16,
///     third: u8
/// }
///
/// assert_eq!(mem::offset_of!(FieldStruct, first), 0);
/// assert_eq!(mem::offset_of!(FieldStruct, second), 2);
/// assert_eq!(mem::offset_of!(FieldStruct, third), 4);
///
/// #[repr(C)]
/// struct NestedA {
///     b: NestedB
/// }
///
/// #[repr(C)]
/// struct NestedB(u8);
///
/// assert_eq!(mem::offset_of!(NestedA, b.0), 0);
/// ```
///
/// [dynamically sized]: https://doc.rust-lang.org/reference/dynamically-sized-types.html
/// [`offset_of_enum`]: https://doc.rust-lang.org/nightly/unstable-book/language-features/offset-of-enum.html
/// [`offset_of_slice`]: https://doc.rust-lang.org/nightly/unstable-book/language-features/offset-of-slice.html
#[stable(feature = "offset_of", since = "1.77.0")]
#[allow_internal_unstable(builtin_syntax, core_intrinsics)]
pub macro offset_of($Container:ty, $($fields:expr)+ $(,)?) {
    // The `{}` is for better error messages
    const {builtin # offset_of($Container, $($fields)+)}
}

/// Create a fresh instance of the inhabited ZST type `T`.
///
/// Prefer this to [`zeroed`] or [`uninitialized`] or [`transmute_copy`]
/// in places where you know that `T` is zero-sized, but don't have a bound
/// (such as [`Default`]) that would allow you to instantiate it using safe code.
///
/// If you're not sure whether `T` is an inhabited ZST, then you should be
/// using [`MaybeUninit`], not this function.
///
/// # Panics
///
/// If `size_of::<T>() != 0`.
///
/// # Safety
///
/// - `T` must be *[inhabited]*, i.e. possible to construct. This means that types
///   like zero-variant enums and [`!`] are unsound to conjure.
/// - You must use the value only in ways which do not violate any *safety*
///   invariants of the type.
///
/// While it's easy to create a *valid* instance of an inhabited ZST, since having
/// no bits in its representation means there's only one possible value, that
/// doesn't mean that it's always *sound* to do so.
///
/// For example, a library could design zero-sized tokens that are `!Default + !Clone`, limiting
/// their creation to functions that initialize some state or establish a scope. Conjuring such a
/// token could break invariants and lead to unsoundness.
///
/// # Examples
///
/// ```
/// #![feature(mem_conjure_zst)]
/// use std::mem::conjure_zst;
///
/// assert_eq!(unsafe { conjure_zst::<()>() }, ());
/// assert_eq!(unsafe { conjure_zst::<[i32; 0]>() }, []);
/// ```
///
/// [inhabited]: https://doc.rust-lang.org/reference/glossary.html#inhabited
#[unstable(feature = "mem_conjure_zst", issue = "95383")]
#[rustc_const_unstable(feature = "mem_conjure_zst", issue = "95383")]
pub const unsafe fn conjure_zst<T>() -> T {
    const_assert!(
        size_of::<T>() == 0,
        "mem::conjure_zst invoked on a non-zero-sized type",
        "mem::conjure_zst invoked on type {name}, which is not zero-sized",
        name: &str = crate::any::type_name::<T>()
    );

    // SAFETY: because the caller must guarantee that it's inhabited and zero-sized,
    // there's nothing in the representation that needs to be set.
    // `assume_init` calls `assert_inhabited`, so we don't need to here.
    unsafe {
        #[allow(clippy::uninit_assumed_init)]
        MaybeUninit::uninit().assume_init()
    }
}