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