#
core/mem/
mod.rs

    1//! Basic functions for dealing with memory.
    2//!
    3//! This module contains functions for querying the size and alignment of
    4//! types, initializing and manipulating memory.
    5
    6#![stable(feature = "rust1", since = "1.0.0")]
    7
    8use crate::alloc::Layout;
    9use crate::marker::DiscriminantKind;
    10use crate::{clone, cmp, fmt, hash, intrinsics, ptr};
    11
    12mod manually_drop;
    13#[stable(feature = "manually_drop", since = "1.20.0")]
    14pub use manually_drop::ManuallyDrop;
    15
    16mod maybe_uninit;
    17#[stable(feature = "maybe_uninit", since = "1.36.0")]
    18pub use maybe_uninit::MaybeUninit;
    19
    20mod transmutability;
    21#[unstable(feature = "transmutability", issue = "99571")]
    22pub use transmutability::{Assume, TransmuteFrom};
    23
    24mod drop_guard;
    25#[unstable(feature = "drop_guard", issue = "144426")]
    26pub use drop_guard::DropGuard;
    27
    28// This one has to be a re-export (rather than wrapping the underlying intrinsic) so that we can do
    29// the special magic "types have equal size" check at the call site.
    30#[stable(feature = "rust1", since = "1.0.0")]
    31#[doc(inline)]
    32pub use crate::intrinsics::transmute;
    33
    34/// Takes ownership and "forgets" about the value **without running its destructor**.
    35///
    36/// Any resources the value manages, such as heap memory or a file handle, will linger
    37/// forever in an unreachable state. However, it does not guarantee that pointers
    38/// to this memory will remain valid.
    39///
    40/// * If you want to leak memory, see [`Box::leak`].
    41/// * If you want to obtain a raw pointer to the memory, see [`Box::into_raw`].
    42/// * If you want to dispose of a value properly, running its destructor, see
    43///   [`mem::drop`].
    44///
    45/// # Safety
    46///
    47/// `forget` is not marked as `unsafe`, because Rust's safety guarantees
    48/// do not include a guarantee that destructors will always run. For example,
    49/// a program can create a reference cycle using [`Rc`][rc], or call
    50/// [`process::exit`][exit] to exit without running destructors. Thus, allowing
    51/// `mem::forget` from safe code does not fundamentally change Rust's safety
    52/// guarantees.
    53///
    54/// That said, leaking resources such as memory or I/O objects is usually undesirable.
    55/// The need comes up in some specialized use cases for FFI or unsafe code, but even
    56/// then, [`ManuallyDrop`] is typically preferred.
    57///
    58/// Because forgetting a value is allowed, any `unsafe` code you write must
    59/// allow for this possibility. You cannot return a value and expect that the
    60/// caller will necessarily run the value's destructor.
    61///
    62/// [rc]: ../../std/rc/struct.Rc.html
    63/// [exit]: ../../std/process/fn.exit.html
    64///
    65/// # Examples
    66///
    67/// The canonical safe use of `mem::forget` is to circumvent a value's destructor
    68/// implemented by the `Drop` trait. For example, this will leak a `File`, i.e. reclaim
    69/// the space taken by the variable but never close the underlying system resource:
    70///
    71/// ```no_run
    72/// use std::mem;
    73/// use std::fs::File;
    74///
    75/// let file = File::open("foo.txt").unwrap();
    76/// mem::forget(file);
    77/// ```
    78///
    79/// This is useful when the ownership of the underlying resource was previously
    80/// transferred to code outside of Rust, for example by transmitting the raw
    81/// file descriptor to C code.
    82///
    83/// # Relationship with `ManuallyDrop`
    84///
    85/// While `mem::forget` can also be used to transfer *memory* ownership, doing so is error-prone.
    86/// [`ManuallyDrop`] should be used instead. Consider, for example, this code:
    87///
    88/// ```
    89/// use std::mem;
    90///
    91/// let mut v = vec![65, 122];
    92/// // Build a `String` using the contents of `v`
    93/// let s = unsafe { String::from_raw_parts(v.as_mut_ptr(), v.len(), v.capacity()) };
    94/// // leak `v` because its memory is now managed by `s`
    95/// mem::forget(v);  // ERROR - v is invalid and must not be passed to a function
    96/// assert_eq!(s, "Az");
    97/// // `s` is implicitly dropped and its memory deallocated.
    98/// ```
    99///
    100/// There are two issues with the above example:
    101///
    102/// * If more code were added between the construction of `String` and the invocation of
    103///   `mem::forget()`, a panic within it would cause a double free because the same memory
    104///   is handled by both `v` and `s`.
    105/// * After calling `v.as_mut_ptr()` and transmitting the ownership of the data to `s`,
    106///   the `v` value is invalid. Even when a value is just moved to `mem::forget` (which won't
    107///   inspect it), some types have strict requirements on their values that
    108///   make them invalid when dangling or no longer owned. Using invalid values in any
    109///   way, including passing them to or returning them from functions, constitutes
    110///   undefined behavior and may break the assumptions made by the compiler.
    111///
    112/// Switching to `ManuallyDrop` avoids both issues:
    113///
    114/// ```
    115/// use std::mem::ManuallyDrop;
    116///
    117/// let v = vec![65, 122];
    118/// // Before we disassemble `v` into its raw parts, make sure it
    119/// // does not get dropped!
    120/// let mut v = ManuallyDrop::new(v);
    121/// // Now disassemble `v`. These operations cannot panic, so there cannot be a leak.
    122/// let (ptr, len, cap) = (v.as_mut_ptr(), v.len(), v.capacity());
    123/// // Finally, build a `String`.
    124/// let s = unsafe { String::from_raw_parts(ptr, len, cap) };
    125/// assert_eq!(s, "Az");
    126/// // `s` is implicitly dropped and its memory deallocated.
    127/// ```
    128///
    129/// `ManuallyDrop` robustly prevents double-free because we disable `v`'s destructor
    130/// before doing anything else. `mem::forget()` doesn't allow this because it consumes its
    131/// argument, forcing us to call it only after extracting anything we need from `v`. Even
    132/// if a panic were introduced between construction of `ManuallyDrop` and building the
    133/// string (which cannot happen in the code as shown), it would result in a leak and not a
    134/// double free. In other words, `ManuallyDrop` errs on the side of leaking instead of
    135/// erring on the side of (double-)dropping.
    136///
    137/// Also, `ManuallyDrop` prevents us from having to "touch" `v` after transferring the
    138/// ownership to `s` — the final step of interacting with `v` to dispose of it without
    139/// running its destructor is entirely avoided.
    140///
    141/// [`Box`]: ../../std/boxed/struct.Box.html
    142/// [`Box::leak`]: ../../std/boxed/struct.Box.html#method.leak
    143/// [`Box::into_raw`]: ../../std/boxed/struct.Box.html#method.into_raw
    144/// [`mem::drop`]: drop
    145/// [ub]: ../../reference/behavior-considered-undefined.html
    146#[inline]
    147#[rustc_const_stable(feature = "const_forget", since = "1.46.0")]
    148#[stable(feature = "rust1", since = "1.0.0")]
    149#[rustc_diagnostic_item = "mem_forget"]
    150pub const fn forget<T>(t: T) {
    151    let _ = ManuallyDrop::new(t);
    152}
    153
    154/// Like [`forget`], but also accepts unsized values.
    155///
    156/// While Rust does not permit unsized locals since its removal in [#111942] it is
    157/// still possible to call functions with unsized values from a function argument
    158/// or place expression.
    159///
    160/// ```rust
    161/// #![feature(unsized_fn_params, forget_unsized)]
    162/// #![allow(internal_features)]
    163///
    164/// use std::mem::forget_unsized;
    165///
    166/// pub fn in_place() {
    167///     forget_unsized(*Box::<str>::from("str"));
    168/// }
    169///
    170/// pub fn param(x: str) {
    171///     forget_unsized(x);
    172/// }
    173/// ```
    174///
    175/// This works because the compiler will alter these functions to pass the parameter
    176/// by reference instead. This trick is necessary to support `Box<dyn FnOnce()>: FnOnce()`.
    177/// See [#68304] and [#71170] for more information.
    178///
    179/// [#111942]: https://github.com/rust-lang/rust/issues/111942
    180/// [#68304]: https://github.com/rust-lang/rust/issues/68304
    181/// [#71170]: https://github.com/rust-lang/rust/pull/71170
    182#[inline]
    183#[unstable(feature = "forget_unsized", issue = "none")]
    184pub fn forget_unsized<T: ?Sized>(t: T) {
    185    intrinsics::forget(t)
    186}
    187
    188/// Returns the size of a type in bytes.
    189///
    190/// More specifically, this is the offset in bytes between successive elements
    191/// in an array with that item type including alignment padding. Thus, for any
    192/// type `T` and length `n`, `[T; n]` has a size of `n * size_of::<T>()`.
    193///
    194/// In general, the size of a type is not stable across compilations, but
    195/// specific types such as primitives are.
    196///
    197/// The following table gives the size for primitives.
    198///
    199/// Type | `size_of::<Type>()`
    200/// ---- | ---------------
    201/// () | 0
    202/// bool | 1
    203/// u8 | 1
    204/// u16 | 2
    205/// u32 | 4
    206/// u64 | 8
    207/// u128 | 16
    208/// i8 | 1
    209/// i16 | 2
    210/// i32 | 4
    211/// i64 | 8
    212/// i128 | 16
    213/// f32 | 4
    214/// f64 | 8
    215/// char | 4
    216///
    217/// Furthermore, `usize` and `isize` have the same size.
    218///
    219/// The types [`*const T`], `&T`, [`Box<T>`], [`Option<&T>`], and `Option<Box<T>>` all have
    220/// the same size. If `T` is `Sized`, all of those types have the same size as `usize`.
    221///
    222/// The mutability of a pointer does not change its size. As such, `&T` and `&mut T`
    223/// have the same size. Likewise for `*const T` and `*mut T`.
    224///
    225/// # Size of `#[repr(C)]` items
    226///
    227/// The `C` representation for items has a defined layout. With this layout,
    228/// the size of items is also stable as long as all fields have a stable size.
    229///
    230/// ## Size of Structs
    231///
    232/// For `struct`s, the size is determined by the following algorithm.
    233///
    234/// For each field in the struct ordered by declaration order:
    235///
    236/// 1. Add the size of the field.
    237/// 2. Round up the current size to the nearest multiple of the next field's [alignment].
    238///
    239/// Finally, round the size of the struct to the nearest multiple of its [alignment].
    240/// The alignment of the struct is usually the largest alignment of all its
    241/// fields; this can be changed with the use of `repr(align(N))`.
    242///
    243/// Unlike `C`, zero sized structs are not rounded up to one byte in size.
    244///
    245/// ## Size of Enums
    246///
    247/// Enums that carry no data other than the discriminant have the same size as C enums
    248/// on the platform they are compiled for.
    249///
    250/// ## Size of Unions
    251///
    252/// The size of a union is the size of its largest field.
    253///
    254/// Unlike `C`, zero sized unions are not rounded up to one byte in size.
    255///
    256/// # Examples
    257///
    258/// ```
    259/// // Some primitives
    260/// assert_eq!(4, size_of::<i32>());
    261/// assert_eq!(8, size_of::<f64>());
    262/// assert_eq!(0, size_of::<()>());
    263///
    264/// // Some arrays
    265/// assert_eq!(8, size_of::<[i32; 2]>());
    266/// assert_eq!(12, size_of::<[i32; 3]>());
    267/// assert_eq!(0, size_of::<[i32; 0]>());
    268///
    269///
    270/// // Pointer size equality
    271/// assert_eq!(size_of::<&i32>(), size_of::<*const i32>());
    272/// assert_eq!(size_of::<&i32>(), size_of::<Box<i32>>());
    273/// assert_eq!(size_of::<&i32>(), size_of::<Option<&i32>>());
    274/// assert_eq!(size_of::<Box<i32>>(), size_of::<Option<Box<i32>>>());
    275/// ```
    276///
    277/// Using `#[repr(C)]`.
    278///
    279/// ```
    280/// #[repr(C)]
    281/// struct FieldStruct {
    282///     first: u8,
    283///     second: u16,
    284///     third: u8
    285/// }
    286///
    287/// // The size of the first field is 1, so add 1 to the size. Size is 1.
    288/// // The alignment of the second field is 2, so add 1 to the size for padding. Size is 2.
    289/// // The size of the second field is 2, so add 2 to the size. Size is 4.
    290/// // The alignment of the third field is 1, so add 0 to the size for padding. Size is 4.
    291/// // The size of the third field is 1, so add 1 to the size. Size is 5.
    292/// // Finally, the alignment of the struct is 2 (because the largest alignment amongst its
    293/// // fields is 2), so add 1 to the size for padding. Size is 6.
    294/// assert_eq!(6, size_of::<FieldStruct>());
    295///
    296/// #[repr(C)]
    297/// struct TupleStruct(u8, u16, u8);
    298///
    299/// // Tuple structs follow the same rules.
    300/// assert_eq!(6, size_of::<TupleStruct>());
    301///
    302/// // Note that reordering the fields can lower the size. We can remove both padding bytes
    303/// // by putting `third` before `second`.
    304/// #[repr(C)]
    305/// struct FieldStructOptimized {
    306///     first: u8,
    307///     third: u8,
    308///     second: u16
    309/// }
    310///
    311/// assert_eq!(4, size_of::<FieldStructOptimized>());
    312///
    313/// // Union size is the size of the largest field.
    314/// #[repr(C)]
    315/// union ExampleUnion {
    316///     smaller: u8,
    317///     larger: u16
    318/// }
    319///
    320/// assert_eq!(2, size_of::<ExampleUnion>());
    321/// ```
    322///
    323/// [alignment]: align_of
    324/// [`*const T`]: primitive@pointer
    325/// [`Box<T>`]: ../../std/boxed/struct.Box.html
    326/// [`Option<&T>`]: crate::option::Option
    327///
    328#[inline(always)]
    329#[must_use]
    330#[stable(feature = "rust1", since = "1.0.0")]
    331#[rustc_promotable]
    332#[rustc_const_stable(feature = "const_mem_size_of", since = "1.24.0")]
    333#[rustc_diagnostic_item = "mem_size_of"]
    334pub const fn size_of<T>() -> usize {
    335    intrinsics::size_of::<T>()
    336}
    337
    338/// Returns the size of the pointed-to value in bytes.
    339///
    340/// This is usually the same as [`size_of::<T>()`]. However, when `T` *has* no
    341/// statically-known size, e.g., a slice [`[T]`][slice] or a [trait object],
    342/// then `size_of_val` can be used to get the dynamically-known size.
    343///
    344/// [trait object]: ../../book/ch17-02-trait-objects.html
    345///
    346/// # Examples
    347///
    348/// ```
    349/// assert_eq!(4, size_of_val(&5i32));
    350///
    351/// let x: [u8; 13] = [0; 13];
    352/// let y: &[u8] = &x;
    353/// assert_eq!(13, size_of_val(y));
    354/// ```
    355///
    356/// [`size_of::<T>()`]: size_of
    357#[inline]
    358#[must_use]
    359#[stable(feature = "rust1", since = "1.0.0")]
    360#[rustc_const_stable(feature = "const_size_of_val", since = "1.85.0")]
    361#[rustc_diagnostic_item = "mem_size_of_val"]
    362pub const fn size_of_val<T: ?Sized>(val: &T) -> usize {
    363    // SAFETY: `val` is a reference, so it's a valid raw pointer
    364    unsafe { intrinsics::size_of_val(val) }
    365}
    366
    367/// Returns the size of the pointed-to value in bytes.
    368///
    369/// This is usually the same as [`size_of::<T>()`]. However, when `T` *has* no
    370/// statically-known size, e.g., a slice [`[T]`][slice] or a [trait object],
    371/// then `size_of_val_raw` can be used to get the dynamically-known size.
    372///
    373/// # Safety
    374///
    375/// This function is only safe to call if the following conditions hold:
    376///
    377/// - If `T` is `Sized`, this function is always safe to call.
    378/// - If the unsized tail of `T` is:
    379///     - a [slice], then the length of the slice tail must be an initialized
    380///       integer, and the size of the *entire value*
    381///       (dynamic tail length + statically sized prefix) must fit in `isize`.
    382///       For the special case where the dynamic tail length is 0, this function
    383///       is safe to call.
    384//        NOTE: the reason this is safe is that if an overflow were to occur already with size 0,
    385//        then we would stop compilation as even the "statically known" part of the type would
    386//        already be too big (or the call may be in dead code and optimized away, but then it
    387//        doesn't matter).
    388///     - a [trait object], then the vtable part of the pointer must point
    389///       to a valid vtable acquired by an unsizing coercion, and the size
    390///       of the *entire value* (dynamic tail length + statically sized prefix)
    391///       must fit in `isize`.
    392///     - an (unstable) [extern type], then this function is always safe to
    393///       call, but may panic or otherwise return the wrong value, as the
    394///       extern type's layout is not known. This is the same behavior as
    395///       [`size_of_val`] on a reference to a type with an extern type tail.
    396///     - otherwise, it is conservatively not allowed to call this function.
    397///
    398/// [`size_of::<T>()`]: size_of
    399/// [trait object]: ../../book/ch17-02-trait-objects.html
    400/// [extern type]: ../../unstable-book/language-features/extern-types.html
    401///
    402/// # Examples
    403///
    404/// ```
    405/// #![feature(layout_for_ptr)]
    406/// use std::mem;
    407///
    408/// assert_eq!(4, size_of_val(&5i32));
    409///
    410/// let x: [u8; 13] = [0; 13];
    411/// let y: &[u8] = &x;
    412/// assert_eq!(13, unsafe { mem::size_of_val_raw(y) });
    413/// ```
    414#[inline]
    415#[must_use]
    416#[unstable(feature = "layout_for_ptr", issue = "69835")]
    417pub const unsafe fn size_of_val_raw<T: ?Sized>(val: *const T) -> usize {
    418    // SAFETY: the caller must provide a valid raw pointer
    419    unsafe { intrinsics::size_of_val(val) }
    420}
    421
    422/// Returns the [ABI]-required minimum alignment of a type in bytes.
    423///
    424/// Every reference to a value of the type `T` must be a multiple of this number.
    425///
    426/// This is the alignment used for struct fields. It may be smaller than the preferred alignment.
    427///
    428/// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface
    429///
    430/// # Examples
    431///
    432/// ```
    433/// # #![allow(deprecated)]
    434/// use std::mem;
    435///
    436/// assert_eq!(4, mem::min_align_of::<i32>());
    437/// ```
    438#[inline]
    439#[must_use]
    440#[stable(feature = "rust1", since = "1.0.0")]
    441#[deprecated(note = "use `align_of` instead", since = "1.2.0", suggestion = "align_of")]
    442pub fn min_align_of<T>() -> usize {
    443    intrinsics::align_of::<T>()
    444}
    445
    446/// Returns the [ABI]-required minimum alignment of the type of the value that `val` points to in
    447/// bytes.
    448///
    449/// Every reference to a value of the type `T` must be a multiple of this number.
    450///
    451/// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface
    452///
    453/// # Examples
    454///
    455/// ```
    456/// # #![allow(deprecated)]
    457/// use std::mem;
    458///
    459/// assert_eq!(4, mem::min_align_of_val(&5i32));
    460/// ```
    461#[inline]
    462#[must_use]
    463#[stable(feature = "rust1", since = "1.0.0")]
    464#[deprecated(note = "use `align_of_val` instead", since = "1.2.0", suggestion = "align_of_val")]
    465pub fn min_align_of_val<T: ?Sized>(val: &T) -> usize {
    466    // SAFETY: val is a reference, so it's a valid raw pointer
    467    unsafe { intrinsics::align_of_val(val) }
    468}
    469
    470/// Returns the [ABI]-required minimum alignment of a type in bytes.
    471///
    472/// Every reference to a value of the type `T` must be a multiple of this number.
    473///
    474/// This is the alignment used for struct fields. It may be smaller than the preferred alignment.
    475///
    476/// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface
    477///
    478/// # Examples
    479///
    480/// ```
    481/// assert_eq!(4, align_of::<i32>());
    482/// ```
    483#[inline(always)]
    484#[must_use]
    485#[stable(feature = "rust1", since = "1.0.0")]
    486#[rustc_promotable]
    487#[rustc_const_stable(feature = "const_align_of", since = "1.24.0")]
    488#[rustc_diagnostic_item = "mem_align_of"]
    489pub const fn align_of<T>() -> usize {
    490    intrinsics::align_of::<T>()
    491}
    492
    493/// Returns the [ABI]-required minimum alignment of the type of the value that `val` points to in
    494/// bytes.
    495///
    496/// Every reference to a value of the type `T` must be a multiple of this number.
    497///
    498/// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface
    499///
    500/// # Examples
    501///
    502/// ```
    503/// assert_eq!(4, align_of_val(&5i32));
    504/// ```
    505#[inline]
    506#[must_use]
    507#[stable(feature = "rust1", since = "1.0.0")]
    508#[rustc_const_stable(feature = "const_align_of_val", since = "1.85.0")]
    509pub const fn align_of_val<T: ?Sized>(val: &T) -> usize {
    510    // SAFETY: val is a reference, so it's a valid raw pointer
    511    unsafe { intrinsics::align_of_val(val) }
    512}
    513
    514/// Returns the [ABI]-required minimum alignment of the type of the value that `val` points to in
    515/// bytes.
    516///
    517/// Every reference to a value of the type `T` must be a multiple of this number.
    518///
    519/// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface
    520///
    521/// # Safety
    522///
    523/// This function is only safe to call if the following conditions hold:
    524///
    525/// - If `T` is `Sized`, this function is always safe to call.
    526/// - If the unsized tail of `T` is:
    527///     - a [slice], then the length of the slice tail must be an initialized
    528///       integer, and the size of the *entire value*
    529///       (dynamic tail length + statically sized prefix) must fit in `isize`.
    530///       For the special case where the dynamic tail length is 0, this function
    531///       is safe to call.
    532///     - a [trait object], then the vtable part of the pointer must point
    533///       to a valid vtable acquired by an unsizing coercion, and the size
    534///       of the *entire value* (dynamic tail length + statically sized prefix)
    535///       must fit in `isize`.
    536///     - an (unstable) [extern type], then this function is always safe to
    537///       call, but may panic or otherwise return the wrong value, as the
    538///       extern type's layout is not known. This is the same behavior as
    539///       [`align_of_val`] on a reference to a type with an extern type tail.
    540///     - otherwise, it is conservatively not allowed to call this function.
    541///
    542/// [trait object]: ../../book/ch17-02-trait-objects.html
    543/// [extern type]: ../../unstable-book/language-features/extern-types.html
    544///
    545/// # Examples
    546///
    547/// ```
    548/// #![feature(layout_for_ptr)]
    549/// use std::mem;
    550///
    551/// assert_eq!(4, unsafe { mem::align_of_val_raw(&5i32) });
    552/// ```
    553#[inline]
    554#[must_use]
    555#[unstable(feature = "layout_for_ptr", issue = "69835")]
    556pub const unsafe fn align_of_val_raw<T: ?Sized>(val: *const T) -> usize {
    557    // SAFETY: the caller must provide a valid raw pointer
    558    unsafe { intrinsics::align_of_val(val) }
    559}
    560
    561/// Returns `true` if dropping values of type `T` matters.
    562///
    563/// This is purely an optimization hint, and may be implemented conservatively:
    564/// it may return `true` for types that don't actually need to be dropped.
    565/// As such always returning `true` would be a valid implementation of
    566/// this function. However if this function actually returns `false`, then you
    567/// can be certain dropping `T` has no side effect.
    568///
    569/// Low level implementations of things like collections, which need to manually
    570/// drop their data, should use this function to avoid unnecessarily
    571/// trying to drop all their contents when they are destroyed. This might not
    572/// make a difference in release builds (where a loop that has no side-effects
    573/// is easily detected and eliminated), but is often a big win for debug builds.
    574///
    575/// Note that [`drop_in_place`] already performs this check, so if your workload
    576/// can be reduced to some small number of [`drop_in_place`] calls, using this is
    577/// unnecessary. In particular note that you can [`drop_in_place`] a slice, and that
    578/// will do a single needs_drop check for all the values.
    579///
    580/// Types like Vec therefore just `drop_in_place(&mut self[..])` without using
    581/// `needs_drop` explicitly. Types like [`HashMap`], on the other hand, have to drop
    582/// values one at a time and should use this API.
    583///
    584/// [`drop_in_place`]: crate::ptr::drop_in_place
    585/// [`HashMap`]: ../../std/collections/struct.HashMap.html
    586///
    587/// # Examples
    588///
    589/// Here's an example of how a collection might make use of `needs_drop`:
    590///
    591/// ```
    592/// use std::{mem, ptr};
    593///
    594/// pub struct MyCollection<T> {
    595/// #   data: [T; 1],
    596///     /* ... */
    597/// }
    598/// # impl<T> MyCollection<T> {
    599/// #   fn iter_mut(&mut self) -> &mut [T] { &mut self.data }
    600/// #   fn free_buffer(&mut self) {}
    601/// # }
    602///
    603/// impl<T> Drop for MyCollection<T> {
    604///     fn drop(&mut self) {
    605///         unsafe {
    606///             // drop the data
    607///             if mem::needs_drop::<T>() {
    608///                 for x in self.iter_mut() {
    609///                     ptr::drop_in_place(x);
    610///                 }
    611///             }
    612///             self.free_buffer();
    613///         }
    614///     }
    615/// }
    616/// ```
    617#[inline]
    618#[must_use]
    619#[stable(feature = "needs_drop", since = "1.21.0")]
    620#[rustc_const_stable(feature = "const_mem_needs_drop", since = "1.36.0")]
    621#[rustc_diagnostic_item = "needs_drop"]
    622pub const fn needs_drop<T: ?Sized>() -> bool {
    623    const { intrinsics::needs_drop::<T>() }
    624}
    625
    626/// Returns the value of type `T` represented by the all-zero byte-pattern.
    627///
    628/// This means that, for example, the padding byte in `(u8, u16)` is not
    629/// necessarily zeroed.
    630///
    631/// There is no guarantee that an all-zero byte-pattern represents a valid value
    632/// of some type `T`. For example, the all-zero byte-pattern is not a valid value
    633/// for reference types (`&T`, `&mut T`) and function pointers. Using `zeroed`
    634/// on such types causes immediate [undefined behavior][ub] because [the Rust
    635/// compiler assumes][inv] that there always is a valid value in a variable it
    636/// considers initialized.
    637///
    638/// This has the same effect as [`MaybeUninit::zeroed().assume_init()`][zeroed].
    639/// It is useful for FFI sometimes, but should generally be avoided.
    640///
    641/// [zeroed]: MaybeUninit::zeroed
    642/// [ub]: ../../reference/behavior-considered-undefined.html
    643/// [inv]: MaybeUninit#initialization-invariant
    644///
    645/// # Examples
    646///
    647/// Correct usage of this function: initializing an integer with zero.
    648///
    649/// ```
    650/// use std::mem;
    651///
    652/// let x: i32 = unsafe { mem::zeroed() };
    653/// assert_eq!(0, x);
    654/// ```
    655///
    656/// *Incorrect* usage of this function: initializing a reference with zero.
    657///
    658/// ```rust,no_run
    659/// # #![allow(invalid_value)]
    660/// use std::mem;
    661///
    662/// let _x: &i32 = unsafe { mem::zeroed() }; // Undefined behavior!
    663/// let _y: fn() = unsafe { mem::zeroed() }; // And again!
    664/// ```
    665#[inline(always)]
    666#[must_use]
    667#[stable(feature = "rust1", since = "1.0.0")]
    668#[rustc_diagnostic_item = "mem_zeroed"]
    669#[track_caller]
    670#[rustc_const_stable(feature = "const_mem_zeroed", since = "1.75.0")]
    671pub const unsafe fn zeroed<T>() -> T {
    672    // SAFETY: the caller must guarantee that an all-zero value is valid for `T`.
    673    unsafe {
    674        intrinsics::assert_zero_valid::<T>();
    675        MaybeUninit::zeroed().assume_init()
    676    }
    677}
    678
    679/// Bypasses Rust's normal memory-initialization checks by pretending to
    680/// produce a value of type `T`, while doing nothing at all.
    681///
    682/// **This function is deprecated.** Use [`MaybeUninit<T>`] instead.
    683/// It also might be slower than using `MaybeUninit<T>` due to mitigations that were put in place to
    684/// limit the potential harm caused by incorrect use of this function in legacy code.
    685///
    686/// The reason for deprecation is that the function basically cannot be used
    687/// correctly: it has the same effect as [`MaybeUninit::uninit().assume_init()`][uninit].
    688/// As the [`assume_init` documentation][assume_init] explains,
    689/// [the Rust compiler assumes][inv] that values are properly initialized.
    690///
    691/// Truly uninitialized memory like what gets returned here
    692/// is special in that the compiler knows that it does not have a fixed value.
    693/// This makes it undefined behavior to have uninitialized data in a variable even
    694/// if that variable has an integer type.
    695///
    696/// Therefore, it is immediate undefined behavior to call this function on nearly all types,
    697/// including integer types and arrays of integer types, and even if the result is unused.
    698///
    699/// [uninit]: MaybeUninit::uninit
    700/// [assume_init]: MaybeUninit::assume_init
    701/// [inv]: MaybeUninit#initialization-invariant
    702#[inline(always)]
    703#[must_use]
    704#[deprecated(since = "1.39.0", note = "use `mem::MaybeUninit` instead")]
    705#[stable(feature = "rust1", since = "1.0.0")]
    706#[rustc_diagnostic_item = "mem_uninitialized"]
    707#[track_caller]
    708pub unsafe fn uninitialized<T>() -> T {
    709    // SAFETY: the caller must guarantee that an uninitialized value is valid for `T`.
    710    unsafe {
    711        intrinsics::assert_mem_uninitialized_valid::<T>();
    712        let mut val = MaybeUninit::<T>::uninit();
    713
    714        // Fill memory with 0x01, as an imperfect mitigation for old code that uses this function on
    715        // bool, nonnull, and noundef types. But don't do this if we actively want to detect UB.
    716        if !cfg!(any(miri, sanitize = "memory")) {
    717            val.as_mut_ptr().write_bytes(0x01, 1);
    718        }
    719
    720        val.assume_init()
    721    }
    722}
    723
    724/// Swaps the values at two mutable locations, without deinitializing either one.
    725///
    726/// * If you want to swap with a default or dummy value, see [`take`].
    727/// * If you want to swap with a passed value, returning the old value, see [`replace`].
    728///
    729/// # Examples
    730///
    731/// ```
    732/// use std::mem;
    733///
    734/// let mut x = 5;
    735/// let mut y = 42;
    736///
    737/// mem::swap(&mut x, &mut y);
    738///
    739/// assert_eq!(42, x);
    740/// assert_eq!(5, y);
    741/// ```
    742#[inline]
    743#[stable(feature = "rust1", since = "1.0.0")]
    744#[rustc_const_stable(feature = "const_swap", since = "1.85.0")]
    745#[rustc_diagnostic_item = "mem_swap"]
    746pub const fn swap<T>(x: &mut T, y: &mut T) {
    747    // SAFETY: `&mut` guarantees these are typed readable and writable
    748    // as well as non-overlapping.
    749    unsafe { intrinsics::typed_swap_nonoverlapping(x, y) }
    750}
    751
    752/// Replaces `dest` with the default value of `T`, returning the previous `dest` value.
    753///
    754/// * If you want to replace the values of two variables, see [`swap`].
    755/// * If you want to replace with a passed value instead of the default value, see [`replace`].
    756///
    757/// # Examples
    758///
    759/// A simple example:
    760///
    761/// ```
    762/// use std::mem;
    763///
    764/// let mut v: Vec<i32> = vec![1, 2];
    765///
    766/// let old_v = mem::take(&mut v);
    767/// assert_eq!(vec![1, 2], old_v);
    768/// assert!(v.is_empty());
    769/// ```
    770///
    771/// `take` allows taking ownership of a struct field by replacing it with an "empty" value.
    772/// Without `take` you can run into issues like these:
    773///
    774/// ```compile_fail,E0507
    775/// struct Buffer<T> { buf: Vec<T> }
    776///
    777/// impl<T> Buffer<T> {
    778///     fn get_and_reset(&mut self) -> Vec<T> {
    779///         // error: cannot move out of dereference of `&mut`-pointer
    780///         let buf = self.buf;
    781///         self.buf = Vec::new();
    782///         buf
    783///     }
    784/// }
    785/// ```
    786///
    787/// Note that `T` does not necessarily implement [`Clone`], so it can't even clone and reset
    788/// `self.buf`. But `take` can be used to disassociate the original value of `self.buf` from
    789/// `self`, allowing it to be returned:
    790///
    791/// ```
    792/// use std::mem;
    793///
    794/// # struct Buffer<T> { buf: Vec<T> }
    795/// impl<T> Buffer<T> {
    796///     fn get_and_reset(&mut self) -> Vec<T> {
    797///         mem::take(&mut self.buf)
    798///     }
    799/// }
    800///
    801/// let mut buffer = Buffer { buf: vec![0, 1] };
    802/// assert_eq!(buffer.buf.len(), 2);
    803///
    804/// assert_eq!(buffer.get_and_reset(), vec![0, 1]);
    805/// assert_eq!(buffer.buf.len(), 0);
    806/// ```
    807#[inline]
    808#[stable(feature = "mem_take", since = "1.40.0")]
    809pub fn take<T: Default>(dest: &mut T) -> T {
    810    replace(dest, T::default())
    811}
    812
    813/// Moves `src` into the referenced `dest`, returning the previous `dest` value.
    814///
    815/// Neither value is dropped.
    816///
    817/// * If you want to replace the values of two variables, see [`swap`].
    818/// * If you want to replace with a default value, see [`take`].
    819///
    820/// # Examples
    821///
    822/// A simple example:
    823///
    824/// ```
    825/// use std::mem;
    826///
    827/// let mut v: Vec<i32> = vec![1, 2];
    828///
    829/// let old_v = mem::replace(&mut v, vec![3, 4, 5]);
    830/// assert_eq!(vec![1, 2], old_v);
    831/// assert_eq!(vec![3, 4, 5], v);
    832/// ```
    833///
    834/// `replace` allows consumption of a struct field by replacing it with another value.
    835/// Without `replace` you can run into issues like these:
    836///
    837/// ```compile_fail,E0507
    838/// struct Buffer<T> { buf: Vec<T> }
    839///
    840/// impl<T> Buffer<T> {
    841///     fn replace_index(&mut self, i: usize, v: T) -> T {
    842///         // error: cannot move out of dereference of `&mut`-pointer
    843///         let t = self.buf[i];
    844///         self.buf[i] = v;
    845///         t
    846///     }
    847/// }
    848/// ```
    849///
    850/// Note that `T` does not necessarily implement [`Clone`], so we can't even clone `self.buf[i]` to
    851/// avoid the move. But `replace` can be used to disassociate the original value at that index from
    852/// `self`, allowing it to be returned:
    853///
    854/// ```
    855/// # #![allow(dead_code)]
    856/// use std::mem;
    857///
    858/// # struct Buffer<T> { buf: Vec<T> }
    859/// impl<T> Buffer<T> {
    860///     fn replace_index(&mut self, i: usize, v: T) -> T {
    861///         mem::replace(&mut self.buf[i], v)
    862///     }
    863/// }
    864///
    865/// let mut buffer = Buffer { buf: vec![0, 1] };
    866/// assert_eq!(buffer.buf[0], 0);
    867///
    868/// assert_eq!(buffer.replace_index(0, 2), 0);
    869/// assert_eq!(buffer.buf[0], 2);
    870/// ```
    871#[inline]
    872#[stable(feature = "rust1", since = "1.0.0")]
    873#[must_use = "if you don't need the old value, you can just assign the new value directly"]
    874#[rustc_const_stable(feature = "const_replace", since = "1.83.0")]
    875#[rustc_diagnostic_item = "mem_replace"]
    876pub const fn replace<T>(dest: &mut T, src: T) -> T {
    877    // It may be tempting to use `swap` to avoid `unsafe` here. Don't!
    878    // The compiler optimizes the implementation below to two `memcpy`s
    879    // while `swap` would require at least three. See PR#83022 for details.
    880
    881    // SAFETY: We read from `dest` but directly write `src` into it afterwards,
    882    // such that the old value is not duplicated. Nothing is dropped and
    883    // nothing here can panic.
    884    unsafe {
    885        // Ideally we wouldn't use the intrinsics here, but going through the
    886        // `ptr` methods introduces two unnecessary UbChecks, so until we can
    887        // remove those for pointers that come from references, this uses the
    888        // intrinsics instead so this stays very cheap in MIR (and debug).
    889
    890        let result = crate::intrinsics::read_via_copy(dest);
    891        crate::intrinsics::write_via_move(dest, src);
    892        result
    893    }
    894}
    895
    896/// Disposes of a value.
    897///
    898/// This does so by calling the argument's implementation of [`Drop`][drop].
    899///
    900/// This effectively does nothing for types which implement `Copy`, e.g.
    901/// integers. Such values are copied and _then_ moved into the function, so the
    902/// value persists after this function call.
    903///
    904/// This function is not magic; it is literally defined as
    905///
    906/// ```
    907/// pub fn drop<T>(_x: T) {}
    908/// ```
    909///
    910/// Because `_x` is moved into the function, it is automatically dropped before
    911/// the function returns.
    912///
    913/// [drop]: Drop
    914///
    915/// # Examples
    916///
    917/// Basic usage:
    918///
    919/// ```
    920/// let v = vec![1, 2, 3];
    921///
    922/// drop(v); // explicitly drop the vector
    923/// ```
    924///
    925/// Since [`RefCell`] enforces the borrow rules at runtime, `drop` can
    926/// release a [`RefCell`] borrow:
    927///
    928/// ```
    929/// use std::cell::RefCell;
    930///
    931/// let x = RefCell::new(1);
    932///
    933/// let mut mutable_borrow = x.borrow_mut();
    934/// *mutable_borrow = 1;
    935///
    936/// drop(mutable_borrow); // relinquish the mutable borrow on this slot
    937///
    938/// let borrow = x.borrow();
    939/// println!("{}", *borrow);
    940/// ```
    941///
    942/// Integers and other types implementing [`Copy`] are unaffected by `drop`.
    943///
    944/// ```
    945/// # #![allow(dropping_copy_types)]
    946/// #[derive(Copy, Clone)]
    947/// struct Foo(u8);
    948///
    949/// let x = 1;
    950/// let y = Foo(2);
    951/// drop(x); // a copy of `x` is moved and dropped
    952/// drop(y); // a copy of `y` is moved and dropped
    953///
    954/// println!("x: {}, y: {}", x, y.0); // still available
    955/// ```
    956///
    957/// [`RefCell`]: crate::cell::RefCell
    958#[inline]
    959#[stable(feature = "rust1", since = "1.0.0")]
    960#[rustc_diagnostic_item = "mem_drop"]
    961pub fn drop<T>(_x: T) {}
    962
    963/// Bitwise-copies a value.
    964///
    965/// This function is not magic; it is literally defined as
    966/// ```
    967/// pub const fn copy<T: Copy>(x: &T) -> T { *x }
    968/// ```
    969///
    970/// It is useful when you want to pass a function pointer to a combinator, rather than defining a new closure.
    971///
    972/// Example:
    973/// ```
    974/// #![feature(mem_copy_fn)]
    975/// use core::mem::copy;
    976/// let result_from_ffi_function: Result<(), &i32> = Err(&1);
    977/// let result_copied: Result<(), i32> = result_from_ffi_function.map_err(copy);
    978/// ```
    979#[inline]
    980#[unstable(feature = "mem_copy_fn", issue = "98262")]
    981pub const fn copy<T: Copy>(x: &T) -> T {
    982    *x
    983}
    984
    985/// Interprets `src` as having type `&Dst`, and then reads `src` without moving
    986/// the contained value.
    987///
    988/// This function will unsafely assume the pointer `src` is valid for [`size_of::<Dst>`][size_of]
    989/// bytes by transmuting `&Src` to `&Dst` and then reading the `&Dst` (except that this is done
    990/// in a way that is correct even when `&Dst` has stricter alignment requirements than `&Src`).
    991/// It will also unsafely create a copy of the contained value instead of moving out of `src`.
    992///
    993/// It is not a compile-time error if `Src` and `Dst` have different sizes, but it
    994/// is highly encouraged to only invoke this function where `Src` and `Dst` have the
    995/// same size. This function triggers [undefined behavior][ub] if `Dst` is larger than
    996/// `Src`.
    997///
    998/// [ub]: ../../reference/behavior-considered-undefined.html
    999///
    1000/// # Examples
    1001///
    1002/// ```
    1003/// use std::mem;
    1004///
    1005/// #[repr(packed)]
    1006/// struct Foo {
    1007///     bar: u8,
    1008/// }
    1009///
    1010/// let foo_array = [10u8];
    1011///
    1012/// unsafe {
    1013///     // Copy the data from 'foo_array' and treat it as a 'Foo'
    1014///     let mut foo_struct: Foo = mem::transmute_copy(&foo_array);
    1015///     assert_eq!(foo_struct.bar, 10);
    1016///
    1017///     // Modify the copied data
    1018///     foo_struct.bar = 20;
    1019///     assert_eq!(foo_struct.bar, 20);
    1020/// }
    1021///
    1022/// // The contents of 'foo_array' should not have changed
    1023/// assert_eq!(foo_array, [10]);
    1024/// ```
    1025#[inline]
    1026#[must_use]
    1027#[track_caller]
    1028#[stable(feature = "rust1", since = "1.0.0")]
    1029#[rustc_const_stable(feature = "const_transmute_copy", since = "1.74.0")]
    1030pub const unsafe fn transmute_copy<Src, Dst>(src: &Src) -> Dst {
    1031    assert!(
    1032        size_of::<Src>() >= size_of::<Dst>(),
    1033        "cannot transmute_copy if Dst is larger than Src"
    1034    );
    1035
    1036    // If Dst has a higher alignment requirement, src might not be suitably aligned.
    1037    if align_of::<Dst>() > align_of::<Src>() {
    1038        // SAFETY: `src` is a reference which is guaranteed to be valid for reads.
    1039        // The caller must guarantee that the actual transmutation is safe.
    1040        unsafe { ptr::read_unaligned(src as *const Src as *const Dst) }
    1041    } else {
    1042        // SAFETY: `src` is a reference which is guaranteed to be valid for reads.
    1043        // We just checked that `src as *const Dst` was properly aligned.
    1044        // The caller must guarantee that the actual transmutation is safe.
    1045        unsafe { ptr::read(src as *const Src as *const Dst) }
    1046    }
    1047}
    1048
    1049/// Opaque type representing the discriminant of an enum.
    1050///
    1051/// See the [`discriminant`] function in this module for more information.
    1052#[stable(feature = "discriminant_value", since = "1.21.0")]
    1053pub struct Discriminant<T>(<T as DiscriminantKind>::Discriminant);
    1054
    1055// N.B. These trait implementations cannot be derived because we don't want any bounds on T.
    1056
    1057#[stable(feature = "discriminant_value", since = "1.21.0")]
    1058impl<T> Copy for Discriminant<T> {}
    1059
    1060#[stable(feature = "discriminant_value", since = "1.21.0")]
    1061impl<T> clone::Clone for Discriminant<T> {
    1062    fn clone(&self) -> Self {
    1063        *self
    1064    }
    1065}
    1066
    1067#[stable(feature = "discriminant_value", since = "1.21.0")]
    1068impl<T> cmp::PartialEq for Discriminant<T> {
    1069    fn eq(&self, rhs: &Self) -> bool {
    1070        self.0 == rhs.0
    1071    }
    1072}
    1073
    1074#[stable(feature = "discriminant_value", since = "1.21.0")]
    1075impl<T> cmp::Eq for Discriminant<T> {}
    1076
    1077#[stable(feature = "discriminant_value", since = "1.21.0")]
    1078impl<T> hash::Hash for Discriminant<T> {
    1079    fn hash<H: hash::Hasher>(&self, state: &mut H) {
    1080        self.0.hash(state);
    1081    }
    1082}
    1083
    1084#[stable(feature = "discriminant_value", since = "1.21.0")]
    1085impl<T> fmt::Debug for Discriminant<T> {
    1086    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
    1087        fmt.debug_tuple("Discriminant").field(&self.0).finish()
    1088    }
    1089}
    1090
    1091/// Returns a value uniquely identifying the enum variant in `v`.
    1092///
    1093/// If `T` is not an enum, calling this function will not result in undefined behavior, but the
    1094/// return value is unspecified.
    1095///
    1096/// # Stability
    1097///
    1098/// The discriminant of an enum variant may change if the enum definition changes. A discriminant
    1099/// of some variant will not change between compilations with the same compiler. See the [Reference]
    1100/// for more information.
    1101///
    1102/// [Reference]: ../../reference/items/enumerations.html#custom-discriminant-values-for-fieldless-enumerations
    1103///
    1104/// The value of a [`Discriminant<T>`] is independent of any *free lifetimes* in `T`. As such,
    1105/// reading or writing a `Discriminant<Foo<'a>>` as a `Discriminant<Foo<'b>>` (whether via
    1106/// [`transmute`] or otherwise) is always sound. Note that this is **not** true for other kinds
    1107/// of generic parameters and for higher-ranked lifetimes; `Discriminant<Foo<A>>` and
    1108/// `Discriminant<Foo<B>>` as well as `Discriminant<Bar<dyn for<'a> Trait<'a>>>` and
    1109/// `Discriminant<Bar<dyn Trait<'static>>>` may be incompatible.
    1110///
    1111/// # Examples
    1112///
    1113/// This can be used to compare enums that carry data, while disregarding
    1114/// the actual data:
    1115///
    1116/// ```
    1117/// use std::mem;
    1118///
    1119/// enum Foo { A(&'static str), B(i32), C(i32) }
    1120///
    1121/// assert_eq!(mem::discriminant(&Foo::A("bar")), mem::discriminant(&Foo::A("baz")));
    1122/// assert_eq!(mem::discriminant(&Foo::B(1)), mem::discriminant(&Foo::B(2)));
    1123/// assert_ne!(mem::discriminant(&Foo::B(3)), mem::discriminant(&Foo::C(3)));
    1124/// ```
    1125///
    1126/// ## Accessing the numeric value of the discriminant
    1127///
    1128/// Note that it is *undefined behavior* to [`transmute`] from [`Discriminant`] to a primitive!
    1129///
    1130/// If an enum has only unit variants, then the numeric value of the discriminant can be accessed
    1131/// with an [`as`] cast:
    1132///
    1133/// ```
    1134/// enum Enum {
    1135///     Foo,
    1136///     Bar,
    1137///     Baz,
    1138/// }
    1139///
    1140/// assert_eq!(0, Enum::Foo as isize);
    1141/// assert_eq!(1, Enum::Bar as isize);
    1142/// assert_eq!(2, Enum::Baz as isize);
    1143/// ```
    1144///
    1145/// If an enum has opted-in to having a [primitive representation] for its discriminant,
    1146/// then it's possible to use pointers to read the memory location storing the discriminant.
    1147/// That **cannot** be done for enums using the [default representation], however, as it's
    1148/// undefined what layout the discriminant has and where it's stored — it might not even be
    1149/// stored at all!
    1150///
    1151/// [`as`]: ../../std/keyword.as.html
    1152/// [primitive representation]: ../../reference/type-layout.html#primitive-representations
    1153/// [default representation]: ../../reference/type-layout.html#the-default-representation
    1154/// ```
    1155/// #[repr(u8)]
    1156/// enum Enum {
    1157///     Unit,
    1158///     Tuple(bool),
    1159///     Struct { a: bool },
    1160/// }
    1161///
    1162/// impl Enum {
    1163///     fn discriminant(&self) -> u8 {
    1164///         // SAFETY: Because `Self` is marked `repr(u8)`, its layout is a `repr(C)` `union`
    1165///         // between `repr(C)` structs, each of which has the `u8` discriminant as its first
    1166///         // field, so we can read the discriminant without offsetting the pointer.
    1167///         unsafe { *<*const _>::from(self).cast::<u8>() }
    1168///     }
    1169/// }
    1170///
    1171/// let unit_like = Enum::Unit;
    1172/// let tuple_like = Enum::Tuple(true);
    1173/// let struct_like = Enum::Struct { a: false };
    1174/// assert_eq!(0, unit_like.discriminant());
    1175/// assert_eq!(1, tuple_like.discriminant());
    1176/// assert_eq!(2, struct_like.discriminant());
    1177///
    1178/// // ⚠️ This is undefined behavior. Don't do this. ⚠️
    1179/// // assert_eq!(0, unsafe { std::mem::transmute::<_, u8>(std::mem::discriminant(&unit_like)) });
    1180/// ```
    1181#[stable(feature = "discriminant_value", since = "1.21.0")]
    1182#[rustc_const_stable(feature = "const_discriminant", since = "1.75.0")]
    1183#[rustc_diagnostic_item = "mem_discriminant"]
    1184#[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
    1185pub const fn discriminant<T>(v: &T) -> Discriminant<T> {
    1186    Discriminant(intrinsics::discriminant_value(v))
    1187}
    1188
    1189/// Returns the number of variants in the enum type `T`.
    1190///
    1191/// If `T` is not an enum, calling this function will not result in undefined behavior, but the
    1192/// return value is unspecified. Equally, if `T` is an enum with more variants than `usize::MAX`
    1193/// the return value is unspecified. Uninhabited variants will be counted.
    1194///
    1195/// Note that an enum may be expanded with additional variants in the future
    1196/// as a non-breaking change, for example if it is marked `#[non_exhaustive]`,
    1197/// which will change the result of this function.
    1198///
    1199/// # Examples
    1200///
    1201/// ```
    1202/// # #![feature(never_type)]
    1203/// # #![feature(variant_count)]
    1204///
    1205/// use std::mem;
    1206///
    1207/// enum Void {}
    1208/// enum Foo { A(&'static str), B(i32), C(i32) }
    1209///
    1210/// assert_eq!(mem::variant_count::<Void>(), 0);
    1211/// assert_eq!(mem::variant_count::<Foo>(), 3);
    1212///
    1213/// assert_eq!(mem::variant_count::<Option<!>>(), 2);
    1214/// assert_eq!(mem::variant_count::<Result<!, !>>(), 2);
    1215/// ```
    1216#[inline(always)]
    1217#[must_use]
    1218#[unstable(feature = "variant_count", issue = "73662")]
    1219#[rustc_const_unstable(feature = "variant_count", issue = "73662")]
    1220#[rustc_diagnostic_item = "mem_variant_count"]
    1221pub const fn variant_count<T>() -> usize {
    1222    const { intrinsics::variant_count::<T>() }
    1223}
    1224
    1225/// Provides associated constants for various useful properties of types,
    1226/// to give them a canonical form in our code and make them easier to read.
    1227///
    1228/// This is here only to simplify all the ZST checks we need in the library.
    1229/// It's not on a stabilization track right now.
    1230#[doc(hidden)]
    1231#[unstable(feature = "sized_type_properties", issue = "none")]
    1232pub trait SizedTypeProperties: Sized {
    1233    /// `true` if this type requires no storage.
    1234    /// `false` if its [size](size_of) is greater than zero.
    1235    ///
    1236    /// # Examples
    1237    ///
    1238    /// ```
    1239    /// #![feature(sized_type_properties)]
    1240    /// use core::mem::SizedTypeProperties;
    1241    ///
    1242    /// fn do_something_with<T>() {
    1243    ///     if T::IS_ZST {
    1244    ///         // ... special approach ...
    1245    ///     } else {
    1246    ///         // ... the normal thing ...
    1247    ///     }
    1248    /// }
    1249    ///
    1250    /// struct MyUnit;
    1251    /// assert!(MyUnit::IS_ZST);
    1252    ///
    1253    /// // For negative checks, consider using UFCS to emphasize the negation
    1254    /// assert!(!<i32>::IS_ZST);
    1255    /// // As it can sometimes hide in the type otherwise
    1256    /// assert!(!String::IS_ZST);
    1257    /// ```
    1258    #[doc(hidden)]
    1259    #[unstable(feature = "sized_type_properties", issue = "none")]
    1260    const IS_ZST: bool = size_of::<Self>() == 0;
    1261
    1262    #[doc(hidden)]
    1263    #[unstable(feature = "sized_type_properties", issue = "none")]
    1264    const LAYOUT: Layout = Layout::new::<Self>();
    1265
    1266    /// The largest safe length for a `[Self]`.
    1267    ///
    1268    /// Anything larger than this would make `size_of_val` overflow `isize::MAX`,
    1269    /// which is never allowed for a single object.
    1270    #[doc(hidden)]
    1271    #[unstable(feature = "sized_type_properties", issue = "none")]
    1272    const MAX_SLICE_LEN: usize = match size_of::<Self>() {
    1273        0 => usize::MAX,
    1274        n => (isize::MAX as usize) / n,
    1275    };
    1276}
    1277#[doc(hidden)]
    1278#[unstable(feature = "sized_type_properties", issue = "none")]
    1279impl<T> SizedTypeProperties for T {}
    1280
    1281/// Expands to the offset in bytes of a field from the beginning of the given type.
    1282///
    1283/// The type may be a `struct`, `enum`, `union`, or tuple.
    1284///
    1285/// The field may be a nested field (`field1.field2`), but not an array index.
    1286/// The field must be visible to the call site.
    1287///
    1288/// The offset is returned as a [`usize`].
    1289///
    1290/// # Offsets of, and in, dynamically sized types
    1291///
    1292/// The field’s type must be [`Sized`], but it may be located in a [dynamically sized] container.
    1293/// If the field type is dynamically sized, then you cannot use `offset_of!` (since the field's
    1294/// alignment, and therefore its offset, may also be dynamic) and must take the offset from an
    1295/// actual pointer to the container instead.
    1296///
    1297/// ```
    1298/// # use core::mem;
    1299/// # use core::fmt::Debug;
    1300/// #[repr(C)]
    1301/// pub struct Struct<T: ?Sized> {
    1302///     a: u8,
    1303///     b: T,
    1304/// }
    1305///
    1306/// #[derive(Debug)]
    1307/// #[repr(C, align(4))]
    1308/// struct Align4(u32);
    1309///
    1310/// assert_eq!(mem::offset_of!(Struct<dyn Debug>, a), 0); // OK — Sized field
    1311/// assert_eq!(mem::offset_of!(Struct<Align4>, b), 4); // OK — not DST
    1312///
    1313/// // assert_eq!(mem::offset_of!(Struct<dyn Debug>, b), 1);
    1314/// // ^^^ error[E0277]: ... cannot be known at compilation time
    1315///
    1316/// // To obtain the offset of a !Sized field, examine a concrete value
    1317/// // instead of using offset_of!.
    1318/// let value: Struct<Align4> = Struct { a: 1, b: Align4(2) };
    1319/// let ref_unsized: &Struct<dyn Debug> = &value;
    1320/// let offset_of_b = unsafe {
    1321///     (&raw const ref_unsized.b).byte_offset_from_unsigned(ref_unsized)
    1322/// };
    1323/// assert_eq!(offset_of_b, 4);
    1324/// ```
    1325///
    1326/// If you need to obtain the offset of a field of a `!Sized` type, then, since the offset may
    1327/// depend on the particular value being stored (in particular, `dyn Trait` values have a
    1328/// dynamically-determined alignment), you must retrieve the offset from a specific reference
    1329/// or pointer, and so you cannot use `offset_of!` to work without one.
    1330///
    1331/// # Layout is subject to change
    1332///
    1333/// Note that type layout is, in general, [subject to change and
    1334/// platform-specific](https://doc.rust-lang.org/reference/type-layout.html). If
    1335/// layout stability is required, consider using an [explicit `repr` attribute].
    1336///
    1337/// Rust guarantees that the offset of a given field within a given type will not
    1338/// change over the lifetime of the program. However, two different compilations of
    1339/// the same program may result in different layouts. Also, even within a single
    1340/// program execution, no guarantees are made about types which are *similar* but
    1341/// not *identical*, e.g.:
    1342///
    1343/// ```
    1344/// struct Wrapper<T, U>(T, U);
    1345///
    1346/// type A = Wrapper<u8, u8>;
    1347/// type B = Wrapper<u8, i8>;
    1348///
    1349/// // Not necessarily identical even though `u8` and `i8` have the same layout!
    1350/// // assert_eq!(mem::offset_of!(A, 1), mem::offset_of!(B, 1));
    1351///
    1352/// #[repr(transparent)]
    1353/// struct U8(u8);
    1354///
    1355/// type C = Wrapper<u8, U8>;
    1356///
    1357/// // Not necessarily identical even though `u8` and `U8` have the same layout!
    1358/// // assert_eq!(mem::offset_of!(A, 1), mem::offset_of!(C, 1));
    1359///
    1360/// struct Empty<T>(core::marker::PhantomData<T>);
    1361///
    1362/// // Not necessarily identical even though `PhantomData` always has the same layout!
    1363/// // assert_eq!(mem::offset_of!(Empty<u8>, 0), mem::offset_of!(Empty<i8>, 0));
    1364/// ```
    1365///
    1366/// [explicit `repr` attribute]: https://doc.rust-lang.org/reference/type-layout.html#representations
    1367///
    1368/// # Unstable features
    1369///
    1370/// The following unstable features expand the functionality of `offset_of!`:
    1371///
    1372/// * [`offset_of_enum`] — allows `enum` variants to be traversed as if they were fields.
    1373/// * [`offset_of_slice`] — allows getting the offset of a field of type `[T]`.
    1374///
    1375/// # Examples
    1376///
    1377/// ```
    1378/// use std::mem;
    1379/// #[repr(C)]
    1380/// struct FieldStruct {
    1381///     first: u8,
    1382///     second: u16,
    1383///     third: u8
    1384/// }
    1385///
    1386/// assert_eq!(mem::offset_of!(FieldStruct, first), 0);
    1387/// assert_eq!(mem::offset_of!(FieldStruct, second), 2);
    1388/// assert_eq!(mem::offset_of!(FieldStruct, third), 4);
    1389///
    1390/// #[repr(C)]
    1391/// struct NestedA {
    1392///     b: NestedB
    1393/// }
    1394///
    1395/// #[repr(C)]
    1396/// struct NestedB(u8);
    1397///
    1398/// assert_eq!(mem::offset_of!(NestedA, b.0), 0);
    1399/// ```
    1400///
    1401/// [dynamically sized]: https://doc.rust-lang.org/reference/dynamically-sized-types.html
    1402/// [`offset_of_enum`]: https://doc.rust-lang.org/nightly/unstable-book/language-features/offset-of-enum.html
    1403/// [`offset_of_slice`]: https://doc.rust-lang.org/nightly/unstable-book/language-features/offset-of-slice.html
    1404#[stable(feature = "offset_of", since = "1.77.0")]
    1405#[allow_internal_unstable(builtin_syntax)]
    1406pub macro offset_of($Container:ty, $($fields:expr)+ $(,)?) {
    1407    // The `{}` is for better error messages
    1408    {builtin # offset_of($Container, $($fields)+)}
    1409}