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