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