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, PhantomData, 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`][leak].
35 /// * If you want to obtain a raw pointer to the memory, see [`Box::into_raw`][into_raw].
36 /// * If you want to dispose of a value properly, running its destructor, see
37 /// [`mem::drop`][drop].
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 dispoe of it without
133 /// running its destructor is entirely avoided.
135 /// [drop]: fn.drop.html
136 /// [uninit]: fn.uninitialized.html
137 /// [clone]: ../clone/trait.Clone.html
138 /// [swap]: fn.swap.html
139 /// [box]: ../../std/boxed/struct.Box.html
140 /// [leak]: ../../std/boxed/struct.Box.html#method.leak
141 /// [into_raw]: ../../std/boxed/struct.Box.html#method.into_raw
142 /// [ub]: ../../reference/behavior-considered-undefined.html
143 /// [`ManuallyDrop`]: struct.ManuallyDrop.html
145 #[rustc_const_unstable(feature = "const_forget", issue = "69616")]
146 #[stable(feature = "rust1", since = "1.0.0")]
147 pub const fn forget<T>(t: T) {
148 ManuallyDrop::new(t);
151 /// Like [`forget`], but also accepts unsized values.
153 /// This function is just a shim intended to be removed when the `unsized_locals` feature gets
156 /// [`forget`]: fn.forget.html
158 #[unstable(feature = "forget_unsized", issue = "none")]
159 pub fn forget_unsized<T: ?Sized>(t: T) {
160 // SAFETY: the forget intrinsic could be safe, but there's no point in making it safe since
161 // we'll be implementing this function soon via `ManuallyDrop`
162 unsafe { intrinsics::forget(t) }
165 /// Returns the size of a type in bytes.
167 /// More specifically, this is the offset in bytes between successive elements
168 /// in an array with that item type including alignment padding. Thus, for any
169 /// type `T` and length `n`, `[T; n]` has a size of `n * size_of::<T>()`.
171 /// In general, the size of a type is not stable across compilations, but
172 /// specific types such as primitives are.
174 /// The following table gives the size for primitives.
176 /// Type | size_of::\<Type>()
177 /// ---- | ---------------
194 /// Furthermore, `usize` and `isize` have the same size.
196 /// The types `*const T`, `&T`, `Box<T>`, `Option<&T>`, and `Option<Box<T>>` all have
197 /// the same size. If `T` is Sized, all of those types have the same size as `usize`.
199 /// The mutability of a pointer does not change its size. As such, `&T` and `&mut T`
200 /// have the same size. Likewise for `*const T` and `*mut T`.
202 /// # Size of `#[repr(C)]` items
204 /// The `C` representation for items has a defined layout. With this layout,
205 /// the size of items is also stable as long as all fields have a stable size.
207 /// ## Size of Structs
209 /// For `structs`, the size is determined by the following algorithm.
211 /// For each field in the struct ordered by declaration order:
213 /// 1. Add the size of the field.
214 /// 2. Round up the current size to the nearest multiple of the next field's [alignment].
216 /// Finally, round the size of the struct to the nearest multiple of its [alignment].
217 /// The alignment of the struct is usually the largest alignment of all its
218 /// fields; this can be changed with the use of `repr(align(N))`.
220 /// Unlike `C`, zero sized structs are not rounded up to one byte in size.
224 /// Enums that carry no data other than the discriminant have the same size as C enums
225 /// on the platform they are compiled for.
227 /// ## Size of Unions
229 /// The size of a union is the size of its largest field.
231 /// Unlike `C`, zero sized unions are not rounded up to one byte in size.
238 /// // Some primitives
239 /// assert_eq!(4, mem::size_of::<i32>());
240 /// assert_eq!(8, mem::size_of::<f64>());
241 /// assert_eq!(0, mem::size_of::<()>());
244 /// assert_eq!(8, mem::size_of::<[i32; 2]>());
245 /// assert_eq!(12, mem::size_of::<[i32; 3]>());
246 /// assert_eq!(0, mem::size_of::<[i32; 0]>());
249 /// // Pointer size equality
250 /// assert_eq!(mem::size_of::<&i32>(), mem::size_of::<*const i32>());
251 /// assert_eq!(mem::size_of::<&i32>(), mem::size_of::<Box<i32>>());
252 /// assert_eq!(mem::size_of::<&i32>(), mem::size_of::<Option<&i32>>());
253 /// assert_eq!(mem::size_of::<Box<i32>>(), mem::size_of::<Option<Box<i32>>>());
256 /// Using `#[repr(C)]`.
262 /// struct FieldStruct {
268 /// // The size of the first field is 1, so add 1 to the size. Size is 1.
269 /// // The alignment of the second field is 2, so add 1 to the size for padding. Size is 2.
270 /// // The size of the second field is 2, so add 2 to the size. Size is 4.
271 /// // The alignment of the third field is 1, so add 0 to the size for padding. Size is 4.
272 /// // The size of the third field is 1, so add 1 to the size. Size is 5.
273 /// // Finally, the alignment of the struct is 2 (because the largest alignment amongst its
274 /// // fields is 2), so add 1 to the size for padding. Size is 6.
275 /// assert_eq!(6, mem::size_of::<FieldStruct>());
278 /// struct TupleStruct(u8, u16, u8);
280 /// // Tuple structs follow the same rules.
281 /// assert_eq!(6, mem::size_of::<TupleStruct>());
283 /// // Note that reordering the fields can lower the size. We can remove both padding bytes
284 /// // by putting `third` before `second`.
286 /// struct FieldStructOptimized {
292 /// assert_eq!(4, mem::size_of::<FieldStructOptimized>());
294 /// // Union size is the size of the largest field.
296 /// union ExampleUnion {
301 /// assert_eq!(2, mem::size_of::<ExampleUnion>());
304 /// [alignment]: ./fn.align_of.html
306 #[stable(feature = "rust1", since = "1.0.0")]
308 #[rustc_const_stable(feature = "const_size_of", since = "1.32.0")]
309 pub const fn size_of<T>() -> usize {
310 intrinsics::size_of::<T>()
313 /// Returns the size of the pointed-to value in bytes.
315 /// This is usually the same as `size_of::<T>()`. However, when `T` *has* no
316 /// statically-known size, e.g., a slice [`[T]`][slice] or a [trait object],
317 /// then `size_of_val` can be used to get the dynamically-known size.
319 /// [slice]: ../../std/primitive.slice.html
320 /// [trait object]: ../../book/ch17-02-trait-objects.html
327 /// assert_eq!(4, mem::size_of_val(&5i32));
329 /// let x: [u8; 13] = [0; 13];
330 /// let y: &[u8] = &x;
331 /// assert_eq!(13, mem::size_of_val(y));
334 #[stable(feature = "rust1", since = "1.0.0")]
335 pub fn size_of_val<T: ?Sized>(val: &T) -> usize {
336 intrinsics::size_of_val(val)
339 /// Returns the size of the pointed-to value in bytes.
341 /// This is usually the same as `size_of::<T>()`. However, when `T` *has* no
342 /// statically-known size, e.g., a slice [`[T]`][slice] or a [trait object],
343 /// then `size_of_val_raw` can be used to get the dynamically-known size.
347 /// This function is only safe to call if the following conditions hold:
349 /// - If `T` is `Sized`, this function is always safe to call.
350 /// - If the unsized tail of `T` is:
351 /// - a [slice], then the length of the slice tail must be an intialized
352 /// integer, and the size of the *entire value*
353 /// (dynamic tail length + statically sized prefix) must fit in `isize`.
354 /// - a [trait object], then the vtable part of the pointer must point
355 /// to a valid vtable acquired by an unsizing coersion, and the size
356 /// of the *entire value* (dynamic tail length + statically sized prefix)
357 /// must fit in `isize`.
358 /// - an (unstable) [extern type], then this function is always safe to
359 /// call, but may panic or otherwise return the wrong value, as the
360 /// extern type's layout is not known. This is the same behavior as
361 /// [`size_of_val`] on a reference to an extern type tail.
362 /// - otherwise, it is conservatively not allowed to call this function.
364 /// [slice]: ../../std/primitive.slice.html
365 /// [trait object]: ../../book/ch17-02-trait-objects.html
366 /// [extern type]: ../../unstable-book/language-features/extern-types.html
371 /// #![feature(layout_for_ptr)]
374 /// assert_eq!(4, mem::size_of_val(&5i32));
376 /// let x: [u8; 13] = [0; 13];
377 /// let y: &[u8] = &x;
378 /// assert_eq!(13, unsafe { mem::size_of_val_raw(y) });
381 #[cfg(not(bootstrap))]
382 #[unstable(feature = "layout_for_ptr", issue = "69835")]
383 pub unsafe fn size_of_val_raw<T: ?Sized>(val: *const T) -> usize {
384 intrinsics::size_of_val(val)
387 /// Returns the [ABI]-required minimum alignment of a type.
389 /// Every reference to a value of the type `T` must be a multiple of this number.
391 /// This is the alignment used for struct fields. It may be smaller than the preferred alignment.
393 /// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface
398 /// # #![allow(deprecated)]
401 /// assert_eq!(4, mem::min_align_of::<i32>());
404 #[stable(feature = "rust1", since = "1.0.0")]
405 #[rustc_deprecated(reason = "use `align_of` instead", since = "1.2.0")]
406 pub fn min_align_of<T>() -> usize {
407 intrinsics::min_align_of::<T>()
410 /// Returns the [ABI]-required minimum alignment of the type of the value that `val` points to.
412 /// Every reference to a value of the type `T` must be a multiple of this number.
414 /// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface
419 /// # #![allow(deprecated)]
422 /// assert_eq!(4, mem::min_align_of_val(&5i32));
425 #[stable(feature = "rust1", since = "1.0.0")]
426 #[rustc_deprecated(reason = "use `align_of_val` instead", since = "1.2.0")]
427 pub fn min_align_of_val<T: ?Sized>(val: &T) -> usize {
428 intrinsics::min_align_of_val(val)
431 /// Returns the [ABI]-required minimum alignment of a type.
433 /// Every reference to a value of the type `T` must be a multiple of this number.
435 /// This is the alignment used for struct fields. It may be smaller than the preferred alignment.
437 /// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface
444 /// assert_eq!(4, mem::align_of::<i32>());
447 #[stable(feature = "rust1", since = "1.0.0")]
449 #[rustc_const_stable(feature = "const_align_of", since = "1.32.0")]
450 pub const fn align_of<T>() -> usize {
451 intrinsics::min_align_of::<T>()
454 /// Returns the [ABI]-required minimum alignment of the type of the value that `val` points to.
456 /// Every reference to a value of the type `T` must be a multiple of this number.
458 /// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface
465 /// assert_eq!(4, mem::align_of_val(&5i32));
468 #[stable(feature = "rust1", since = "1.0.0")]
470 pub fn align_of_val<T: ?Sized>(val: &T) -> usize {
471 min_align_of_val(val)
474 /// Returns the [ABI]-required minimum alignment of the type of the value that `val` points to.
476 /// Every reference to a value of the type `T` must be a multiple of this number.
478 /// [ABI]: https://en.wikipedia.org/wiki/Application_binary_interface
482 /// This function is only safe to call if the following conditions hold:
484 /// - If `T` is `Sized`, this function is always safe to call.
485 /// - If the unsized tail of `T` is:
486 /// - a [slice], then the length of the slice tail must be an intialized
487 /// integer, and the size of the *entire value*
488 /// (dynamic tail length + statically sized prefix) must fit in `isize`.
489 /// - a [trait object], then the vtable part of the pointer must point
490 /// to a valid vtable acquired by an unsizing coersion, and the size
491 /// of the *entire value* (dynamic tail length + statically sized prefix)
492 /// must fit in `isize`.
493 /// - an (unstable) [extern type], then this function is always safe to
494 /// call, but may panic or otherwise return the wrong value, as the
495 /// extern type's layout is not known. This is the same behavior as
496 /// [`align_of_val`] on a reference to an extern type tail.
497 /// - otherwise, it is conservatively not allowed to call this function.
499 /// [slice]: ../../std/primitive.slice.html
500 /// [trait object]: ../../book/ch17-02-trait-objects.html
501 /// [extern type]: ../../unstable-book/language-features/extern-types.html
506 /// #![feature(layout_for_ptr)]
509 /// assert_eq!(4, unsafe { mem::align_of_val_raw(&5i32) });
512 #[cfg(not(bootstrap))]
513 #[unstable(feature = "layout_for_ptr", issue = "69835")]
514 pub unsafe fn align_of_val_raw<T: ?Sized>(val: *const T) -> usize {
515 intrinsics::min_align_of_val(val)
518 /// Returns `true` if dropping values of type `T` matters.
520 /// This is purely an optimization hint, and may be implemented conservatively:
521 /// it may return `true` for types that don't actually need to be dropped.
522 /// As such always returning `true` would be a valid implementation of
523 /// this function. However if this function actually returns `false`, then you
524 /// can be certain dropping `T` has no side effect.
526 /// Low level implementations of things like collections, which need to manually
527 /// drop their data, should use this function to avoid unnecessarily
528 /// trying to drop all their contents when they are destroyed. This might not
529 /// make a difference in release builds (where a loop that has no side-effects
530 /// is easily detected and eliminated), but is often a big win for debug builds.
532 /// Note that [`drop_in_place`] already performs this check, so if your workload
533 /// can be reduced to some small number of [`drop_in_place`] calls, using this is
534 /// unnecessary. In particular note that you can [`drop_in_place`] a slice, and that
535 /// will do a single needs_drop check for all the values.
537 /// Types like Vec therefore just `drop_in_place(&mut self[..])` without using
538 /// `needs_drop` explicitly. Types like [`HashMap`], on the other hand, have to drop
539 /// values one at a time and should use this API.
541 /// [`drop_in_place`]: ../ptr/fn.drop_in_place.html
542 /// [`HashMap`]: ../../std/collections/struct.HashMap.html
546 /// Here's an example of how a collection might make use of `needs_drop`:
549 /// use std::{mem, ptr};
551 /// pub struct MyCollection<T> {
555 /// # impl<T> MyCollection<T> {
556 /// # fn iter_mut(&mut self) -> &mut [T] { &mut self.data }
557 /// # fn free_buffer(&mut self) {}
560 /// impl<T> Drop for MyCollection<T> {
561 /// fn drop(&mut self) {
564 /// if mem::needs_drop::<T>() {
565 /// for x in self.iter_mut() {
566 /// ptr::drop_in_place(x);
569 /// self.free_buffer();
575 #[stable(feature = "needs_drop", since = "1.21.0")]
576 #[rustc_const_stable(feature = "const_needs_drop", since = "1.36.0")]
577 pub const fn needs_drop<T>() -> bool {
578 intrinsics::needs_drop::<T>()
581 /// Returns the value of type `T` represented by the all-zero byte-pattern.
583 /// This means that, for example, the padding byte in `(u8, u16)` is not
584 /// necessarily zeroed.
586 /// There is no guarantee that an all-zero byte-pattern represents a valid value of
587 /// some type `T`. For example, the all-zero byte-pattern is not a valid value
588 /// for reference types (`&T` and `&mut T`). Using `zeroed` on such types
589 /// causes immediate [undefined behavior][ub] because [the Rust compiler assumes][inv]
590 /// that there always is a valid value in a variable it considers initialized.
592 /// This has the same effect as [`MaybeUninit::zeroed().assume_init()`][zeroed].
593 /// It is useful for FFI sometimes, but should generally be avoided.
595 /// [zeroed]: union.MaybeUninit.html#method.zeroed
596 /// [ub]: ../../reference/behavior-considered-undefined.html
597 /// [inv]: union.MaybeUninit.html#initialization-invariant
601 /// Correct usage of this function: initializing an integer with zero.
606 /// let x: i32 = unsafe { mem::zeroed() };
607 /// assert_eq!(0, x);
610 /// *Incorrect* usage of this function: initializing a reference with zero.
613 /// # #![allow(invalid_value)]
616 /// let _x: &i32 = unsafe { mem::zeroed() }; // Undefined behavior!
619 #[stable(feature = "rust1", since = "1.0.0")]
620 #[allow(deprecated_in_future)]
622 #[rustc_diagnostic_item = "mem_zeroed"]
623 pub unsafe fn zeroed<T>() -> T {
624 #[cfg(not(bootstrap))]
625 intrinsics::assert_zero_valid::<T>();
627 intrinsics::panic_if_uninhabited::<T>();
628 MaybeUninit::zeroed().assume_init()
631 /// Bypasses Rust's normal memory-initialization checks by pretending to
632 /// produce a value of type `T`, while doing nothing at all.
634 /// **This function is deprecated.** Use [`MaybeUninit<T>`] instead.
636 /// The reason for deprecation is that the function basically cannot be used
637 /// correctly: it has the same effect as [`MaybeUninit::uninit().assume_init()`][uninit].
638 /// As the [`assume_init` documentation][assume_init] explains,
639 /// [the Rust compiler assumes][inv] that values are properly initialized.
640 /// As a consequence, calling e.g. `mem::uninitialized::<bool>()` causes immediate
641 /// undefined behavior for returning a `bool` that is not definitely either `true`
642 /// or `false`. Worse, truly uninitialized memory like what gets returned here
643 /// is special in that the compiler knows that it does not have a fixed value.
644 /// This makes it undefined behavior to have uninitialized data in a variable even
645 /// if that variable has an integer type.
646 /// (Notice that the rules around uninitialized integers are not finalized yet, but
647 /// until they are, it is advisable to avoid them.)
649 /// [`MaybeUninit<T>`]: union.MaybeUninit.html
650 /// [uninit]: union.MaybeUninit.html#method.uninit
651 /// [assume_init]: union.MaybeUninit.html#method.assume_init
652 /// [inv]: union.MaybeUninit.html#initialization-invariant
654 #[rustc_deprecated(since = "1.39.0", reason = "use `mem::MaybeUninit` instead")]
655 #[stable(feature = "rust1", since = "1.0.0")]
656 #[allow(deprecated_in_future)]
658 #[rustc_diagnostic_item = "mem_uninitialized"]
659 pub unsafe fn uninitialized<T>() -> T {
660 #[cfg(not(bootstrap))]
661 intrinsics::assert_uninit_valid::<T>();
663 intrinsics::panic_if_uninhabited::<T>();
664 MaybeUninit::uninit().assume_init()
667 /// Swaps the values at two mutable locations, without deinitializing either one.
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.
696 /// A simple example:
701 /// let mut v: Vec<i32> = vec![1, 2];
703 /// let old_v = mem::take(&mut v);
704 /// assert_eq!(vec![1, 2], old_v);
705 /// assert!(v.is_empty());
708 /// `take` allows taking ownership of a struct field by replacing it with an "empty" value.
709 /// Without `take` you can run into issues like these:
711 /// ```compile_fail,E0507
712 /// struct Buffer<T> { buf: Vec<T> }
714 /// impl<T> Buffer<T> {
715 /// fn get_and_reset(&mut self) -> Vec<T> {
716 /// // error: cannot move out of dereference of `&mut`-pointer
717 /// let buf = self.buf;
718 /// self.buf = Vec::new();
724 /// Note that `T` does not necessarily implement [`Clone`], so it can't even clone and reset
725 /// `self.buf`. But `take` can be used to disassociate the original value of `self.buf` from
726 /// `self`, allowing it to be returned:
731 /// # struct Buffer<T> { buf: Vec<T> }
732 /// impl<T> Buffer<T> {
733 /// fn get_and_reset(&mut self) -> Vec<T> {
734 /// mem::take(&mut self.buf)
738 /// let mut buffer = Buffer { buf: vec![0, 1] };
739 /// assert_eq!(buffer.buf.len(), 2);
741 /// assert_eq!(buffer.get_and_reset(), vec![0, 1]);
742 /// assert_eq!(buffer.buf.len(), 0);
745 /// [`Clone`]: ../../std/clone/trait.Clone.html
747 #[stable(feature = "mem_take", since = "1.40.0")]
748 pub fn take<T: Default>(dest: &mut T) -> T {
749 replace(dest, T::default())
752 /// Moves `src` into the referenced `dest`, returning the previous `dest` value.
754 /// Neither value is dropped.
758 /// A simple example:
763 /// let mut v: Vec<i32> = vec![1, 2];
765 /// let old_v = mem::replace(&mut v, vec![3, 4, 5]);
766 /// assert_eq!(vec![1, 2], old_v);
767 /// assert_eq!(vec![3, 4, 5], v);
770 /// `replace` allows consumption of a struct field by replacing it with another value.
771 /// Without `replace` you can run into issues like these:
773 /// ```compile_fail,E0507
774 /// struct Buffer<T> { buf: Vec<T> }
776 /// impl<T> Buffer<T> {
777 /// fn replace_index(&mut self, i: usize, v: T) -> T {
778 /// // error: cannot move out of dereference of `&mut`-pointer
779 /// let t = self.buf[i];
786 /// Note that `T` does not necessarily implement [`Clone`], so we can't even clone `self.buf[i]` to
787 /// avoid the move. But `replace` can be used to disassociate the original value at that index from
788 /// `self`, allowing it to be returned:
791 /// # #![allow(dead_code)]
794 /// # struct Buffer<T> { buf: Vec<T> }
795 /// impl<T> Buffer<T> {
796 /// fn replace_index(&mut self, i: usize, v: T) -> T {
797 /// mem::replace(&mut self.buf[i], v)
801 /// let mut buffer = Buffer { buf: vec![0, 1] };
802 /// assert_eq!(buffer.buf[0], 0);
804 /// assert_eq!(buffer.replace_index(0, 2), 0);
805 /// assert_eq!(buffer.buf[0], 2);
808 /// [`Clone`]: ../../std/clone/trait.Clone.html
810 #[stable(feature = "rust1", since = "1.0.0")]
811 #[must_use = "if you don't need the old value, you can just assign the new value directly"]
812 pub fn replace<T>(dest: &mut T, mut src: T) -> T {
813 swap(dest, &mut src);
817 /// Disposes of a value.
819 /// This does call the argument's implementation of [`Drop`][drop].
821 /// This effectively does nothing for types which implement `Copy`, e.g.
822 /// integers. Such values are copied and _then_ moved into the function, so the
823 /// value persists after this function call.
825 /// This function is not magic; it is literally defined as
828 /// pub fn drop<T>(_x: T) { }
831 /// Because `_x` is moved into the function, it is automatically dropped before
832 /// the function returns.
834 /// [drop]: ../ops/trait.Drop.html
841 /// let v = vec![1, 2, 3];
843 /// drop(v); // explicitly drop the vector
846 /// Since [`RefCell`] enforces the borrow rules at runtime, `drop` can
847 /// release a [`RefCell`] borrow:
850 /// use std::cell::RefCell;
852 /// let x = RefCell::new(1);
854 /// let mut mutable_borrow = x.borrow_mut();
855 /// *mutable_borrow = 1;
857 /// drop(mutable_borrow); // relinquish the mutable borrow on this slot
859 /// let borrow = x.borrow();
860 /// println!("{}", *borrow);
863 /// Integers and other types implementing [`Copy`] are unaffected by `drop`.
866 /// #[derive(Copy, Clone)]
871 /// drop(x); // a copy of `x` is moved and dropped
872 /// drop(y); // a copy of `y` is moved and dropped
874 /// println!("x: {}, y: {}", x, y.0); // still available
877 /// [`RefCell`]: ../../std/cell/struct.RefCell.html
878 /// [`Copy`]: ../../std/marker/trait.Copy.html
880 #[stable(feature = "rust1", since = "1.0.0")]
881 pub fn drop<T>(_x: T) {}
883 /// Interprets `src` as having type `&U`, and then reads `src` without moving
884 /// the contained value.
886 /// This function will unsafely assume the pointer `src` is valid for
887 /// [`size_of::<U>`][size_of] bytes by transmuting `&T` to `&U` and then reading
888 /// the `&U`. It will also unsafely create a copy of the contained value instead of
889 /// moving out of `src`.
891 /// It is not a compile-time error if `T` and `U` have different sizes, but it
892 /// is highly encouraged to only invoke this function where `T` and `U` have the
893 /// same size. This function triggers [undefined behavior][ub] if `U` is larger than
896 /// [ub]: ../../reference/behavior-considered-undefined.html
897 /// [size_of]: fn.size_of.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 ptr::read_unaligned(src as *const T as *const U)
930 /// Opaque type representing the discriminant of an enum.
932 /// See the [`discriminant`] function in this module for more information.
934 /// [`discriminant`]: fn.discriminant.html
935 #[stable(feature = "discriminant_value", since = "1.21.0")]
936 pub struct Discriminant<T>(u64, PhantomData<fn() -> T>);
938 // N.B. These trait implementations cannot be derived because we don't want any bounds on T.
940 #[stable(feature = "discriminant_value", since = "1.21.0")]
941 impl<T> Copy for Discriminant<T> {}
943 #[stable(feature = "discriminant_value", since = "1.21.0")]
944 impl<T> clone::Clone for Discriminant<T> {
945 fn clone(&self) -> Self {
950 #[stable(feature = "discriminant_value", since = "1.21.0")]
951 impl<T> cmp::PartialEq for Discriminant<T> {
952 fn eq(&self, rhs: &Self) -> bool {
957 #[stable(feature = "discriminant_value", since = "1.21.0")]
958 impl<T> cmp::Eq for Discriminant<T> {}
960 #[stable(feature = "discriminant_value", since = "1.21.0")]
961 impl<T> hash::Hash for Discriminant<T> {
962 fn hash<H: hash::Hasher>(&self, state: &mut H) {
967 #[stable(feature = "discriminant_value", since = "1.21.0")]
968 impl<T> fmt::Debug for Discriminant<T> {
969 fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
970 fmt.debug_tuple("Discriminant").field(&self.0).finish()
974 /// Returns a value uniquely identifying the enum variant in `v`.
976 /// If `T` is not an enum, calling this function will not result in undefined behavior, but the
977 /// return value is unspecified.
981 /// The discriminant of an enum variant may change if the enum definition changes. A discriminant
982 /// of some variant will not change between compilations with the same compiler.
986 /// This can be used to compare enums that carry data, while disregarding
992 /// enum Foo { A(&'static str), B(i32), C(i32) }
994 /// assert_eq!(mem::discriminant(&Foo::A("bar")), mem::discriminant(&Foo::A("baz")));
995 /// assert_eq!(mem::discriminant(&Foo::B(1)), mem::discriminant(&Foo::B(2)));
996 /// assert_ne!(mem::discriminant(&Foo::B(3)), mem::discriminant(&Foo::C(3)));
998 #[stable(feature = "discriminant_value", since = "1.21.0")]
999 #[rustc_const_unstable(feature = "const_discriminant", issue = "69821")]
1000 pub const fn discriminant<T>(v: &T) -> Discriminant<T> {
1001 Discriminant(intrinsics::discriminant_value(v), PhantomData)