1 //! Basic functions for dealing with memory.
3 //! This module contains functions for querying the size and alignment of
4 //! types, initializing and manipulating memory.
6 #![stable(feature = "rust1", since = "1.0.0")]
12 use crate::intrinsics;
13 use crate::marker::{Copy, DiscriminantKind, Sized};
17 #[stable(feature = "manually_drop", since = "1.20.0")]
18 pub use manually_drop::ManuallyDrop;
21 #[stable(feature = "maybe_uninit", since = "1.36.0")]
22 pub use maybe_uninit::MaybeUninit;
24 #[stable(feature = "rust1", since = "1.0.0")]
26 pub use crate::intrinsics::transmute;
28 /// Takes ownership and "forgets" about the value **without running its destructor**.
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.
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
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
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.
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.
56 /// [rc]: ../../std/rc/struct.Rc.html
57 /// [exit]: ../../std/process/fn.exit.html
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:
67 /// use std::fs::File;
69 /// let file = File::open("foo.txt").unwrap();
70 /// mem::forget(file);
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.
77 /// # Relationship with `ManuallyDrop`
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:
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.
94 /// There are two issues with the above example:
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.
106 /// Switching to `ManuallyDrop` avoids both issues:
109 /// use std::mem::ManuallyDrop;
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.
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.
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.
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
141 #[rustc_const_stable(feature = "const_forget", since = "1.46.0")]
142 #[stable(feature = "rust1", since = "1.0.0")]
143 pub const fn forget<T>(t: T) {
144 let _ = ManuallyDrop::new(t);
147 /// Like [`forget`], but also accepts unsized values.
149 /// This function is just a shim intended to be removed when the `unsized_locals` feature gets
152 #[unstable(feature = "forget_unsized", issue = "none")]
153 pub fn forget_unsized<T: ?Sized>(t: T) {
154 // SAFETY: the forget intrinsic could be safe, but there's no point in making it safe since
155 // we'll be implementing this function soon via `ManuallyDrop`
156 unsafe { intrinsics::forget(t) }
159 /// Returns the size of a type in bytes.
161 /// More specifically, this is the offset in bytes between successive elements
162 /// in an array with that item type including alignment padding. Thus, for any
163 /// type `T` and length `n`, `[T; n]` has a size of `n * size_of::<T>()`.
165 /// In general, the size of a type is not stable across compilations, but
166 /// specific types such as primitives are.
168 /// The following table gives the size for primitives.
170 /// Type | size_of::\<Type>()
171 /// ---- | ---------------
188 /// Furthermore, `usize` and `isize` have the same size.
190 /// The types `*const T`, `&T`, `Box<T>`, `Option<&T>`, and `Option<Box<T>>` all have
191 /// the same size. If `T` is Sized, all of those types have the same size as `usize`.
193 /// The mutability of a pointer does not change its size. As such, `&T` and `&mut T`
194 /// have the same size. Likewise for `*const T` and `*mut T`.
196 /// # Size of `#[repr(C)]` items
198 /// The `C` representation for items has a defined layout. With this layout,
199 /// the size of items is also stable as long as all fields have a stable size.
201 /// ## Size of Structs
203 /// For `structs`, the size is determined by the following algorithm.
205 /// For each field in the struct ordered by declaration order:
207 /// 1. Add the size of the field.
208 /// 2. Round up the current size to the nearest multiple of the next field's [alignment].
210 /// Finally, round the size of the struct to the nearest multiple of its [alignment].
211 /// The alignment of the struct is usually the largest alignment of all its
212 /// fields; this can be changed with the use of `repr(align(N))`.
214 /// Unlike `C`, zero sized structs are not rounded up to one byte in size.
218 /// Enums that carry no data other than the discriminant have the same size as C enums
219 /// on the platform they are compiled for.
221 /// ## Size of Unions
223 /// The size of a union is the size of its largest field.
225 /// Unlike `C`, zero sized unions are not rounded up to one byte in size.
232 /// // Some primitives
233 /// assert_eq!(4, mem::size_of::<i32>());
234 /// assert_eq!(8, mem::size_of::<f64>());
235 /// assert_eq!(0, mem::size_of::<()>());
238 /// assert_eq!(8, mem::size_of::<[i32; 2]>());
239 /// assert_eq!(12, mem::size_of::<[i32; 3]>());
240 /// assert_eq!(0, mem::size_of::<[i32; 0]>());
243 /// // Pointer size equality
244 /// assert_eq!(mem::size_of::<&i32>(), mem::size_of::<*const i32>());
245 /// assert_eq!(mem::size_of::<&i32>(), mem::size_of::<Box<i32>>());
246 /// assert_eq!(mem::size_of::<&i32>(), mem::size_of::<Option<&i32>>());
247 /// assert_eq!(mem::size_of::<Box<i32>>(), mem::size_of::<Option<Box<i32>>>());
250 /// Using `#[repr(C)]`.
256 /// struct FieldStruct {
262 /// // The size of the first field is 1, so add 1 to the size. Size is 1.
263 /// // The alignment of the second field is 2, so add 1 to the size for padding. Size is 2.
264 /// // The size of the second field is 2, so add 2 to the size. Size is 4.
265 /// // The alignment of the third field is 1, so add 0 to the size for padding. Size is 4.
266 /// // The size of the third field is 1, so add 1 to the size. Size is 5.
267 /// // Finally, the alignment of the struct is 2 (because the largest alignment amongst its
268 /// // fields is 2), so add 1 to the size for padding. Size is 6.
269 /// assert_eq!(6, mem::size_of::<FieldStruct>());
272 /// struct TupleStruct(u8, u16, u8);
274 /// // Tuple structs follow the same rules.
275 /// assert_eq!(6, mem::size_of::<TupleStruct>());
277 /// // Note that reordering the fields can lower the size. We can remove both padding bytes
278 /// // by putting `third` before `second`.
280 /// struct FieldStructOptimized {
286 /// assert_eq!(4, mem::size_of::<FieldStructOptimized>());
288 /// // Union size is the size of the largest field.
290 /// union ExampleUnion {
295 /// assert_eq!(2, mem::size_of::<ExampleUnion>());
298 /// [alignment]: align_of
300 #[stable(feature = "rust1", since = "1.0.0")]
302 #[rustc_const_stable(feature = "const_size_of", since = "1.32.0")]
303 pub const fn size_of<T>() -> usize {
304 intrinsics::size_of::<T>()
307 /// Returns the size of the pointed-to value in bytes.
309 /// This is usually the same as `size_of::<T>()`. However, when `T` *has* no
310 /// statically-known size, e.g., a slice [`[T]`][slice] or a [trait object],
311 /// then `size_of_val` can be used to get the dynamically-known size.
313 /// [slice]: ../../std/primitive.slice.html
314 /// [trait object]: ../../book/ch17-02-trait-objects.html
321 /// assert_eq!(4, mem::size_of_val(&5i32));
323 /// let x: [u8; 13] = [0; 13];
324 /// let y: &[u8] = &x;
325 /// assert_eq!(13, mem::size_of_val(y));
328 #[stable(feature = "rust1", since = "1.0.0")]
329 #[rustc_const_unstable(feature = "const_size_of_val", issue = "46571")]
330 pub const fn size_of_val<T: ?Sized>(val: &T) -> usize {
331 intrinsics::size_of_val(val)
334 /// Returns the size of the pointed-to value in bytes.
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_raw` can be used to get the dynamically-known size.
342 /// This function is only safe to call if the following conditions hold:
344 /// - If `T` is `Sized`, this function is always safe to call.
345 /// - If the unsized tail of `T` is:
346 /// - a [slice], then the length of the slice tail must be an initialized
347 /// integer, and the size of the *entire value*
348 /// (dynamic tail length + statically sized prefix) must fit in `isize`.
349 /// - a [trait object], then the vtable part of the pointer must point
350 /// to a valid vtable acquired by an unsizing coercion, and the size
351 /// of the *entire value* (dynamic tail length + statically sized prefix)
352 /// must fit in `isize`.
353 /// - an (unstable) [extern type], then this function is always safe to
354 /// call, but may panic or otherwise return the wrong value, as the
355 /// extern type's layout is not known. This is the same behavior as
356 /// [`size_of_val`] on a reference to a type with an extern type tail.
357 /// - otherwise, it is conservatively not allowed to call this function.
359 /// [slice]: ../../std/primitive.slice.html
360 /// [trait object]: ../../book/ch17-02-trait-objects.html
361 /// [extern type]: ../../unstable-book/language-features/extern-types.html
366 /// #![feature(layout_for_ptr)]
369 /// assert_eq!(4, mem::size_of_val(&5i32));
371 /// let x: [u8; 13] = [0; 13];
372 /// let y: &[u8] = &x;
373 /// assert_eq!(13, unsafe { mem::size_of_val_raw(y) });
376 #[unstable(feature = "layout_for_ptr", issue = "69835")]
377 pub unsafe fn size_of_val_raw<T: ?Sized>(val: *const T) -> usize {
378 intrinsics::size_of_val(val)
381 /// Returns the [ABI]-required minimum alignment of a type.
383 /// Every reference to a value of the type `T` must be a multiple of this number.
385 /// This is the alignment used for struct fields. It may be smaller than the preferred alignment.
387 /// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface
392 /// # #![allow(deprecated)]
395 /// assert_eq!(4, mem::min_align_of::<i32>());
398 #[stable(feature = "rust1", since = "1.0.0")]
399 #[rustc_deprecated(reason = "use `align_of` instead", since = "1.2.0")]
400 pub fn min_align_of<T>() -> usize {
401 intrinsics::min_align_of::<T>()
404 /// Returns the [ABI]-required minimum alignment of the type of the value that `val` points to.
406 /// Every reference to a value of the type `T` must be a multiple of this number.
408 /// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface
413 /// # #![allow(deprecated)]
416 /// assert_eq!(4, mem::min_align_of_val(&5i32));
419 #[stable(feature = "rust1", since = "1.0.0")]
420 #[rustc_deprecated(reason = "use `align_of_val` instead", since = "1.2.0")]
421 pub fn min_align_of_val<T: ?Sized>(val: &T) -> usize {
422 intrinsics::min_align_of_val(val)
425 /// Returns the [ABI]-required minimum alignment of a type.
427 /// Every reference to a value of the type `T` must be a multiple of this number.
429 /// This is the alignment used for struct fields. It may be smaller than the preferred alignment.
431 /// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface
438 /// assert_eq!(4, mem::align_of::<i32>());
441 #[stable(feature = "rust1", since = "1.0.0")]
443 #[rustc_const_stable(feature = "const_align_of", since = "1.32.0")]
444 pub const fn align_of<T>() -> usize {
445 intrinsics::min_align_of::<T>()
448 /// Returns the [ABI]-required minimum alignment of the type of the value that `val` points to.
450 /// Every reference to a value of the type `T` must be a multiple of this number.
452 /// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface
459 /// assert_eq!(4, mem::align_of_val(&5i32));
462 #[stable(feature = "rust1", since = "1.0.0")]
463 #[rustc_const_unstable(feature = "const_align_of_val", issue = "46571")]
465 pub const fn align_of_val<T: ?Sized>(val: &T) -> usize {
466 intrinsics::min_align_of_val(val)
469 /// Returns the [ABI]-required minimum alignment of the type of the value that `val` points to.
471 /// Every reference to a value of the type `T` must be a multiple of this number.
473 /// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface
477 /// This function is only safe to call if the following conditions hold:
479 /// - If `T` is `Sized`, this function is always safe to call.
480 /// - If the unsized tail of `T` is:
481 /// - a [slice], then the length of the slice tail must be an initialized
482 /// integer, and the size of the *entire value*
483 /// (dynamic tail length + statically sized prefix) must fit in `isize`.
484 /// - a [trait object], then the vtable part of the pointer must point
485 /// to a valid vtable acquired by an unsizing coercion, and the size
486 /// of the *entire value* (dynamic tail length + statically sized prefix)
487 /// must fit in `isize`.
488 /// - an (unstable) [extern type], then this function is always safe to
489 /// call, but may panic or otherwise return the wrong value, as the
490 /// extern type's layout is not known. This is the same behavior as
491 /// [`align_of_val`] on a reference to a type with an extern type tail.
492 /// - otherwise, it is conservatively not allowed to call this function.
494 /// [slice]: ../../std/primitive.slice.html
495 /// [trait object]: ../../book/ch17-02-trait-objects.html
496 /// [extern type]: ../../unstable-book/language-features/extern-types.html
501 /// #![feature(layout_for_ptr)]
504 /// assert_eq!(4, unsafe { mem::align_of_val_raw(&5i32) });
507 #[unstable(feature = "layout_for_ptr", issue = "69835")]
508 pub unsafe fn align_of_val_raw<T: ?Sized>(val: *const T) -> usize {
509 intrinsics::min_align_of_val(val)
512 /// Returns `true` if dropping values of type `T` matters.
514 /// This is purely an optimization hint, and may be implemented conservatively:
515 /// it may return `true` for types that don't actually need to be dropped.
516 /// As such always returning `true` would be a valid implementation of
517 /// this function. However if this function actually returns `false`, then you
518 /// can be certain dropping `T` has no side effect.
520 /// Low level implementations of things like collections, which need to manually
521 /// drop their data, should use this function to avoid unnecessarily
522 /// trying to drop all their contents when they are destroyed. This might not
523 /// make a difference in release builds (where a loop that has no side-effects
524 /// is easily detected and eliminated), but is often a big win for debug builds.
526 /// Note that [`drop_in_place`] already performs this check, so if your workload
527 /// can be reduced to some small number of [`drop_in_place`] calls, using this is
528 /// unnecessary. In particular note that you can [`drop_in_place`] a slice, and that
529 /// will do a single needs_drop check for all the values.
531 /// Types like Vec therefore just `drop_in_place(&mut self[..])` without using
532 /// `needs_drop` explicitly. Types like [`HashMap`], on the other hand, have to drop
533 /// values one at a time and should use this API.
535 /// [`drop_in_place`]: crate::ptr::drop_in_place
536 /// [`HashMap`]: ../../std/collections/struct.HashMap.html
540 /// Here's an example of how a collection might make use of `needs_drop`:
543 /// use std::{mem, ptr};
545 /// pub struct MyCollection<T> {
549 /// # impl<T> MyCollection<T> {
550 /// # fn iter_mut(&mut self) -> &mut [T] { &mut self.data }
551 /// # fn free_buffer(&mut self) {}
554 /// impl<T> Drop for MyCollection<T> {
555 /// fn drop(&mut self) {
558 /// if mem::needs_drop::<T>() {
559 /// for x in self.iter_mut() {
560 /// ptr::drop_in_place(x);
563 /// self.free_buffer();
569 #[stable(feature = "needs_drop", since = "1.21.0")]
570 #[rustc_const_stable(feature = "const_needs_drop", since = "1.36.0")]
571 #[rustc_diagnostic_item = "needs_drop"]
572 pub const fn needs_drop<T>() -> bool {
573 intrinsics::needs_drop::<T>()
576 /// Returns the value of type `T` represented by the all-zero byte-pattern.
578 /// This means that, for example, the padding byte in `(u8, u16)` is not
579 /// necessarily zeroed.
581 /// There is no guarantee that an all-zero byte-pattern represents a valid value
582 /// of some type `T`. For example, the all-zero byte-pattern is not a valid value
583 /// for reference types (`&T`, `&mut T`) and functions pointers. Using `zeroed`
584 /// on such types causes immediate [undefined behavior][ub] because [the Rust
585 /// compiler assumes][inv] that there always is a valid value in a variable it
586 /// considers initialized.
588 /// This has the same effect as [`MaybeUninit::zeroed().assume_init()`][zeroed].
589 /// It is useful for FFI sometimes, but should generally be avoided.
591 /// [zeroed]: MaybeUninit::zeroed
592 /// [ub]: ../../reference/behavior-considered-undefined.html
593 /// [inv]: MaybeUninit#initialization-invariant
597 /// Correct usage of this function: initializing an integer with zero.
602 /// let x: i32 = unsafe { mem::zeroed() };
603 /// assert_eq!(0, x);
606 /// *Incorrect* usage of this function: initializing a reference with zero.
609 /// # #![allow(invalid_value)]
612 /// let _x: &i32 = unsafe { mem::zeroed() }; // Undefined behavior!
613 /// let _y: fn() = unsafe { mem::zeroed() }; // And again!
616 #[stable(feature = "rust1", since = "1.0.0")]
617 #[allow(deprecated_in_future)]
619 #[rustc_diagnostic_item = "mem_zeroed"]
620 pub unsafe fn zeroed<T>() -> T {
621 // SAFETY: the caller must guarantee that an all-zero value is valid for `T`.
623 intrinsics::assert_zero_valid::<T>();
624 MaybeUninit::zeroed().assume_init()
628 /// Bypasses Rust's normal memory-initialization checks by pretending to
629 /// produce a value of type `T`, while doing nothing at all.
631 /// **This function is deprecated.** Use [`MaybeUninit<T>`] instead.
633 /// The reason for deprecation is that the function basically cannot be used
634 /// correctly: it has the same effect as [`MaybeUninit::uninit().assume_init()`][uninit].
635 /// As the [`assume_init` documentation][assume_init] explains,
636 /// [the Rust compiler assumes][inv] that values are properly initialized.
637 /// As a consequence, calling e.g. `mem::uninitialized::<bool>()` causes immediate
638 /// undefined behavior for returning a `bool` that is not definitely either `true`
639 /// or `false`. Worse, truly uninitialized memory like what gets returned here
640 /// is special in that the compiler knows that it does not have a fixed value.
641 /// This makes it undefined behavior to have uninitialized data in a variable even
642 /// if that variable has an integer type.
643 /// (Notice that the rules around uninitialized integers are not finalized yet, but
644 /// until they are, it is advisable to avoid them.)
646 /// [`MaybeUninit<T>`]: MaybeUninit
647 /// [uninit]: MaybeUninit::uninit
648 /// [assume_init]: MaybeUninit::assume_init
649 /// [inv]: MaybeUninit#initialization-invariant
651 #[rustc_deprecated(since = "1.39.0", reason = "use `mem::MaybeUninit` instead")]
652 #[stable(feature = "rust1", since = "1.0.0")]
653 #[allow(deprecated_in_future)]
655 #[rustc_diagnostic_item = "mem_uninitialized"]
656 pub unsafe fn uninitialized<T>() -> T {
657 // SAFETY: the caller must guarantee that an unitialized value is valid for `T`.
659 intrinsics::assert_uninit_valid::<T>();
660 MaybeUninit::uninit().assume_init()
664 /// Swaps the values at two mutable locations, without deinitializing either one.
666 /// * If you want to swap with a default or dummy value, see [`take`].
667 /// * If you want to swap with a passed value, returning the old value, see [`replace`].
677 /// mem::swap(&mut x, &mut y);
679 /// assert_eq!(42, x);
680 /// assert_eq!(5, y);
683 #[stable(feature = "rust1", since = "1.0.0")]
684 pub fn swap<T>(x: &mut T, y: &mut T) {
685 // SAFETY: the raw pointers have been created from safe mutable references satisfying all the
686 // constraints on `ptr::swap_nonoverlapping_one`
688 ptr::swap_nonoverlapping_one(x, y);
692 /// Replaces `dest` with the default value of `T`, returning the previous `dest` value.
694 /// * If you want to replace the values of two variables, see [`swap`].
695 /// * If you want to replace with a passed value instead of the default value, see [`replace`].
699 /// A simple example:
704 /// let mut v: Vec<i32> = vec![1, 2];
706 /// let old_v = mem::take(&mut v);
707 /// assert_eq!(vec![1, 2], old_v);
708 /// assert!(v.is_empty());
711 /// `take` allows taking ownership of a struct field by replacing it with an "empty" value.
712 /// Without `take` you can run into issues like these:
714 /// ```compile_fail,E0507
715 /// struct Buffer<T> { buf: Vec<T> }
717 /// impl<T> Buffer<T> {
718 /// fn get_and_reset(&mut self) -> Vec<T> {
719 /// // error: cannot move out of dereference of `&mut`-pointer
720 /// let buf = self.buf;
721 /// self.buf = Vec::new();
727 /// Note that `T` does not necessarily implement [`Clone`], so it can't even clone and reset
728 /// `self.buf`. But `take` can be used to disassociate the original value of `self.buf` from
729 /// `self`, allowing it to be returned:
734 /// # struct Buffer<T> { buf: Vec<T> }
735 /// impl<T> Buffer<T> {
736 /// fn get_and_reset(&mut self) -> Vec<T> {
737 /// mem::take(&mut self.buf)
741 /// let mut buffer = Buffer { buf: vec![0, 1] };
742 /// assert_eq!(buffer.buf.len(), 2);
744 /// assert_eq!(buffer.get_and_reset(), vec![0, 1]);
745 /// assert_eq!(buffer.buf.len(), 0);
748 #[stable(feature = "mem_take", since = "1.40.0")]
749 pub fn take<T: Default>(dest: &mut T) -> T {
750 replace(dest, T::default())
753 /// Moves `src` into the referenced `dest`, returning the previous `dest` value.
755 /// Neither value is dropped.
757 /// * If you want to replace the values of two variables, see [`swap`].
758 /// * If you want to replace with a default value, see [`take`].
762 /// A simple example:
767 /// let mut v: Vec<i32> = vec![1, 2];
769 /// let old_v = mem::replace(&mut v, vec![3, 4, 5]);
770 /// assert_eq!(vec![1, 2], old_v);
771 /// assert_eq!(vec![3, 4, 5], v);
774 /// `replace` allows consumption of a struct field by replacing it with another value.
775 /// Without `replace` you can run into issues like these:
777 /// ```compile_fail,E0507
778 /// struct Buffer<T> { buf: Vec<T> }
780 /// impl<T> Buffer<T> {
781 /// fn replace_index(&mut self, i: usize, v: T) -> T {
782 /// // error: cannot move out of dereference of `&mut`-pointer
783 /// let t = self.buf[i];
790 /// Note that `T` does not necessarily implement [`Clone`], so we can't even clone `self.buf[i]` to
791 /// avoid the move. But `replace` can be used to disassociate the original value at that index from
792 /// `self`, allowing it to be returned:
795 /// # #![allow(dead_code)]
798 /// # struct Buffer<T> { buf: Vec<T> }
799 /// impl<T> Buffer<T> {
800 /// fn replace_index(&mut self, i: usize, v: T) -> T {
801 /// mem::replace(&mut self.buf[i], v)
805 /// let mut buffer = Buffer { buf: vec![0, 1] };
806 /// assert_eq!(buffer.buf[0], 0);
808 /// assert_eq!(buffer.replace_index(0, 2), 0);
809 /// assert_eq!(buffer.buf[0], 2);
812 #[stable(feature = "rust1", since = "1.0.0")]
813 #[must_use = "if you don't need the old value, you can just assign the new value directly"]
814 pub fn replace<T>(dest: &mut T, mut src: T) -> T {
815 swap(dest, &mut src);
819 /// Disposes of a value.
821 /// This does so by calling the argument's implementation of [`Drop`][drop].
823 /// This effectively does nothing for types which implement `Copy`, e.g.
824 /// integers. Such values are copied and _then_ moved into the function, so the
825 /// value persists after this function call.
827 /// This function is not magic; it is literally defined as
830 /// pub fn drop<T>(_x: T) { }
833 /// Because `_x` is moved into the function, it is automatically dropped before
834 /// the function returns.
843 /// let v = vec![1, 2, 3];
845 /// drop(v); // explicitly drop the vector
848 /// Since [`RefCell`] enforces the borrow rules at runtime, `drop` can
849 /// release a [`RefCell`] borrow:
852 /// use std::cell::RefCell;
854 /// let x = RefCell::new(1);
856 /// let mut mutable_borrow = x.borrow_mut();
857 /// *mutable_borrow = 1;
859 /// drop(mutable_borrow); // relinquish the mutable borrow on this slot
861 /// let borrow = x.borrow();
862 /// println!("{}", *borrow);
865 /// Integers and other types implementing [`Copy`] are unaffected by `drop`.
868 /// #[derive(Copy, Clone)]
873 /// drop(x); // a copy of `x` is moved and dropped
874 /// drop(y); // a copy of `y` is moved and dropped
876 /// println!("x: {}, y: {}", x, y.0); // still available
879 /// [`RefCell`]: crate::cell::RefCell
881 #[stable(feature = "rust1", since = "1.0.0")]
882 pub fn drop<T>(_x: T) {}
884 /// Interprets `src` as having type `&U`, and then reads `src` without moving
885 /// the contained value.
887 /// This function will unsafely assume the pointer `src` is valid for [`size_of::<U>`][size_of]
888 /// bytes by transmuting `&T` to `&U` and then reading the `&U` (except that this is done in a way
889 /// that is correct even when `&U` makes stricter alignment requirements than `&T`). It will also
890 /// unsafely create a copy of the contained value instead of moving out of `src`.
892 /// It is not a compile-time error if `T` and `U` have different sizes, but it
893 /// is highly encouraged to only invoke this function where `T` and `U` have the
894 /// same size. This function triggers [undefined behavior][ub] if `U` is larger than
897 /// [ub]: ../../reference/behavior-considered-undefined.html
909 /// let foo_array = [10u8];
912 /// // Copy the data from 'foo_array' and treat it as a 'Foo'
913 /// let mut foo_struct: Foo = mem::transmute_copy(&foo_array);
914 /// assert_eq!(foo_struct.bar, 10);
916 /// // Modify the copied data
917 /// foo_struct.bar = 20;
918 /// assert_eq!(foo_struct.bar, 20);
921 /// // The contents of 'foo_array' should not have changed
922 /// assert_eq!(foo_array, [10]);
925 #[stable(feature = "rust1", since = "1.0.0")]
926 pub unsafe fn transmute_copy<T, U>(src: &T) -> U {
927 // If U has a higher alignment requirement, src may not be suitably aligned.
928 if align_of::<U>() > align_of::<T>() {
929 // SAFETY: `src` is a reference which is guaranteed to be valid for reads.
930 // The caller must guarantee that the actual transmutation is safe.
931 unsafe { ptr::read_unaligned(src as *const T as *const U) }
933 // SAFETY: `src` is a reference which is guaranteed to be valid for reads.
934 // We just checked that `src as *const U` was properly aligned.
935 // The caller must guarantee that the actual transmutation is safe.
936 unsafe { ptr::read(src as *const T as *const U) }
940 /// Opaque type representing the discriminant of an enum.
942 /// See the [`discriminant`] function in this module for more information.
943 #[stable(feature = "discriminant_value", since = "1.21.0")]
944 pub struct Discriminant<T>(<T as DiscriminantKind>::Discriminant);
946 // N.B. These trait implementations cannot be derived because we don't want any bounds on T.
948 #[stable(feature = "discriminant_value", since = "1.21.0")]
949 impl<T> Copy for Discriminant<T> {}
951 #[stable(feature = "discriminant_value", since = "1.21.0")]
952 impl<T> clone::Clone for Discriminant<T> {
953 fn clone(&self) -> Self {
958 #[stable(feature = "discriminant_value", since = "1.21.0")]
959 impl<T> cmp::PartialEq for Discriminant<T> {
960 fn eq(&self, rhs: &Self) -> bool {
965 #[stable(feature = "discriminant_value", since = "1.21.0")]
966 impl<T> cmp::Eq for Discriminant<T> {}
968 #[stable(feature = "discriminant_value", since = "1.21.0")]
969 impl<T> hash::Hash for Discriminant<T> {
970 fn hash<H: hash::Hasher>(&self, state: &mut H) {
975 #[stable(feature = "discriminant_value", since = "1.21.0")]
976 impl<T> fmt::Debug for Discriminant<T> {
977 fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
978 fmt.debug_tuple("Discriminant").field(&self.0).finish()
982 /// Returns a value uniquely identifying the enum variant in `v`.
984 /// If `T` is not an enum, calling this function will not result in undefined behavior, but the
985 /// return value is unspecified.
989 /// The discriminant of an enum variant may change if the enum definition changes. A discriminant
990 /// of some variant will not change between compilations with the same compiler.
994 /// This can be used to compare enums that carry data, while disregarding
1000 /// enum Foo { A(&'static str), B(i32), C(i32) }
1002 /// assert_eq!(mem::discriminant(&Foo::A("bar")), mem::discriminant(&Foo::A("baz")));
1003 /// assert_eq!(mem::discriminant(&Foo::B(1)), mem::discriminant(&Foo::B(2)));
1004 /// assert_ne!(mem::discriminant(&Foo::B(3)), mem::discriminant(&Foo::C(3)));
1006 #[stable(feature = "discriminant_value", since = "1.21.0")]
1007 #[rustc_const_unstable(feature = "const_discriminant", issue = "69821")]
1008 pub const fn discriminant<T>(v: &T) -> Discriminant<T> {
1009 Discriminant(intrinsics::discriminant_value(v))
1012 /// Returns the number of variants in the enum type `T`.
1014 /// If `T` is not an enum, calling this function will not result in undefined behavior, but the
1015 /// return value is unspecified. Equally, if `T` is an enum with more variants than `usize::MAX`
1016 /// the return value is unspecified. Uninhabited variants will be counted.
1021 /// # #![feature(never_type)]
1022 /// # #![feature(variant_count)]
1027 /// enum Foo { A(&'static str), B(i32), C(i32) }
1029 /// assert_eq!(mem::variant_count::<Void>(), 0);
1030 /// assert_eq!(mem::variant_count::<Foo>(), 3);
1032 /// assert_eq!(mem::variant_count::<Option<!>>(), 2);
1033 /// assert_eq!(mem::variant_count::<Result<!, !>>(), 2);
1036 #[unstable(feature = "variant_count", issue = "73662")]
1037 #[rustc_const_unstable(feature = "variant_count", issue = "73662")]
1038 pub const fn variant_count<T>() -> usize {
1039 intrinsics::variant_count::<T>()